J. Appl. Entomol. 131(6), 400–405 (2007) doi: 10.1111/j.1439-0418.2007.01205.x
2007 The Authors
Journal compilation 2007 Blackwell Verlag, Berlin
Western corn rootworm adult movement and possible egg laying
in fields bordering maize
J. Igrc Barčić1, R. Bažok1, C. R. Edwards2 and T. Kos1
1
Faculty of Agriculture, Department for Agricultural Zoology, Svetošimunska, Zagreb, Croatia; 2Department of
Entomology, Purdue University, West Lafayette, IN, USA
Ms. received: December 15, 2006; accepted: May 11, 2007
Abstract: Western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, larval damage in maize following
soybean was observed in Croatia in 2003 along the edges and within soybean fields which bordered continuous maize
fields in previous year. The explanation was that WCR adults moved from the continuous maize to the neighbouring
soybean fields to lay eggs. This study was designed to measure how far WCR adults will enter into neighbouring fields
to lay eggs. The WCR adult population was monitored in continuous maize fields in 2003 and 2005 by using Pherocon
AM non-baited yellow sticky traps in the middle and on the borders of the maize field and at different distances and
directions into neighbouring fields planted by wheat and soybean. Larval presence and root damage ratings (Iowa State
University 1–6 ) were recorded at different locations within the maize field in following years. Approximately, the same
concentration of WCR adults was recorded along the edges of the maize fields as recorded in the centres of those fields.
A significant number of WCR adults was recorded up to a distance of 50 m into neighbouring fields. Regression
analysis showed medium negative correlation between distance from previous maize field and root damage in the
following year. Findings indicate that WCR egg lying can reach approximately 20 m into fields neighbouring maize
fields and that significant root damage caused by WCR larvae in first-year maize following soybean and wheat can
happen up to a distance of 20 m into those fields. Most farmers’s fields in Croatia are up to approximately 50 m wide.
As an edge effect for WCR egg laying can reach approximately 20 m into fields neighbouring maize fields, our research
results indicate that it is possible to see WCR larval damage in rotated fields without those WCR’s being the variant
form.
Key words: adult movement, first-year maize, root damage, soybean, western corn rootworm, wheat
1 Introduction
The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is a new pest in Europe, as well
as in Croatia. WCR was first observed in eastern
Croatia in 1995. Since 1995, the pest has spread
westward across Croatia and now infests about
24 500 km2. Within the infested area, approximately
328 000 ha of maize are sown. The total maize
production surface in Croatia is 350 000–400 000 ha.
Crop rotation has provided an alternative means of
control for WCR in regions where it is feasible to
rotate maize with other crops. The tendency of adult
females to lay their eggs almost exclusively in maize,
together with the basic inability of WCR larvae to
complete their development on the roots of most crops
other than maize, has resulted in the successful control
of WCR using this cultural method. In the USA Corn
Belt, annual rotation of maize with other crops such as
soybean has been utilized as a major strategy for
managing WCR (Gray et al. 1998).
However, since the mid-to-late 1990s throughout
east-central Illinois and north-western Indiana, and
more recently other parts of the eastern Midwest,
WCR has been laying eggs in soybean fields regardless
of whether or not volunteer maize or WCR-attracting
weeds are present (Edwards 1996; Levine and OloumiSadeghi 1996; Barna et al. 1998). This ÔnewÕ WCR is
referred as the ÔvariantÕ of the wild type. The new
WCR, or ÔvariantÕ has developed behavioural resistance to crop rotation (Levine et al. 2002). Economic
thresholds for the WCR variant in soybean to predict
subsequent larval damage to maize the following year
have been developed by Gerber et al. (2001).
Studies of WCR movement and egg laying habits
were conducted by various authors (VanWoerkom
et al. 1980; Coats et al. 1986, 1987; Grant and Seevers
1989, etc.). It was initially thought that the movement
is due to beetles searching for food or oviposition sites.
Godfrey and Turpin (1983) found that most of WCR
adults moving from field to field were females. Coats
et al. (1986) has defined the flight potential in WCR.
Female beetles according the same authors exhibit two
types of flight: trivial and sustained (or migratory).
Several days after mating, newly-mated females engage
in sustained, migratory flights that facilitate
Western corn rootworm adult movement
long-distance dispersal from their native field. The
maximum distances of 24 km for one flight and
39.6 km for all flights during 24 h were recorded by
Coats et al. (1986, 1987) showed that females treated
with juvenile hormone mimic and inhibitor flew both
trivial and sustained flights that were significantly
longer than those of the controls. However, the short
distance movement of WCR prior to the ÔvariantÕ
appearance was not studied by researches.
Spencer et al. (2005) stated that the rotation tolerance is a problem of movement and understanding
WCR movement patