ECOLOGY AND BEHAVIOR
Role of Egg Density on Establishment and Plant-to-Plant Movement by
Western Corn Rootworm Larvae (Coleoptera: Chrysomelidae)
B. E. HIBBARD,1, 2 M. L. HIGDON,1 D. P. DURAN,2 Y. M. SCHWEIKERT,2
AND
M. R. ELLERSIECK3
J. Econ. Entomol. 97(3): 871Ð882 (2004)
ABSTRACT The effect of egg density on establishment and dispersal of larvae of the western corn
rootworm, Diabrotica virgifera virgifera LeConte, was evaluated in a 3-yr Þeld study. Implications of
these data for resistance management plans for Bt crops are discussed. Viable egg levels of 100, 200,
400, 800, and 1,600 eggs per infested plant were evaluated in 2000, 2001, and 2002. A 3,200 viable egg
level was also tested in 2001 and 2002. All eggs were infested on one plant per subplot in a Þeld that
was planted to soybean, Glycine max (L.), in the previous year. For each subplot, the infested plant,
three plants down the row, the closest plant in the adjacent row of the plot, and a control plant at least
1.5 m from any infested plant (six plants total) were sampled. In 2000, there were Þve sample dates
between egg hatch and pupation, and in 2001 and 2002, there were six sample dates. On each sample
date, four replications of each egg density were sampled for both larval recovery and plant damage.
Initial establishment on a corn plant seemed to not be density-dependent because a similar percentage
of larvae was recovered from all infestation rates. Plant damage and, secondarily, subsequent postestablishment larval movement were density-dependent. Very little damage and postestablishment
movement occurred at lower infestation levels, but signiÞcant damage and movement occurred at
higher infestation rates. Movement generally occurred at a similar time as signiÞcant plant damage and
not at initial establishment, so timing of movement seemed to be motivated by available food resources
rather than crowding. At the highest infestation level in 2001, signiÞcant movement three plants down
the row and across the 0.76 m row was detected, perhaps impacting refuge strategies for transgenic
corn.
KEY WORDS larval movement, larval establishment, resistance management, Diabrotica virgifera
virgifera, maize
THE WESTERN CORN ROOTWORM, Diabrotica virgifera virgifera LeConte, is commonly considered the most important insect pest of corn, Zea mays L., throughout
much of the primary corn-growing regions of the
United States (Krysan and Miller 1986). Damage is
caused by larvae feeding directly on the roots of the
corn plants, which hinders water and nutrient uptake.
Corn hybrids that regrow roots after damage have less
yield loss under drought conditions but can be lower
yielding under adequate moisture (Gray and Steffey
1998). Since the emergence of the western corn rootworm as a major pest of corn ⬎50 yr ago, a variety of
management tactics have been implemented, but
many have failed, at least in certain regions. Because
their eggs overwinter in the soil and hatch the following spring, rotating corn to a nonhost, such as
This article reports the results of research only. Mention of a
proprietary product does not constitute an endorsement or recommendation for its use by the USDA or the University of Missouri.
1 USDAÐARS, Plant Genetics Research Unit, 205 Curtis Hall, University of Missouri, Columbia, MO 65211.
2 Department of Entomology, 1-87 Agriculture Bldg., University of
Missouri, Columbia, MO 65211.
3 Agricultural Experiment Station Statistician, 307E Middlebush,
University of Missouri, Columbia, MO 65211.
soybean, Glycine max (L.), has generally been an effective management tactic. In parts of the eastern
Corn Belt, the western corn rootworm adapted by
laying eggs in Þelds adjacent to corn (Levine and
Oloumi-Sadeghi 1996). In areas where continuous
corn is grown, soil-applied insecticide is the most common management tactic (Mayo 1986). Resistance developed to larval treatment with cyclodiene insecticides ⬎40 yr ago (Ball and Weekman 1962) and more
recently to adult treatment with organophosphate and
carbamate insecticides (Meinke et al. 1998). The population that developed resistance to adult sprays is also
resistant as larvae to the same compounds (Wright et
al. 2000). These problems, possible implications of the
Food Quality Protection Act of 1996 (Public Law
104 Ð170), and expansion of their range into Europe
(Sivcev et al. 1994) make additional strategies to manage corn rootworms highly desirable.
Transgenic corn that expresses endotoxins from the
bacterium Bacillus thuringiensis Berliner (Bt) has
been developed by several seed companies to control
damage from corn rootworm larvae (Moellenbeck et
al. 2001, Ellis et al. 2002). Due to the behavioral and
genetic plasticity of these insects, adaptation to
sources of native and transgenic resistance is a con-
872
JOURNAL OF ECONOMIC ENTOMOLOGY
cern. As part of the registration process for Bt crops in
the United States, all registrants must submit an Insect
Resistance Management Plan to the Environmental
Protection Agency (EPA). Unfortunately, a number
of gaps exist in our knowledge of corn rootworm
biology that hinder the development of an optimal
resistance management plan for these species. Efforts
to model adaptation of corn rootworms to transgenic
corn (Onstad et al. 2001, Storer 2003) have renewed
interest in rootworm basic biology because these efforts have illustrated how little we know about this
major insect pest (EPA ScientiÞc Advisory Panel
2002).
Recently, Hibbard et al. (2003) demonstrated that
western corn rootworm larvae could move at least
three plants down a row and across a 0.46-m row after
initial establishment on a plant. As discussed by Hibbard et al. (2003), information on the dispersal of
western corn rootworm larvae is limited. It has been
published (Suttle et al. 1967, Short and Luedtke 1970)
that western corn rootworm larvae can move up to
100 cm from egg hatch to pupation, but procedural
problems make the data questionable (Branson 1986).
Movement of western corn rootworm larvae through
the soil is affected by soil bulk density (Strnad and
Bergman 1987a, Ellsbury et al. 1994), soil moisture
(MacDonald and Ellis 1990), and macropores in the
soil (Gustin and Schumacher 1989). Plant damage and
lodging decreased when an artiÞcial infestation point
was 22.5 cm or farther from the plant compared with
infestation points 15 or 7.5 cm (Chaddha 1990). Compaction of inter-row soil from wheel trafÞc can help
prevent larval movement into corn from soybean areas
planted to corn the previous year in a strip intercropping system (Ellsbury et al. 1999). Other factors may
also inßuence larval movement; for instance, western
corn rootworm larvae are strongly attracted by carbon
dioxide (Strnad et al. 1986, Hibbard and Bjostad 1988)
that is released from respiring roots (Massimino et al.
1980). Factors in host roots trigger a localized search
behavior when larvae are removed from the host, and
this localized search behavior is not triggered by nonhost roots (Strnad and Dunn 1990). Larval migration
is not complete when the neonate reaches the plant.
Strnad and Bergman (1987b) demonstrated that as
larvae grow, they redistribute, moving to younger root
whorls that emerge from the stalk.
We recently studied larval movement within plots
of Bt (Cry3Bb1) corn, isoline, corn, and mixtures of
the two (B.E.H., unpublished data). We demonstrated
that western corn rootworm larvae did not move from
infested, isoline roots to adjacent Bt roots until the
isoline roots were severely damaged from larval feeding (movement from infested isoline plants to adjacent isoline plants occurred sooner). Although it was
clear that larvae had the potential to move and that Bt
could affect their choice of whether and/or when to
move, it was not clear how other factors such as initial
egg density might inßuence larval establishment and
dispersal. The goal of the current work was to determine the effect of larval density on establishment and
movement of western corn rootworm larvae.
Table 1.
trials
Vol. 97, no. 3
Key dates associated with the 2000, 2001, and 2002
Event of
importance
Corn
phenology
Planting
Infestation
Eggs documented
not hatching
1st documentation
of egg hatch
1st sampling
2nd sampling
3rd sampling
4th sampling
5th sampling
6th sampling
2000
2001
2002
V1Ð2
V4
8 May
23 May
7 June
25 April
4 May
9 June
18 April
15 May
8 June
V4Ð5
9 June
12 June
11 June
12 June
16 June
21 June
27 June
30 June
n/a
13 June
18 June
22 June
26 June
2 July
6 July
12 June
17 June
20 June
24 June
28 June
3 July
V5Ð6
V6Ð8
V7Ð10
V10Ð14
V12Ð15
V14Ð16
n/a, not applicable.
Materials and Methods
The study was conducted at the University of Missouri (MU) Bradford Research and Extension Center,
9 km east of Columbia, MO. The Mexico silt loam soil
type was determined to be 2% sand, 70% silt, and 28%
clay (Missouri University Soils Testing Laboratory).
The Þelds selected for research had been planted with
soybean in the previous year, and unlike parts of the
eastern Corn Belt, egg laying by western corn rootworm adults outside of corn has not been detected in
Missouri. Because of these two factors, we assumed
that feral western corn rootworms would not be found
in our plots, but we veriÞed this with uninfested controls. The experiment was set up in a 5 by 5 by six
(sample date ⫻ egg level ⫻ plant category) factorial
treatment arrangement in a randomized complete
block split-split-plot design in space as outlined in
Steel et al. (1997) in 2000. In 2001 and 2002, a sixth egg
level and sample date were added. The main-plot
effect was sample date, the subplot effect was egg
level, and the sub-subplot effect was plant category.
Key dates associated with planting, infestation, and
sampling were documented (Table 1). Both years,
plots were planted using an 18-cm seed spacing and
0.76-m rows by using Pioneer Brand 3394 hybrid seed
(susceptible to western corn rootworm larval feeding,
but with some tolerance). There were four replications representing a 10- (2000) or 12 (2001 and 2002)row block across the entire Þeld. In each replication,
there were six 9.15-m ranges separated by a 1.22-m
alley. Within each replication and range, there were
Þve (2000) or six (2001 and 2002) two-row plots that
were used for each egg density evaluated. At plant
emergence, two subplots were created in each tworow plot by placing a labeled stake on one of the end
plants where good germination of at least four plants
in a row had occurred. One of the subplots (chosen
randomly) was later used for larval recovery and one
was used to evaluate plant damage. A total of Þve egg
densities were evaluated in 2000: 100, 200, 400, 800, and
1,600 viable eggs per infested plant. In 2001 and 2002,
a 3,200 viable egg treatment was added. The correct
number of eggs was infested in each subplot, half on
either side of each staked plant (between rows) on
June 2004
HIBBARD ET AL.: WESTERN CORN ROOTWORM LARVAL MOVEMENT
Fig. 1. Sampling plan within a subplot.
the dates indicated (Table 1). Eggs were suspended in
dilute (0.15%) agar (AEP Colloids, Inc., Saratoga
Springs, NY), and application was calibrated so that
10 ml of solution went on either side of the infested
plant. The agar was injected into a 10-cm deep hole
that was made with a 1-cm-diameter steel rod. In
addition, two petri dishes containing eggs and soil
were buried at infestation depth away from any infested plots and were periodically examined to determine egg hatch initiation and duration. For each subplot, the infested plant, three consecutive plants down
the row, the closest plant in the adjacent row of the
plot, and a control plant at least 1.5 m from any infested
plant (six plants total), were sampled (Fig. 1, Inf, P1,
P2, P3, Row, and Cnt). Enough plots were set up for
four replications for root ratings and four replications
for larval recovery at each of six sampling times.
Within each replication, a sample date was randomly
chosen from one of the six ranges available (sampling
was destructive).
Each year, sampling was initiated shortly after larval
hatch was detected (Table 1) in petri dishes of eggs
described above. Each plant that was sampled was
initially labeled with the plot location, subplot type,
plant type, and a random code, but information on the
plant and subplot type was removed before evaluating
for root damage or searching for larvae (this information was tied to the random code for that plot but was
not known when doing root ratings or searching for
larvae). Plant phenology was also noted (Ritchie et al.
1992). Half of the subplots sampled were washed and
rated for damage by using a 0 Ð3 scale based on the
number of pruned root nodes (Oleson 1998). With the
remaining roots in each plot, whole root balls were
gently placed in Þne mesh polyethylene (onion) bags
and hung over water pans in a greenhouse. This technique, under Missouri summer conditions, was analogous to use of a Berlése funnel. Greenhouse temperatures were typically 38 Ð50⬚C during the heat of the
day. Most larvae were recovered within the Þrst 4 d
with this technique (usually peaking on days 2 and 3).
Occasionally, especially when roots were sampled just
after a rain and/or cool, cloudy days followed, larvae
were still recovered up to the sixth day. Western corn
rootworm larvae falling from the onion bags into the
water pans below were collected once or more per day
and were stored in 95% ethanol until they could be
processed. Roots were allowed to hang for larval recovery for a minimum of 7 d. During processing, each
873
larva recovered was closely examined for the presence
of urogomphi, small appendages on the posterior margin of the anal plate, which are present on southern
corn rootworm larvae, Diabrotica undecimpunctata
howardi Barber, but not on western corn rootworm
larvae (Krysan 1986). Any southern corn rootworms
found were counted and discarded. The western corn
rootworm larvae from each sample were counted, and
the total sample from an individual root ball was
weighed (wet).
Statistical Analysis. PROC MIXED of the SAS statistical package (SAS Institute 1990) was used for data
analysis. A separate analysis was done each year for
larval recovery and plant damage ratings. The 2000
data were analyzed as a 5 by 5 by 6 (sample dates ⫻
egg levels ⫻ plant categories) factorial treatment arrangement in a randomized complete block split-splitplot design in space as outlined in Steel et al. (1997).
In 2001 and 2002, an additional egg level and sample
date were added. The linear model contained the
main-plot effect of sample dates, the subplot effect of
egg levels, and the sub-subplot of plant categories, and
all possible interactions of sample dates, egg levels, and
plant categories. Replications within dates served as
the denominator of the F-test for sample dates. Replications within egg levels and sample dates were used
as the denominator of F-test for egg levels and the
interaction of egg levels ⫻ sample dates. All other
effects used the residual error for the denominator of
F. Beyond the standard analysis of variance
(ANOVA), we preplanned comparisons of egg levels
within plant categories and sampling dates, plant categories within egg levels and sampling dates, and sampling dates within egg levels and plant categories. This
was done with the t-test output from PROC MIXED.
Although nontransformed data are shown in the tables, all data were transformed by log (x ⫹ 1) for
analyses to meet the assumptions of equal variance.
The percentage of larvae recovered versus eggs
infested was also analyzed. The number of larvae recovered from each plant (Fig. 1, Inf, P1, P2, P3, and
Row, but not the control plant) from each egg level
within each sample date were Þrst totaled and then
divided by the number of eggs infested. The percentage data were then transformed by arcsine (公x) to
meet analysis assumptions (Snedecor and Cochran
1989). The transformed data were then analyzed as a
5 by 5 (2000) or 6 by 6 (2001 and 2002) split-plot in
space. The linear model contained the main-plot effect of sample dates and the subplot effect of egg levels
and the interaction of sample dates ⫻ egg levels. Replications within dates served as the denominator of F
for the main plot effect.
Data for average weight were considered as a missing value in the analysis when no larvae were recovered from a particular plant (division by zero). Because of the large number of missing values for this
analysis, combining data from the 3 yr was desirable.
For analysis of average weight, the data were Þrst
analyzed as above by year. An F-ratio was calculated
by dividing the largest error mean (year 2001) by the
smallest error mean square (year 2000); the result was
874
JOURNAL OF ECONOMIC ENTOMOLOGY
Table 2.
2000 analysis of variance
Analysis
Table 4.
Effect
df
Percent recovery Replications
Dates
Eggs
Dates ⫻ eggs
No. larvae
Replications
Dates
Plants
Dates ⫻ plants
Egg levels
Dates ⫻ eggs
Plants ⫻ eggs
Dates ⫻ plants ⫻ eggs
Damage
Replications
Dates
Plants
Dates ⫻ plants
Eggs
Dates ⫻ eggs
Plants ⫻ eggs
Dates ⫻ plants ⫻ eggs
F value
P⬎F
3, 12
0.66
0.5932
4, 12
5.14
0.0120
4, 60
2.98
0.0259
16, 60
1.21
0.2910
3, 12
0.39
0.7647
4, 12
4.90
0.0141
5, 375 106.81 ⬍0.0001
20, 375
3.06 ⬍0.0001
4, 60
11.72 ⬍0.0001
16, 60
0.83
0.6480
20, 375
6.46 ⬍0.0001
80, 375
1.14
0.2053
3, 12
0.71
0.5657
4, 12
3.55
0.0392
5, 375 24.73 ⬍0.0001
20, 375
7.96 ⬍0.0001
4, 60
4.29
0.0400
16, 60
1.76
0.0597
20, 375
2.08
0.0045
80, 375
1.06
0.3541
nonsigniÞcant. This analysis indicated years could be
pooled. Average weights of the pooled data were log
(x ⫹ 1) transformed and analyzed as a 5 by 5 by 6
(sample dates ⫻ egg levels ⫻ plant categories) factorial treatment arrangement in a randomized complete block split-split-plot design in space as outlined
above.
Results
The percentage of larvae recovered larvae was signiÞcantly affected by sample dates in all 3 yr and was
signiÞcantly affected by egg infestation levels in 2000
and 2002 (Tables 2, 3, and 4). The effect of replication
and the interaction of sample dates ⫻ egg infestation
levels were not signiÞcant in any year. Timing of larval
sampling was important for highest percentage of recovery. In all 3 yr of the study, more larvae, when
Table 3.
2001 analysis of variance
Analysis
Effect
df
F value
P⬎F
Percent recovery
Replications
Dates
Eggs
Dates ⫻ eggs
Replications
Dates
Plants
Dates ⫻ plants
Eggs
Dates ⫻ eggs
Plants ⫻ eggs
Dates ⫻ plants
⫻ eggs
Replications
Dates
Plants
Dates ⫻ plant
Eggs
Dates ⫻ eggs
Plants ⫻ eggs
Dates ⫻ plants
⫻ eggs
3, 15
5, 15
5, 90
25, 90
3, 15
5, 15
5, 540
25, 540
5, 90
25, 90
25, 540
125, 540
1.60
12.46
1.54
1.00
0.20
11.72
112.75
13.30
21.28
1.93
6.54
1.42
0.2305
⬍0.0001
0.1849
0.4779
0.8923
⬍0.0001
⬍0.0001
⬍0.0001
⬍0.0001
0.0129
⬍0.0001
0.0048
3, 15
5, 15
5, 539
25, 539
5, 90
25, 90
25, 539
125, 539
1.66
28.60
23.21
5.06
10.14
5.74
4.13
1.80
0.8923
⬍0.0001
⬍0.0001
⬍0.0001
⬍0.0001
⬍0.0001
⬍0.0001
⬍0.0001
No. larvae
Damage
Vol. 97, no. 3
2002 analysis of variance
Analysis
Effect
df
F Value
P⬎F
Percent recovery
Replications
Dates
Eggs
Dates ⫻ eggs
Replications
Dates
Plants
Dates ⫻ plant
Eggs
Dates ⫻ eggs
Plants ⫻ eggs
Dates ⫻ plants
⫻ eggs
Replications
Dates
Plants
Dates ⫻ plants
Eggs
Dates ⫻ eggs
Plants ⫻ eggs
Dates ⫻ plants
⫻ eggs
3, 15
5, 15
5, 90
25, 90
3, 15
5, 15
5, 540
25, 540
5, 90
25, 90
25, 540
125, 540
0.36
6.84
15.29
1.02
0.77
11.84
106.29
8.13
7.30
0.74
4.41
1.11
0.7797
0.0016
⬍0.0001
0.4512
0.5290
⬍0.0001
⬍0.0001
⬍0.0001
⬍0.0001
0.8017
⬍0.0001
0.2201
3, 15
5, 15
5, 540
25, 540
5, 90
25, 90
25, 540
125, 540
0.83
30.43
64.93
8.15
9.53
1.31
7.45
1.43
0.4995
⬍0.0001
⬍0.0001
⬍0.0001
⬍0.0001
0.1797
⬍0.0001
0.0039
No. larvae
Damage
expressed as a percentage of the number of eggs infested, were recovered on the third sample date than
any other sample date, signiÞcantly so in 2001 (Table
5). In general, the last sample dates had the lowest
percentage of larvae recovered (some larvae may have
pupated), and early sample dates had the next lowest
percentage of larvae recovered (some eggs may not
have hatched and small larvae are more difÞcult to
recover by using our techniques). By the third sample
date, ⬇9 Ð12 d after egg hatch was Þrst documented
from petri dishes (Table 1), all viable eggs should have
hatched, and larvae were generally larger and easy to
detect.
Overall, the number of eggs infested did not have a
consistent effect on percentage of larval establishment. In 2000, percentage of larval recovery was highest from plots infested with 800 or 400 eggs (Table 5).
In 2001, there were no signiÞcant differences in percentage of recovery between infestation levels, and
percentage of recovery from plots infested with 200
eggs was lower than percentage of recovery from even
Table 5. Percentage of recovery ⴞ SE of larvae from different
egg infestation rates
Main effect
2000
2001
2002
100 eggs
200 eggs
400 eggs
800 eggs
1,600 eggs
3,200 eggs
Sample date 1
Sample date 2
Sample date 3
Sample date 4
Sample date 5
Sample date 6
1.40 ⫾ 0.46b
1.78 ⫾ 0.41ab
2.61 ⫾ 0.69a
2.62 ⫾ 0.50a
1.07 ⫾ 0.27b
n/a
1.14 ⫾ 0.58c
1.56 ⫾ 0.40bc
3.15 ⫾ 0.60a
2.51 ⫾ 0.38ab
1.12 ⫾ 0.33c
n/a
1.63 ⫾ 0.61a
0.60 ⫾ 0.21a
1.49 ⫾ 0.42a
0.77 ⫾ 0.20a
0.57 ⫾ 0.19a
0.75 ⫾ 0.25a
1.27 ⫾ 0.36b
1.25 ⫾ 0.30b
2.50 ⫾ 0.60a
0.68 ⫾ 0.14b
0.08 ⫾ 0.04c
0.03 ⫾ 0.01c
8.37 ⫾ 1.52a
5.17 ⫾ 1.25b
2.71 ⫾ 0.48c
2.61 ⫾ 0.56c
1.95 ⫾ 0.53c
1.49 ⫾ 0.36c
5.01 ⫾ 1.19a
4.75 ⫾ 0.65a
5.73 ⫾ 1.28a
3.85 ⫾ 1.25ab
2.38 ⫾ 0.66b
0.57 ⫾ 0.15c
n/a, not applicable.
SigniÞcant differences within a year and main effect are indicated
by different lowercase letters.
June 2004
HIBBARD ET AL.: WESTERN CORN ROOTWORM LARVAL MOVEMENT
875
Table 6. Number of western corn rootworm larvae ⴞ SE recovered in 2000 from varying infestation levels of western corn rootworm
eggs on the infested (Inf) plant
Date
Plant
6/12
6/16
6/21
6/27
6/30
6/12
6/16
6/21
6/27
6/30
6/12
6/16
6/21
6/27
6/30
6/12
6/16
6/21
6/27
6/30
6/12
6/16
6/21
6/27
6/30
6/12
6/16
6/21
6/27
6/30
All
Inf
Inf
Inf
Inf
Inf
P1
P1
P1
P1
P1
P2
P2
P2
P2
P2
P3
P3
P3
P3
P3
Row
Row
Row
Row
Row
Cnt
Cnt
Cnt
Cnt
Cnt
All
No. of eggs infested
100
200
400
800
1,600
0.0 ⫾ 0.0bCm
0.3 ⫾ 0.3bBCm
3.0 ⫾ 1.7bAm
1.3 ⫾ 0.3cABm
0.8 ⫾ 0.8bABCm
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0bAm
0.8 ⫾ 0.8bcAn
0.0 ⫾ 0.0cAn
0.0 ⫾ 0.0bAm
0.0 ⫾ 0.0aAm
0.3 ⫾ 0.3aAm
0.0 ⫾ 0.0aAn
0.8 ⫾ 0.8bAmn
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAm
0.3 ⫾ 0.3aAm
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAm
0.24 ⫾ 0.08c
2.0 ⫾ 1.1aBCm
0.5 ⫾ 0.3bCm
5.8 ⫾ 3.1bAm
1.0 ⫾ 0.7cBCmn
3.5 ⫾ 1.8aABm
0.0 ⫾ 0.0aBn
0.0 ⫾ 0.0bBm
0.0 ⫾ 0.0cBn
2.0 ⫾ 1.2bAm
1.0 ⫾ 0.7abABn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAn
0.5 ⫾ 0.5bAmn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAn
1.5 ⫾ 1.2aAmn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.3 ⫾ 0.3aAm
0.0 ⫾ 0.0aAn
0.3 ⫾ 0.3aAmn
0.0 ⫾ 0.0aAn
0.61 ⫾ 0.17c
12.0 ⫾ 11.0aABm
12.8 ⫾ 5.5aAm
7.0 ⫾ 4.1bABm
3.8 ⫾ 1.0bBm
2.0 ⫾ 1.4abBm
0.0 ⫾ 0.0aBn
0.3 ⫾ 0.3bBn
1.8 ⫾ 1.8bcBn
4.8 ⫾ 1.5aAm
2.0 ⫾ 2.0abBmn
0.0 ⫾ 0.0aBn
0.3 ⫾ 0.3aBn
0.3 ⫾ 0.3aBn
4.8 ⫾ 3.5aAm
0.0 ⫾ 0.0aBn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.5 ⫾ 0.3aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.3 ⫾ 0.3aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.3 ⫾ 0.3aAn
0.3 ⫾ 0.3aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
1.76 ⫾ 0.51b
9.5 ⫾ 7.2aCDm
13.8 ⫾ 4.9aBCm
30.0 ⫾ 7.9aAm
22.8 ⫾ 7.5aABm
4.3 ⫾ 1.9aDm
1.0 ⫾ 0.7aBn
3.5 ⫾ 2.6aABn
7.3 ⫾ 3.3aAn
2.3 ⫾ 0.6abABn
3.5 ⫾ 3.5aABn
0.0 ⫾ 0.0aBn
0.0 ⫾ 0.0aBo
0.8 ⫾ 0.5aABo
2.5 ⫾ 1.0aAn
1.5 ⫾ 1.5aABn
0.0 ⫾ 0.0aAn
0.3 ⫾ 0.3aAo
0.5 ⫾ 0.5aAo
0.5 ⫾ 0.3aAo
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.3 ⫾ 0.3aAo
0.8 ⫾ 0.8aAo
0.0 ⫾ 0.0aAo
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.3 ⫾ 0.3aAo
0.0 ⫾ 0.0aAo
0.0 ⫾ 0.0aAo
0.0 ⫾ 0.0aAn
3.50 ⫾ 0.78a
4.5 ⫾ 2.7aBm
17.8 ⫾ 8.2aAm
27.0 ⫾ 11.9aAm
12.8 ⫾ 7.6bAm
4.0 ⫾ 3.4aBm
1.5 ⫾ 1.2aBm
6.8 ⫾ 2.7aAm
3.3 ⫾ 2.3bABn
3.8 ⫾ 3.1abABn
2.8 ⫾ 2.1aABm
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.3 ⫾ 0.3aAo
0.5 ⫾ 0.5bAno
0.5 ⫾ 0.3aAmn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAo
0.3 ⫾ 0.3aAo
0.0 ⫾ 0.0aAn
0.3 ⫾ 0.3aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAo
0.3 ⫾ 0.3aAo
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAn
0.0 ⫾ 0.0aAo
0.0 ⫾ 0.0aAo
0.0 ⫾ 0.0aAn
2.87 ⫾ 0.75ab
Although untransformed data are shown, statistics were performed using log(x ⫹ 1) data. Different lowercase letters starting with an “a”
indicate a signiÞcant difference within a row. Different uppercase letters indicate a signiÞcant difference within a column and plant. Different
lowercase letters starting with “m” indicate a signiÞcant difference between plants, but within a treatment and date.
the 3,200 egg infestations (Table 5). In 2002, there
seemed to be a clear trend of recovering a slightly
higher percentage of larvae from infestation rates with
fewer eggs infested (Table 5), although other explanations are likely for the discrepancy of 2002 data from
2000 and 2001 (see Discussion).
Egg density was an important factor in plant damage
and subsequent larval movement. In all 3 yr, main
effects for sample dates, plant categories, and egg
levels in addition to the interaction of plant categories ⫻ dates and plant categories ⫻ egg levels, significantly affected plant damage and the number of larvae recovered (Tables 2Ð 4). In 2001, the interaction of
sample dates ⫻ egg levels and the three-way interaction of sample dates ⫻ plant categories ⫻ egg levels
also signiÞcantly affected larval recovery and plant
damage (Table 3). The three-way interaction was also
signiÞcant for plant damage in 2002 (Table 4). Differences among replication were not signiÞcant in any
analysis.
SigniÞcantly more western corn rootworm larvae
were recovered from P2 plants on the fourth sample
date than the Þrst sample date for those plots infested
with 400 or 800 viable western corn rootworm eggs in
2000, but no such differences were observed with
other infestation rates (Table 6). Similar differences
occurred for P1 plants in 2000 from plots infested with
200, 400, 800, or 1,600 viable eggs, but no apparent
larval movement occurred after initial establishment
when infested with 100 viable eggs in 2000. In 2001,
signiÞcantly more western corn rootworm larvae were
recovered from P3 plants on the third sample date
than the Þrst sample date when the plots were infested
with 3,200 viable eggs (Table 7). SigniÞcantly more
larvae were also recovered across the row on the
fourth sample date than the Þrst sample date when the
plots were infested with 3,200 viable eggs, the Þrst
documentation across a 76-cm row. Such postestablishment movement to the P3 and across row plants
did not occur when infested with lower numbers of
eggs in 2001; however, movement to P2 plants was
documented on the fourth sample date when the plot
was infested with 800 and 3,200 viable eggs (Table 7).
Postestablishment movement to P1 plants occurred
with all infestation levels except 200 viable eggs in
2001. In 2002, no statistically signiÞcant postestablishment larval movement was documented at any infestation level (Table 8).
Postestablishment movement was also demonstrated by plant damage results. In 2000, statistically
signiÞcant damage occurred to the infested plant by
the last sample date with all infestation levels except
the lowest level (Table 9). SigniÞcant damage also
occurred on the last sample date on P1 plants when the
876
JOURNAL OF ECONOMIC ENTOMOLOGY
Vol. 97, no. 3
Table 7. Number of western corn rootworm larvae ⴞ SE recovered in 2001 from varying infestation levels of western corn rootworm
eggs on the infested (Inf) plant
Date
Plant
6/18
6/22
6/26
7/02
7/06
7/12
6/18
6/22
6/26
7/02
7/06
7/12
6/18
6/22
6/26
7/02
7/06
7/12
6/18
6/22
6/26
7/02
7/06
7/12
6/18
6/22
6/26
7/02
7/06
7/12
6/18
6/22
6/26
7/02
7/06
7/12
All
Inf
Inf
Inf
Inf
Inf
Inf
P1
P1
P1
P1
P1
P1
P2
P2
P2
P2
P2
P2
P3
P3
P3
P3
P3
P3
Row
Row
Row
Row
Row
Row
Cnt
Cnt
Cnt
Cnt
Cnt
Cnt
All
No. of eggs infested
100
2.0 ⫾ 1.7 bABm
0.5 ⫾ 0.5 cBCm
3.8 ⫾ 2.3 cAm
0.5 ⫾ 0.3 dBCm
0.0 ⫾ 0.0 bCm
0.0 ⫾ 0.0 aCm
0.0 ⫾ 0.0 aBn
0.3 ⫾ 0.3 bABm
1.3 ⫾ 0.6 bAm
0.0 ⫾ 0.0 bBm
0.0 ⫾ 0.0 aBm
0.0 ⫾ 0.0 aBm
0.5 ⫾ 0.5 aAmn
0.5 ⫾ 0.5 aAm
0.0 ⫾ 0.0 aAn
0.3 ⫾ 0.3 bcAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 bAn
0.0 ⫾ 0.0 bAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 bAm
0.3 ⫾ 0.3 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
0.5 ⫾ 0.5 aAm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.028 ⫹ 0.10c
200
400
800
1,600
3,200
1.5 ⫾ 0.6 bAm
1.0 ⫾ 0.4 cAm
0.8 ⫾ 0.8 cABm
0.8 ⫾ 0.5 cdABm
0.0 ⫾ 0.0 bBm
0.0 ⫾ 0.0 aBm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 bAn
0.0 ⫾ 0.0 cAm
0.0 ⫾ 0.0 bAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 cAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAn
0.8 ⫾ 0.8 abAm
1.3 ⫾ 0.9 abAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAn
1.3 ⫾ 1.3 aAm
0.0 ⫾ 0.0 bAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
0.3 ⫾ 0.3 aAmn
0.3 ⫾ 0.3 aAm
0.3 ⫾ 0.3 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.22 ⫹ 0.06c
4.3 ⫾ 2.4 bBm
7.3 ⫾ 2.9 bBm
17.0 ⫾ 3.1 abAm
1.3 ⫾ 0.9 cdCm
0.0 ⫾ 0.0 bCm
0.0 ⫾ 0.0 aCm
0.0 ⫾ 0.0 aCn
0.0 ⫾ 0.0 bCn
1.8 ⫾ 1.1 bAn
1.0 ⫾ 0.4 aABm
0.0 ⫾ 0.0 aCm
0.3 ⫾ 0.3 aBCm
0.0 ⫾ 0.0 aAn
0.8 ⫾ 0.8 aAn
0.0 ⫾ 0.0 aAo
0.3 ⫾ 0.3 bcAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
1.3 ⫾ 1.3 aAn
0.0 ⫾ 0.0 bAo
0.0 ⫾ 0.0 bAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAn
0.8 ⫾ 0.8 aAno
0.0 ⫾ 0.0 bAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAo
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.99 ⫹ 0.29b
11.8 ⫾ 8.0 abAm
4.3 ⫾ 1.7 bABm
9.3 ⫾ 0.6 bAm
2.5 ⫾ 1.3 bcBm
0.8 ⫾ 0.5 abBm
0.0 ⫾ 0.0 aBm
0.0 ⫾ 0.0 aCn
1.0 ⫾ 0.6 bABCn
2.3 ⫾ 1.1 abAn
1.3 ⫾ 0.5 aABmn
0.3 ⫾ 0.3 aBCm
0.3 ⫾ 0.3 aBCm
0.0 ⫾ 0.0 aBn
0.3 ⫾ 0.3 aABn
0.3 ⫾ 0.3 aABo
1.5 ⫾ 1.0 bAmn
0.0 ⫾ 0.0 aBm
0.3 ⫾ 0.3 aABm
0.3 ⫾ 0.3 aAn
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 bAo
0.3 ⫾ 0.3 bAn
0.3 ⫾ 0.3 aAm
0.0 ⫾ 0.0 aAm
0.3 ⫾ 0.3 aAn
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAo
0.3 ⫾ 0.3 abAn
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAn
0.3 ⫾ 0.3 aAo
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
1.03 ⫹ 0.29b
23.8 ⫾ 16.2 aAm
8.3 ⫾ 4.8 bBm
11.0 ⫾ 4.8 bAm
3.8 ⫾ 0.9 abBm
0.0 ⫾ 0.0 bCm
0.3 ⫾ 0.3 aCm
0.0 ⫾ 0.0 aBn
0.5 ⫾ 0.5 bBn
2.3 ⫾ 1.3 abAn
2.3 ⫾ 1.3 aAm
0.0 ⫾ 0.0 aBm
0.0 ⫾ 0.0 aBm
0.3 ⫾ 0.3 aABn
0.3 ⫾ 0.3 aABn
0.5 ⫾ 0.3 aABno
1.0 ⫾ 0.4 bAn
0.0 ⫾ 0.0 aBm
0.0 ⫾ 0.0 aBm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 bAo
0.3 ⫾ 0.3 bAn
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
0.3 ⫾ 0.3 aAn
0.3 ⫾ 0.3 aAo
0.3 ⫾ 0.3 abAn
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAo
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAm
0.0 ⫾ 0.0 aAm
1.53 ⫹ 0.56b
9.3 ⫾ 3.6 abBm
65.5 ⫾ 36.8 aAm
36.8 ⫾ 12.4 aAm
6.8 ⫾ 2.6 aBm
1.8 ⫾ 0.9 aCm
0.5 ⫾ 0.5 aCm
0.3 ⫾ 0.3 aBn
4.3 ⫾ 1.8 aAn
4.5 ⫾ 2.0 aAn
2.3 ⫾ 1.3 aAn
0.3 ⫾ 0.3 aBn
0.3 ⫾ 0.3 aBm
0.0 ⫾ 0.0 aBn
0.0 ⫾ 0.0 aBo
0.5 ⫾ 0.5 aBp
3.8 ⫾ 1.5 aAmn
0.5 ⫾ 0.3 aBmn
0.8 ⫾ 0.8 aBm
0.0 ⫾ 0.0 aBn
0.0 ⫾ 0.0 aBo
3.0 ⫾ 2.7 aAo
2.3 ⫾ 1.0 aAn
0.0 ⫾ 0.0 aBn
0.0 ⫾ 0.0 aBm
0.0 ⫾ 0.0 aBn
0.0 ⫾ 0.0 aBo
0.3 ⫾ 0.3 aABp
1.0 ⫾ 0.4 aAno
0.3 ⫾ 0.3 aABn
0.0 ⫾ 0.0 aBm
0.0 ⫾ 0.0 aAn
0.0 ⫾ 0.0 aAo
0.0 ⫾ 0.0 aAp
0.3 ⫾ 0.3 aAo
0.5 ⫾ 0.5 aAn
0.0 ⫾ 0.0 aAm
4.03 ⫹ 1.39a
Although untransformed data are shown, statistics were performed using log (x ⫹ 1) data. Different lowercase letters starting with an “a”
indicate a signiÞcant difference within a row. Different uppercase letters indicate a signiÞcant difference within a column and plant. Different
lowercase letters starting with “m” indicate a signiÞcant difference between plants, but within a treatment and date.
plots were infested with 800 and 1,600 viable western
corn rootworm eggs, but signiÞcant damage did not
occur to P1 plants at other infestation levels in 2000.
SigniÞcant damage did not occur on P2, P3, control, or
plants across the row in 2000 (Table 9). When data
from all plants were averaged together in 2000, signiÞcantly more damage occurred when the infested
plant had 800 or 1,600 viable eggs than with 100 or
200 viable eggs. In 2001, statistically signiÞcant damage
occurred on the P2 plant on the Þfth sample date
when the plots were infested with 3,200 viable eggs
(Table 10). SigniÞcant damage also occurred to P1
plants when infested with 400, 800, or 3,200 viable
eggs. SigniÞcant damage occurred to the infested plant
at all infestation levels except 100 and 200 viable eggs.
In 2002, signiÞcant damage occurred to the infested
plant at all infestation levels, to the P1 plant at infestation rates of 800, 1,600, and 3,200 viable eggs and to
the P2 plant with infestation rates of 1,600 and 3,200
viable eggs (Table 11). The average damage rating to
the P2 plant on the last sample data when infested with
3,200 viable eggs was 0.71.
The effect of egg density did not signiÞcantly affect
average larval weight when averaged across years,
sample dates, and plant categories (F ⫽ 0.38; df ⫽ 4,
248; P ⫽ 0.82). As expected, average larval weight
signiÞcantly increased with later sample dates (F ⫽
21.3; df ⫽ 4, 248; P ⬍ 0.0001). Larvae recovered from
the infested plant had an average larval weight that
was signiÞcantly lower than larvae recovered from all
other plants within a subplot when averaged across
years, sample dates, and egg levels (F ⫽ 3.51; df ⫽ 5,
248; P ⫽ 0.0044).
Discussion
Branson and Sutter (1985) evaluated the performance of western corn rootworm adults from Þeld
plots with varying densities of western corn rootworm
larvae. They documented that high densities of eggs
resulted in not only greater plant damage but also a
lower percentage of adults being recovered (densitydependent mortality). The adults that they did recover had a smaller head capsule width, died earlier,
June 2004
HIBBARD ET AL.: WESTERN CORN ROOTWORM LARVAL MOVEMENT
877
Table 8. Number of western corn rootworm larvae ⴞ SE recovered in 2002 from varying infestation levels of western corn rootworm
eggs on the infested (Inf) plant
Date
Plant
6/12
6/17
6/20
6/24
6/28
7/3
6/12
6/17
6/20
6/24
6/28
7/3
6/12
6/17
6/20
6/24
6/28
7/3
6/12
6/17
6/20
6/24
6/28
7/3
6/12
6/17
6/20
6/24
6/28
7/3
6/12
6/17
6/20
6/24
6/28
7/3
All
Inf
Inf
Inf
Inf
Inf
Inf
P1
P1
P1
P1
P1
P1
P2
P2
P2
P2
P2
P2
P3
P3
P3
P3
P3
P3
Row
Row
Row
Row
Row
Row
Cnt
Cnt
Cnt
Cnt
Cnt
Cnt
All
No. of eggs infested
100
200
400
800
1,600
3,200
7.0 ⫾ 1.6dAm
4.8 ⫾ 1.4deAm
6.5 ⫾ 1.6cAm
2.3 ⫾ 1.3aAm
2.5 ⫾ 1.9aAm
0.3 ⫾ 0.3aAm
0.8 ⫾ 0.5aAm
1.0 ⫾ 0.7aAm
3.8 ⫾ 1.5aAm
2.5 ⫾ 1.0aAm
2.0 ⫾ 1.4aAm
0.0 ⫾ 0.0aAm
3.0 ⫾ 2.3aAm
0.3 ⫾ 0.3aAm
2.3 ⫾ 1.3aAm
1.8 ⫾ 0.8aAm
1.3 ⫾ 0.8aAm
0.3 ⫾ 0.3aAm
0.8 ⫾ 0.8aAm
0.8 ⫾ 0.8aAm
1.0 ⫾ 0.6aAm
0.8 ⫾ 0.5aAm
1.0 ⫾ 0.7aAm
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAm
0.3 ⫾ 0.3aAm
1.5 ⫾ 1.2aAm
0.0 ⫾ 0.0aAm
0.8 ⫾ 0.5aAm
1.3 ⫾ 0.9aAm
6.5 ⫾ 5.3aAm
1.0 ⫾ 1.0aAm
0.0 ⫾ 0.0aAm
0.5 ⫾ 0.3aAm
0.3 ⫾ 0.3aAm
1.65 ⫾ 0.25c
5.8 ⫾ 2.4dAm
3.3 ⫾ 2.0eAm
5.0 ⫾ 1.1cAm
3.0 ⫾ 2.7aAm
1.0 ⫾ 0.6aAm
0.0 ⫾ 0.0aAm
1.0 ⫾ 0.6aAm
0.8 ⫾ 0.3aAm
1.5 ⫾ 0.5aAm
9.5 ⫾ 5.7aAm
1.8 ⫾ 1.2aAm
1.0 ⫾ 0.4aAm
1.8 ⫾ 0.9aAm
2.5 ⫾ 1.0aAm
3.3 ⫾ 1.0aAm
2.3 ⫾ 1.0aAm
0.5 ⫾ 0.5aAm
0.3 ⫾ 0.3aAm
0.5 ⫾ 0.3aAm
0.8 ⫾ 0.3aAm
9.0 ⫾ 7.7aAm
0.8 ⫾ 0.5aAm
1.0 ⫾ 1.0aAm
0.5 ⫾ 0.5aAm
1.3 ⫾ 0.8aAm
0.8 ⫾ 0.5aAm
1.5 ⫾ 1.2aAm
0.5 ⫾ 0.3aAm
0.0 ⫾ 0.0aAm
0.8 ⫾ 0.3aAm
2.0 ⫾ 1.7aAm
0.5 ⫾ 0.3aAm
0.8 ⫾ 0.5aAm
0.0 ⫾ 0.0aAm
1.3 ⫾ 0.9aAm
0.0 ⫾ 0.0aAm
1.85 ⫾ 0.33c
7.0 ⫾ 3.9dABm
17.3 ⫾ 7.9cdAm
7.3 ⫾ 1.3cABm
4.0 ⫾ 1.1aABm
5.3 ⫾ 2.8aABm
1.3 ⫾ 0.9aBm
1.0 ⫾ 0.6aAm
1.8 ⫾ 1.2aAn
1.5 ⫾ 0.6aAm
2.0 ⫾ 0.9aAm
1.5 ⫾ 0.6aAm
0.5 ⫾ 0.5aAm
1.5 ⫾ 1.0aAm
1.5 ⫾ 1.2aAn
1.5 ⫾ 1.5aAm
2.0 ⫾ 1.4aAm
1.5 ⫾ 1.5aAm
0.0 ⫾ 0.0aAm
0.5 ⫾ 0.5aAm
0.3 ⫾ 0.3aAn
0.5 ⫾ 0.5aAm
1.0 ⫾ 1.0aAm
1.0 ⫾ 0.7aAm
0.3 ⫾ 0.3aAm
0.8 ⫾ 0.8aAm
0.3 ⫾ 0.3aAn
0.3 ⫾ 0.3aAm
0.0 ⫾ 0.0aAm
1.3 ⫾ 0.9aAm
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAn
0.3 ⫾ 0.3aAm
0.5 ⫾ 0.5aAm
1.3 ⫾ 0.9aAm
0.0 ⫾ 0.0aAm
1.86 ⫾ 0.36c
28.0 ⫾ 20.2cAm
30.3 ⫾ 5.8bcAm
14.3 ⫾ 6.9cABm
4.3 ⫾ 1.3aBm
6.0 ⫾ 1.5aBm
1.5 ⫾ 0.9aBm
1.0 ⫾ 0.4aAn
2.5 ⫾ 0.5aAn
2.5 ⫾ 1.3aAm
3.8 ⫾ 1.1aAm
3.3 ⫾ 1.4aAm
0.5 ⫾ 0.3aAm
0.5 ⫾ 0.3aAn
3.8 ⫾ 3.1aAn
1.3 ⫾ 0.9aAm
1.3 ⫾ 0.6aAm
0.3 ⫾ 0.3aAm
0.5 ⫾ 0.5aAm
1.0 ⫾ 0.7aAn
0.5 ⫾ 0.3aAn
5.8 ⫾ 4.2aAm
1.3 ⫾ 0.8aAm
2.0 ⫾ 1.1aAm
0.0 ⫾ 0.0aAm
2.0 ⫾ 1.2aAn
3.5 ⫾ 2.5aAn
0.3 ⫾ 0.3aAm
2.5 ⫾ 1.4aAm
0.8 ⫾ 0.8aAm
0.0 ⫾ 0.0aAm
0.8 ⫾ 0.3aAn
1.5 ⫾ 0.9aAn
1.3 ⫾ 1.3aAm
0.8 ⫾ 0.8aAm
3.8 ⫾ 3.4aAm
0.3 ⫾ 0.3aAm
3.72 ⫾ 0.80bc
71.3 ⫾ 36.2aAm
31.5 ⫾ 12.5bBm
29.3 ⫾ 14.0bBm
5.8 ⫾ 1.4aCm
2.3 ⫾ 0.5aCm
0.3 ⫾ 0.3aCm
4.5 ⫾ 3.2aAn
5.0 ⫾ 0.9aAn
6.3 ⫾ 3.7aAn
2.3 ⫾ 0.9aAm
3.8 ⫾ 2.1aAm
0.3 ⫾ 0.3aAm
0.3 ⫾ 0.3aAn
1.3 ⫾ 0.6aAn
2.8 ⫾ 1.0aAn
3.3 ⫾ 2.3aAm
1.5 ⫾ 0.9aAm
0.0 ⫾ 0.0aAm
0.0 ⫾ 0.0aAn
0.3 ⫾ 0.3aAn
1.0 ⫾ 0.7aAn
1.3 ⫾ 0.8aAm
1.5 ⫾ 0.9aAm
0.3 ⫾ 0.3aAm
1.0 ⫾ 0.4aAn
3.3 ⫾ 1.7aAn
4.8 ⫾ 3.5aAn
1.3 ⫾ 0.9aAm
0.3 ⫾ 0.3aAm
0.3 ⫾ 0.3aAm
1.0 ⫾ 1.0aAn
0.0 ⫾ 0.0aAn
0.8 ⫾ 0.8aAn
2.0 ⫾ 2.0aAm
0.5 ⫾ 0.5aAm
0.0 ⫾ 0.0aAm
5.31 ⫾ 1.48ab
41.3 ⫾ 6.1bBm
126.0 ⫾ 43.3aAm
43.3 ⫾ 19.8aBm
9.5 ⫾ 3.1aCm
8.3 ⫾ 3.0aCm
0.5 ⫾ 0.5aCm
4.3 ⫾ 1.9aAn
9.8 ⫾ 2.4aAn
7.3 ⫾ 4.1aAn
4.8 ⫾ 1.9aAm
5.3 ⫾ 2.3aAm
1.5 ⫾ 0.9aAm
1.0 ⫾ 0.6aAn
1.8 ⫾ 1.8aAn
2.0 ⫾ 0.9aAn
6.0 ⫾ 2.0aAm
2.5 ⫾ 1.2aAm
0.3 ⫾ 0.3aAm
0.3 ⫾ 0.3aAn
1.3 ⫾ 0.6aAn
0.8 ⫾ 0.8aAn
2.3 ⫾ 0.6aAm
1.8 ⫾ 1.2aAm
0.0 ⫾ 0.0aAm
0.3 ⫾ 0.3aAn
0.0 ⫾ 0.0aAn
2.5 ⫾ 1.8aAn
1.8 ⫾ 0.9aAm
0.8 ⫾ 0.5aAm
0.0 ⫾ 0.0aAm
2.8 ⫾ 2.4aAn
0.0 ⫾ 0.0aAn
0.3 ⫾ 0.3aAn
1.0 ⫾ 0.7aAm
2.8 ⫾ 1.4aAm
1.3 ⫾ 1.3aAm
8.18 ⫾ 2.19a
Although untransformed data are shown, statistics were performed using log (x ⫹ 1) data. Different lowercase letters starting with an “a”
indicate a signiÞcant difference within a row. Different uppercase letters indicate a signiÞcant difference within a column and plant. Different
lowercase letters starting with “m” indicate a signiÞcant difference between plants, but within a treatment and date.
and laid fewer eggs per female. In a similar set of
studies conducted in the greenhouse, Weiss et al.
(1985) documented that higher infestation rates resulted in longer developmental times and an altered
sex ratio of adults to produce a higher percentage of
males. Stressed larvae, in general, will produce inferior
adults (Peters and Barbosa 1977); however, the timing
of stress (whether during host Þnding or peak larval
feeding) has not been evaluated previously with the
western corn rootworm.
In his model, Storer (2003) included an assumption
that “most density-dependence occurs after the larvae
have become established but before they have
reached adulthood.” Data from the current study support this previously undocumented assumption. In
2001, egg density did not signiÞcantly affect percentage of larval recovery (Tables 3 and 5), and percentage
of recovery from plots infested with 200 eggs was
actually nominally lower than percentage of recovery
from even the 3,200 egg infestations (Table 5). In 2000,
densities of 400 and 800 eggs had the highest percentage of recovery, a signiÞcantly higher percent-
age recovery than from 100 viable eggs (Table 5). In
2002, there was the appearance of signiÞcant densitydependent mortality during establishment; however,
⬎1100 morphologically distinguished southern corn
rootworm larvae were recovered from the experiment
that year. This compared with only 23 in 2000 and
seven in 2001. Unfortunately, as indicated by the number of larvae recovered from control plants in 2002
(identiÞed morphologically as western corn rootworms), some of the larvae morphologically identiÞed
as western corn rootworm larvae that year were likely
actually southern corn rootworm larvae. B.E.H. (unpublished data) used the methods of Clark et al.
(2001a, b) to evaluate known western corn rootworm
larvae, known southern corn rootworm larvae, and
unknown larvae (morphologically identiÞed as western corn rootworm larvae from the same 2002 Bradford Research and Extension Center Þeld as the current study). Of 83 larvae morphologically identiÞed as
western corn rootworm in 2002, 14 were actually
southern corn rootworm larvae. Given the nature of
the percentage of recovery equation, the effect of any
878
JOURNAL OF ECONOMIC ENTOMOLOGY
Vol. 97, no. 3
Table 9. Plant damage (0 –3 scale) ⴞ SE from western corn rootworm larvae in 2000 from varying infestation levels of western corn
rootworm eggs on the infested (Inf) plant
Date
Plant
6/12
6/16
6/21
6/27
6/30
6/12
6/16
6/21
6/27
6/30
6/12
6/16
6/21
6/27
6/30
6/12
6/16
6/21
6/27
6/30
6/12
6/16
6/21
6/27
6/30
6/12
6/16
6/21
6/27
6/30
All
Inf
Inf
Inf
Inf
Inf
P1
P1
P1
P1
P1
P2
P2
P2
P2
P2
P3
P3
P3
P3
P3
Row
Row
Row
Row
Row
Cnt
Cnt
Cnt
Cnt
Cnt
All
No. of eggs infested
100
200
400
800
1,600
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.01 bAm
0.04 ⫾ 0.01 cAm
0.06 ⫾ 0.03 cAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.00 aAm
0.02 ⫾ 0.00 aAm
0.00 ⫾ 0.00 bAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.00 aAm
0.01 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.01 aAm
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.00 aAm
0.02 ⫾ 0.01 aAm
0.01 ⫾ 0.01 aAm
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.009 ⫾ 0.001b
0.00 ⫾ 0.00 aBm
0.01 ⫾ 0.01 aBm
0.02 ⫾ 0.01 bBm
0.02 ⫾ 0.01 cBm
0.22 ⫾ 0.18 cAm
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.01 bAn
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAn
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.01 aAm
0.00 ⫾ 0.00 aAn
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAm
0.03 ⫾ 0.03 aAm
0.00 ⫾ 0.00 aAn
0.01 ⫾ 0.01 aAm
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAn
0.013 ⫾ 0.006b
0.00 ⫾ 0.00 aBm
0.03 ⫾ 0.01 aBm
0.01 ⫾ 0.01 bBm
0.33 ⫾ 0.18 aAm
0.33 ⫾ 0.11 bAm
0.01 ⫾ 0.01 aAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAm
0.05 ⫾ 0.02 aAn
0.02 ⫾ 0.01 bAn
0.03 ⫾ 0.02 aAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.00 aAm
0.02 ⫾ 0.00 aAn
0.02 ⫾ 0.01 aAn
0.02 ⫾ 0.01 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.00 aAn
0.01 ⫾ 0.01 aAn
0.01 ⫾ 0.01 aAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAm
0.02 ⫾ 0.00 aAn
0.01 ⫾ 0.01 aAn
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.00 aAn
0.00 ⫾ 0.00 aAn
0.031 ⫾ 0.010ab
0.03 ⫾ 0.01 aCm
0.03 ⫾ 0.01 aCm
0.16 ⫾ 0.06 aBm
0.22 ⫾ 0.18 abBm
0.54 ⫾ 0.49 bAm
0.00 ⫾ 0.00 aBm
0.01 ⫾ 0.01 aBm
0.02 ⫾ 0.01 aBn
0.09 ⫾ 0.05 aABmn
0.16 ⫾ 0.11 aAn
0.03 ⫾ 0.01 aAm
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAn
0.03 ⫾ 0.02 aAn
0.02 ⫾ 0.01 aAo
0.06 ⫾ 0.06 aAm
0.01 ⫾ 0.01 aAm
0.00 ⫾ 0.00 aAn
0.02 ⫾ 0.01 aAn
0.01 ⫾ 0.01 aAo
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.01 aAm
0.00 ⫾ 0.00 aAn
0.02 ⫾ 0.01 aAn
0.00 ⫾ 0.00 aAo
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.01 aAm
0.00 ⫾ 0.00 aAn
0.02 ⫾ 0.00 aAn
0.00 ⫾ 0.00 aAo
0.050 ⫾ 0.018a
0.01 ⫾ 0.01 aBCm
0.00 ⫾ 0.00 aCm
0.06 ⫾ 0.02 abBCm
0.11 ⫾ 0.05 bcBm
0.63 ⫾ 0.22 aAm
0.01 ⫾ 0.01 aBm
0.01 ⫾ 0.01 aBm
0.02 ⫾ 0.01 aBm
0.02 ⫾ 0.01 aBm
0.23 ⫾ 0.10 aAn
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAm
0.03 ⫾ 0.02 aAm
0.09 ⫾ 0.06 aAo
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.00 ⫾ 0.00 aAm
0.02 ⫾ 0.01 aAm
0.03 ⫾ 0.03 aAo
0.01 ⫾ 0.01 aAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.00 aAm
0.01 ⫾ 0.01 aAm
0.01 ⫾ 0.01 aAo
0.01 ⫾ 0.01 aAm
0.00 ⫾ 0.00 aAm
0.01 ⫾ 0.00 aAm
0.02 ⫾ 0.01 aAm
0.00 ⫾ 0.00 aAo
0.044 ⫾ 0.013a
Although untransformed data are shown, statistics were performed using log (x ⫹ 1) data. Different lowercase letters starting with an “a”
indicate a signiÞcant difference within a row. Different uppercase letters indicate a signiÞcant difference within a column and plant. Different
lowercase letters starting with “m” indicate a signiÞcant difference between plants, but within a treatment and date.
southern corn rootworm (which were presumably
found randomly on all egg levels and plants) from the
100 egg treatment would be magniÞed 32-fold compared with the 3,200 egg treatment. Given this, Table
5 is more accurate in 2000 and 2001, when southern
corn rootworm populations were very low and this
likely also accounts for the lack of statistically significant larval movement in 2002 (zeros were not found
where western corn rootworm larvae were unlikely to
be recovered). Damage to the infested plant was
lower than we would have preferred for the highest
infestation levels in 2000, so we added an additional,
higher infestation level in 2001. Again, we would have
preferred more damage to the highest infestation levels in 2001, so we repeated the experiment an additional year. Given the caveat that we would have
preferred more damage in 2000 and 2001, our data
support the assumption of Storer (2003) that densitydependent mortality is negligible during establishment because the percentage of larvae recovered did
not vary consistently with infestation density.
Onstad et al. (2001) plotted a graph of western corn
rootworm survival to the adult stage versus the number of eggs per hectare. If converted to eggs per plant
by using a reasonable plant population, the carrying
capacity of an average corn Þeld could be estimated at
⬇200 eggs per plant. At this egg level in our study, very
few larvae were recovered, even from the infested
plant, and relatively minor damage occurred (Tables
6 Ð11). Given the extremely high damage that can be
found in farmerÕs Þelds (Gray and Steffey 1998), egg
populations far exceeding 200 eggs per plant are not
rare. We chose not to evaluate egg levels lower than
100 per infested plant because of the number of larvae
recovered was not signiÞcant in preliminary trials. We
chose to evaluate 3,200 eggs in 2001 and 2002 because
we wanted to ensure a high damage level to the infested plant at at least one infestation level. Plant
damage to the infested plant reached only 0.62 on the
last sample date in 2000 when it was infested with 1,600
viable eggs (Table 9), which was less than we would
have preferred at our highest infestation level. It
should be noted that egg populations have not been
observed to reach 3,200 eggs per plant across an entire
Þeld, but it is not unreasonable, given the clumped
distribution of rootworm eggs in the Þeld (Branson
1986) that 3,200 eggs may be found on a given plant.
Density-independent factors such unsuccessful
host Þnding, ßooding, dry soil and/or unknown factors
caused mortality that was considerable, but comparable to other studies (Branson and Sutter 1985,
Spike and Tollefson 1989, Riedell and Sutter 1995,
Hoback et al. 2002). After citing data on egg predation,
survival after establishment, and adult emergence
June 2004
HIBBARD ET AL.: WESTERN CORN ROOTWORM LARVAL MOVEMENT
879
Table 10. Plant damage (0 –3 scale) ⴞ SE from western corn rootworm larvae in 2001 from varying infestation levels of western corn
rootworm eggs on the infested (Inf) plant
No. of eggs infested
Date
Plant
6/18
6/22
6/26
7/02
7/06
7/12
6/18
6/22
6/26
7/02
7/06
7/12
6/18
6/22
6/26
7/02
7/06
7/12
6/18
6/22
6/26
7/02
7/06
7/12
6/18
6/22
6/26
7/02
7/06
7/12
6/18
6/22
6/26
7/02
7/06
7/12
All
Inf
0.02 ⫾ 0.01aAm
0.00 ⫾ 0.00aAm
0.01 ⫾ 0.01aCm
0.01 ⫾ 0.01aCm
0.01 ⫾ 0.01aCm
0.01 ⫾ 0.00aDm
Inf
0.03 ⫾ 0.01aAm
0.01 ⫾ 0.00aAm
0.00 ⫾ 0.00aCm
0.01 ⫾ 0.00aCm
0.02 ⫾ 0.00aCm
0.02 ⫾ 0.01aDm
Inf
0.02 ⫾ 0.01bAm
0.01 ⫾ 0.01bAm
0.02 ⫾ 0.01bCm
0.09 ⫾ 0.06abBCm
0.12 ⫾ 0.05aBm
0.09 ⫾ 0.06abCDm
Inf
0.03 ⫾ 0.02cAm
0.09 ⫾ 0.06bcAm
0.10 ⫾ 0.06bcBCm 0.15 ⫾ 0.12abABm
0.08 ⫾ 0.06cBCm 0.26 ⫾ 0.16aBm
Inf
0.06 ⫾ 0.01cAm
0.01 ⫾ 0.01cAm
0.24 ⫾ 0.17bAm
0.22 ⫾ 0.11bAm
0.29 ⫾ 0.24bAm
0.94 ⫾ 0.16aAm
Inf
0.05 ⫾ 0.02bcAmn 0.02 ⫾ 0.01cAn
0.14 ⫾ 0.07abABm 0.07 ⫾ 0.02abcBCn 0.14 ⫾ 0.06abBm 0.16 ⫾ 0.06aBCm
P1
0.00 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.00aBm
0.02 ⫾ 0.01aBm
0.01 ⫾ 0.01aAm
0.01 ⫾ 0.01aBm
P1
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.00 ⫾ 0.00aBm
0.00 ⫾ 0.00aBm
0.01 ⫾ 0.00aAm
0.03 ⫾ 0.02aBm
P1
0.02 ⫾ 0.01aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.00aBm
0.01 ⫾ 0.00aBm
0.02 ⫾ 0.01aAn
0.02 ⫾ 0.01aBm
P1
0.00 ⫾ 0.00aAm
0.01 ⫾ 0.01aAm
0.03 ⫾ 0.02aBmn
0.03 ⫾ 0.01aBmn
0.04 ⫾ 0.02aAm
0.09 ⫾ 0.06aBn
P1
0.01 ⫾ 0.01cAm
0.04 ⫾ 0.02cAm
0.27 ⫾ 0.17bAm
0.04 ⫾ 0.02cBn
0.04 ⫾ 0.02cAn
0.63 ⫾ 0.30aAn
P1
0.02 ⫾ 0.01bAmn
0.04 ⫾ 0.02bAmn
0.06 ⫾ 0.03bBmn
0.18 ⫾ 0.04aAm
0.04 ⫾ 0.02bAn
0.09 ⫾ 0.01abBmn
P2
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.00aBm
0.00 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.02 ⫾ 0.01aAm
0.00 ⫾ 0.00aBm
P2
0.03 ⫾ 0.02aAm
0.01 ⫾ 0.01aBm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.00aBm
P2
0.00 ⫾ 0.00aAm
0.04 ⫾ 0.02aABm
0.00 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.00aAn
0.02 ⫾ 0.01aBm
P2
0.07 ⫾ 0.06aAm
0.01 ⫾ 0.00aBm
0.00 ⫾ 0.00aAn
0.01 ⫾ 0.00aAn
0.01 ⫾ 0.00aAm
0.03 ⫾ 0.02aBn
P2
0.01 ⫾ 0.00bAm
0.01 ⫾ 0.01bBm
0.03 ⫾ 0.01bAn
0.04 ⫾ 0.02bAn
0.01 ⫾ 0.00bAn
0.26 ⫾ 0.17aAo
P2
0.02 ⫾ 0.01bAmn
0.13 ⫾ 0.05aAm
0.02 ⫾ 0.01bAn
0.01 ⫾ 0.01bAn
0.01 ⫾ 0.01bAno
0.04 ⫾ 0.02bBn
P3
0.00 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.01aAm
0.01 ⫾ 0.01aAm
0.00 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
P3
0.01 ⫾ 0.00aAm
0.00 ⫾ 0.00aAm
0.01 ⫾ 0.01aAm
0.03 ⫾ 0.02aAm
0.01 ⫾ 0.00aAm
0.00 ⫾ 0.00aAm
P3
0.04 ⫾ 0.02aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.03 ⫾ 0.02aAm
0.02 ⫾ 0.01aAn
0.02 ⫾ 0.01aAm
P3
0.02 ⫾ 0.01aAm
0.01 ⫾ 0.00aAm
0.00 ⫾ 0.00aAn
0.01 ⫾ 0.01aAn
0.02 ⫾ 0.01aAm
0.03 ⫾ 0.02aAn
P3
0.03 ⫾ 0.03abAm
0.00 ⫾ 0.00bAm
0.10 ⫾ 0.05aAn
0.06 ⫾ 0.06abAn
0.01 ⫾ 0.00abAn
0.04 ⫾ 0.02abAp
P3
0.01 ⫾ 0.00aAn
0.02 ⫾ 0.01aAn
0.04 ⫾ 0.01aAn
0.01 ⫾ 0.01aAn
0.03 ⫾ 0.02aAno
0.04 ⫾ 0.02aAn
Row 0.01 ⫾ 0.01aAm
0.01 ⫾ 0.01aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.01aAm
0.01 ⫾ 0.01aAm
0.01 ⫾ 0.00aAm
Row 0.01 ⫾ 0.01aAm
0.01 ⫾ 0.01aAm
0.00 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.00 ⫾ 0.00aAm
Row 0.01 ⫾ 0.01aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.01aAm
0.01 ⫾ 0.01aAn
0.01 ⫾ 0.01aAm
Row 0.02 ⫾ 0.01aAm
0.02 ⫾ 0.01aAm
0.02 ⫾ 0.01aAn
0.02 ⫾ 0.00aAn
0.02 ⫾ 0.01aAm
0.03 ⫾ 0.01aAn
Row 0.02 ⫾ 0.01aAm
0.03 ⫾ 0.02aAm
0.04 ⫾ 0.02aAn
0.04 ⫾ 0.02aAn
0.06 ⫾ 0.03aAn
0.03 ⫾ 0.02aAp
Row 0.09 ⫾ 0.06aAmn
0.02 ⫾ 0.01aAn
0.01 ⫾ 0.00aAn
0.04 ⫾ 0.02aAn
0.00 ⫾ 0.00aAo
0.01 ⫾ 0.01aAn
Cnt
0.01 ⫾ 0.00aBm
0.01 ⫾ 0.00aAm
0.03 ⫾ 0.03aAm
0.01 ⫾ 0.01aAm
0.00 ⫾ 0.00aAm
0.00 ⫾ 0.00aAm
Cnt
0.02 ⫾ 0.01aBm
0.00 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.00 ⫾ 0.00aAm
0.01 ⫾ 0.01aAm
Cnt
0.01 ⫾ 0.00aBm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.01aAm
0.01 ⫾ 0.01aAm
0.01 ⫾ 0.01aAn
0.01 ⫾ 0.01aAm
Cnt
0.04 ⫾ 0.02aABm
0.01 ⫾ 0.00aAm
0.00 ⫾ 0.00aAn
0.00 ⫾ 0.00aAn
0.04 ⫾ 0.02aAm
0.03 ⫾ 0.01aAn
Cnt
0.03 ⫾ 0.01aABm
0.02 ⫾ 0.01aAm
0.04 ⫾ 0.02aAn
0.03 ⫾ 0.02aAn
0.02 ⫾ 0.01aAn
0.04 ⫾ 0.02aAp
Cnt
0.11 ⫾ 0.05aAm
0.04 ⫾ 0.02abAmn 0.04 ⫾ 0.02abAn
0.01 ⫾ 0.01bAn
0.01 ⫾ 0.01bAno
0.01 ⫾ 0.01bAn
All
0.023 ⫾ 0.04b
0.018 ⫾ 0.003b
0.036 ⫾ 0.008b
0.034 ⫾ 0.006b
0.031 ⫾ 0.08b
0.083 ⫾ 0.019a
100
200
400
800
1,600
3,200
Although untransformed data are shown, statistics were performed using log (x ⫹ 1) data. Different lowercase letters starting with an “a”
indicate a signiÞcant difference within a row. Different uppercase letters indicate a signiÞcant difference within a column and plant. Different
lowercase letters starting with “m” indicate a signiÞcant difference between plants, but within a treatment and date.
from artiÞcial infestations, Storer (2003) set densityindependent survival at 5% in his model. Presumably,
this Þgure included nonviable eggs, and egg, larval,
and pupal predation, in addition to unsuccessful
establishment on a host. In the current study, densityindependent establishment was between 2.5 and
5.7% when plants were sampled on the optimal date as
estimated by larval recovery by using our sampling
technique (Table 5). This Þgure did not include any
overwintering or pupal mortality, but the 5% Þgure
used by Storer (2003) is very close given the data of
other studies cited by Storer (2003) in combination
with ours. Interestingly, density-independent mortality in greenhouse situations is nearly 10-fold less
(Weiss et al. 1985; B.E.H., unpublished data). An understanding of the differences between densityindependent mortality between Þeld and greenhouse
conditions is an important gap in our understanding
of corn rootworm biology. Regardless, density-independent mortality in larval establishment is an important factor that must be considered when selection
intensity (an important component in resistance man-
agement models) of Bt corn or insecticides is calculated.
Previously, Hibbard et al. (2003) demonstrated that
western corn rootworm larvae could move at least
three plants down a corn row and across a 0.46-m row.
Part of the impetus for the current study was to help
understand the driving force behind postestablishment larval movement by western corn rootworm
larvae. Although egg density seemed not to be an
important factor in percentage of larval establishment,
it was an important factor in plant damage and, secondarily, subsequent larval movement. In general,
damage was highest in the plots infested with the most
viable eggs and decreased as distance from the infested plant decreased. Postestablishment movement
generally occurred about the time that signiÞcant
damage began to occur, rather than at the time of
establishment. These data imply that plant-to-plant
movement was motivated by a search for food and was
density-dependent only because damage was densitydependent. If crowding caused larval movement
rather than a lack of food, movement would be ex-
880
JOURNAL OF ECONOMIC ENTOMOLOGY
Table 11.
plant
Date
Plant
6/12
6/17
6/20
6/24
6/28
7/3
6/12
6/17
6/20
6/24
6/28
7/3
6/12
6/17
6/20
6/24
6/28
7/3
6/12
6/17
6/20
6/24
6/28
7/3
6/12
6/17
6/20
6/24
6/28
7/3
6/12
6/17
6/20
6/24
6/28
7/3
All
Inf
Inf
Inf
Inf
Inf
Inf
P1
P1
P1
P1
P1
P1
P2
P2
P2
P2
P2
P2
P3
P3
P3
P3
P3
P3
Row
Row
Row
Row
Row
Row
Cnt
Cnt
Cnt
Cnt
Cnt
Cnt
All
Vol. 97, no. 3
Plant damage (0 –3 scale) ⴞ SE in 2002 from varying infestation levels of western corn rootworm eggs on the infested (Inf)
No. of eggs infested
100
200
400
800
1,600
3,200
0.03 ⫾ 0.01aBm
0.03 ⫾ 0.03aBm
0.04 ⫾ 0.02bABm
0.22 ⫾ 0.12cABm
0.23 ⫾ 0.10cABm
0.36 ⫾ 0.16cAmn
0.01 ⫾ 0.01aAm
0.02 ⫾ 0.01aAm
0.13 ⫾ 0.12aAm
0.02 ⫾ 0.01bAm
0.21 ⫾ 0.18abcAm
0.26 ⫾ 0.14bAno
0.00 ⫾ 0.00aBm
0.02 ⫾ 0.00aBm
0.06 ⫾ 0.06aBm
0.02 ⫾ 0.00aBm
0.04 ⫾ 0.02aBm
0.56 ⫾ 0.36abAm
0.00 ⫾ 0.00aBm
0.00 ⫾ 0.00aBm
0.04 ⫾ 0.02aBm
0.02 ⫾ 0.00aBm
0.14 ⫾ 0.06aBm
0.60 ⫾ 0.55aAm
0.00 ⫾ 0.00aAm
0.03 ⫾ 0.01aAm
0.02 ⫾ 0.01aAm
0.03 ⫾ 0.01aAm
0.05 ⫾ 0.02aAm
0.09 ⫾ 0.01aAo
0.01 ⫾ 0.01aAm
0.03 ⫾ 0.01aAm
0.02 ⫾ 0.00aAm
0.03 ⫾ 0.01aAm
0.14 ⫾ 0.04aAm
0.05 ⫾ 0.02aAo
0.096 ⫾ 0.022cd
0.00 ⫾ 0.00aBm
0.02 ⫾ 0.01aBm
0.04 ⫾ 0.02bBm
0.04 ⫾ 0.02cBm
0.22 ⫾ 0.18cABm
0.38 ⫾ 0.13cAm
0.01 ⫾ 0.00aAm
0.02 ⫾ 0.01aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.01bAm
0.11 ⫾ 0.05bcAm
0.13 ⫾ 0.04bAmn
0.01 ⫾ 0.01aAm
0.03 ⫾ 0.01aAm
0.02 ⫾ 0.01aAm
0.03 ⫾ 0.01aAm
0.12 ⫾ 0.05aAm
0.10 ⫾ 0.05cAmn
0.03 ⫾ 0.02aAm
0.02 ⫾ 0.01aAm
0.01 ⫾ 0.01aAm
0.04 ⫾ 0.01aAm
0.02 ⫾ 0.01aAm
0.21 ⫾ 0.11bAmn
0.01 ⫾ 0.01aAm
0.01 ⫾ 0.00aAm
0.02 ⫾ 0.01aAm
0.02 ⫾ 0.01aAm
0.06 ⫾ 0.02aAm
0.05 ⫾ 0.00aAn
0.03 ⫾ 0.03aAm
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.01aAm
0.02 ⫾ 0.00aAm
0.08 ⫾ 0.06aAm
0.06 ⫾ 0.02aAn
0.054 ⫾ 0.009d
0.02 ⫾ 0.01aBm
0.01 ⫾ 0.00aBm
0.06 ⫾ 0.02bBm
0.11 ⫾ 0.05cBm
0.27 ⫾ 0.17cABm
0.46 ⫾ 0.20cAm
0.02 ⫾ 0.01aAm
0.03 ⫾ 0.01aAm
0.00 ⫾ 0.00aAm
0.04 ⫾ 0.02bAm
0.07 ⫾ 0.02cAm
0.18 ⫾ 0.11bAmn
0.01 ⫾ 0.01aAm
0.04 ⫾ 0.02aAm
0.04 ⫾ 0.02aAm
0.02 ⫾ 0.01aAm
0.04 ⫾ 0.02aAm
0.14 ⫾ 0.06cAn
0.01 ⫾ 0.00aAm
0.03 ⫾ 0.01aAm
0.07 ⫾ 0.06aAm
0.05 ⫾ 0.02aAm
0.04 ⫾ 0.01aAm
0.08 ⫾ 0.02bAn
0.00 ⫾ 0.00aAm
0.01 ⫾ 0.00aAm
0.07 ⫾ 0.06aAm
0.01 ⫾ 0.01aAm
0.10 ⫾ 0.05aAm
0.11 ⫾ 0.05aAn
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.01aAm
0.03 ⫾ 0.01aAm
0.01 ⫾ 0.00aAm
0.02 ⫾ 0.01aAm
0.08 ⫾ 0.01aAn
0.062 ⫾ 0.011d
0.01 ⫾ 0.00aCm
0.04 ⫾ 0.02aCm
0.14 ⫾ 0.12bCm
0.56 ⫾ 0.16bBm
0.64 ⫾ 0.25bBm
1.14 ⫾ 0.42bAm
0.03 ⫾ 0.02aBm
0.02 ⫾ 0.01aBm
0.07 ⫾ 0.06aBm
0.24 ⫾ 0.17abABn
0.44 ⫾ 0.12aAm
0.18 ⫾ 0.04bABn
0.01 ⫾ 0.01aAm
0.01 ⫾ 0.00aAm
0.02 ⫾ 0.01aAm
0.23 ⫾ 0.18aAn
0.10 ⫾ 0.05aAn
0.04 ⫾ 0.01cAn
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.01aAm
0.07 ⫾ 0.06aAm
0.03 ⫾ 0.01aAn
0.04 ⫾ 0.02aAn
0.28 ⫾ 0.24abAn
0.01 ⫾ 0.00aAm
0.01 ⫾ 0.01aAm
0.03 ⫾ 0.01aAm
0.03 ⫾ 0.02aAn
0.08 ⫾ 0.06aAn
0.06 ⫾ 0.02aAn
0.03 ⫾ 0.02aAm
0.03 ⫾ 0.02aAm
0.04 ⫾ 0.02aAm
0.02 ⫾ 0.01aAn
0.09 ⫾ 0.06aAn
0.29 ⫾ 0.24aAn
0.140 ⫾ 0.025bc
0.04 ⫾ 0.01aDm
0.06 ⫾ 0.02aDm
0.70 ⫾ 0.25aCm
0.55 ⫾ 0.26bCm
1.06 ⫾ 0.16aBm
1.75 ⫾ 0.31aAm
0.01 ⫾ 0.00aBm
0.02 ⫾ 0.01aBm
0.04 ⫾ 0.02aBn
0.21 ⫾ 0.18bBn
0.28 ⫾ 0.24abcBn
0.88 ⫾ 0.07aAn
0.01 ⫾ 0.01aBm
0.02 ⫾ 0.01aBm
0.08 ⫾ 0.06aABn
0.22 ⫾ 0.18aABn
0.06 ⫾ 0.02aABn
0.35 ⫾ 0.17bcAo
0.01 ⫾ 0.01aAm
0.01 ⫾ 0.01aAm
0.02 ⫾ 0.00aAn
0.05 ⫾ 0.02aAn
0.03 ⫾ 0.01aAn
0.18 ⫾ 0.11bAop
0.01 ⫾ 0.00aAm
0.03 ⫾ 0.01aAm
0.08 ⫾ 0.06aAn
0.06 ⫾ 0.03aAn
0.04 ⫾ 0.02aAn
0.11 ⫾ 0.05aAop
0.03 ⫾ 0.03aAm
0.01 ⫾ 0.01aAm
0.01 ⫾ 0.00aAn
0.03 ⫾ 0.01aAn
0.06 ⫾ 0.02aAn
0.06 ⫾ 0.02aAp
0.197 ⫾ 0.034ab
0.05 ⫾ 0.02aDm
0.15 ⫾ 0.06aDm
0.81 ⫾ 0.28aBCm
1.13 ⫾ 0.31aBm
0.76 ⫾ 0.30abCm
1.75 ⫾ 0.52aAm
0.02 ⫾ 0.01aCm
0.01 ⫾ 0.00aCm
0.32 ⫾ 0.12aBCn
0.55 ⫾ 0.33aBn
0.40 ⫾ 0.37abBn
1.00 ⫾ 0.37aAn
0.03 ⫾ 0.03aBm
0.02 ⫾ 0.01aBm
0.02 ⫾ 0.01aBo
0.06 ⫾ 0.02aBo
0.14 ⫾ 0.12aBnop
0.71 ⫾ 0.51aAn
0.03 ⫾ 0.03aAm
0.00 ⫾ 0.00aAm
0.03 ⫾ 0.02aAno
0.11 ⫾ 0.05aAo
0.07 ⫾ 0.02aAop
0.21 ⫾ 0.04bAo
0.00 ⫾ 0.00aAm
0.08 ⫾ 0.06aAm
0.07 ⫾ 0.06aAno
0.02 ⫾ 0.00aAo
0.14 ⫾ 0.12aAnop
0.08 ⫾ 0.02aAo
0.01 ⫾ 0.01aAm
0.02 ⫾ 0.00aAm
0.02 ⫾ 0.01aAo
0.02 ⫾ 0.01aAo
0.04 ⫾ 0.02aAp
0.10 ⫾ 0.05aAo
0.248 ⫾ 0.043a
Although untransformed data are shown, statistics were performed using log (x ⫹ 1) data. Different lowercase letters starting with an “a”
indicate a signiÞcant difference within a row. Different uppercase letters indicate a signiÞcant difference within a column and plant. Different
lowercase letters starting with “m” indicate a signiÞcant difference between plants, but within a treatment and date.
pected to have occurred earlier than found here. Despite procedural matters that put the data in question
(Branson 1986) previous reports of larval movement
from egg hatch to adult emergence had been reported
to be as high as 100 cm (Suttle et al. 1967, Short and
Luedtke 1970). Data from the current study and from
Hibbard et al. (2003) have documented postestablishment larval movement as far as 61 cm. Chaddha (1990)
demonstrated that although establishment success diminished the farther away infestation points were
from corn plants, signiÞcant establishment occurred at
distances up to 30 cm. Combining our data and data
from Chaddha (1990), it is not inconceivable that
movement up to 100 cm from egg hatch to adult
emergence could take place, but we speculate that
such movement is rare and may involve movement of
third instar larvae searching for pupation sites as well.
Factors not investigated here, such as soil type, soil
moisture, soil bulk density, and others likely also play
a role in larval movement.
Efforts to model adaptation of corn rootworms to
transgenic corn (Onstad et al. 2001, Storer 2003) have
illustrated that much of the basic larval biology and
ecology of this pest is unknown (EPA ScientiÞc Advisory Panel 2002). Part of the impetus for this study
was to help address the need for additional basic biological information for resistance management models. Movement of larvae from susceptible to transgenic
plants and vice versa could adversely affect resistance
management in several ways (Mallet and Porter 1992,
Davis and Onstad 2000). Because larger larvae are
more tolerant to toxins, initial development on a susceptible plant (a grassy weed or corn plant) followed
by subsequent migration to a nearby transgenic plant
might accelerate the rate of adaptation if heterozygotes with the resistance gene survived exposure to
the toxin at greater rates. Alternatively, if a larva
brießy fed on a transgenic root and then moved to a
nearby susceptible root, this, too, could accelerate the
rate of resistance development if heterozygotes for
the resistance gene were preferentially selected.
However, if a low-dose product produced susceptible
beetles, movement of larger larvae onto transgenic
roots from less suitable alternate hosts or highly dam-
June 2004
HIBBARD ET AL.: WESTERN CORN ROOTWORM LARVAL MOVEMENT
aged corn in a seed mix could actually increase product durability by producing additional susceptible insects from within the transgenic Þeld. Whether larval
movement to Bt plants from alternate hosts or refuge
plants would increase or slow the development of
resistance may depend on the dose or selection intensity of the event. The current study suggests that
under low-to-moderate infestation levels, little to no
larval movement occurs after initial establishment on
a susceptible host. However, under higher infestation
levels, signiÞcant movement will occur from damaged
to undamaged plants down the row. Under very high
infestation rates, statistically signiÞcant larval movement was documented across a 76-cm row. B.E.H.
(unpublished data) documented that movement to
Cry3Bb1 expressing roots was less than movement to
similarly situated isoline roots. The true test for evaluating whether larval movement would have a positive
or negative impact on resistance management would
be to evaluate the progeny of those completing only
a portion of their life on transgenic plants.
Acknowledgments
We thank Arnulfo Antonio, Charles Thiel, Julie Barry, and
Tim Praiswater (USDAÐARS, Plant Genetics Research Unit)
and a number of summer laborers for technical assistance in
this research. We thank Tom Clark, Ted Wilson, and Isaac
Oyediran (Department of Entomology, University of Missouri); Larry Darrah (USDAÐARS, Plant Genetics Unit); and
David Onstad (University of Illinois) for comments on earlier
versions of this manuscript. Funding, in part, was provided by
USDAÐCSREES NRI CGP Project Award Numbers 200135316-10000 and 2002-35316-12282.
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Received 7 October 2003; accepted 28 January 2004.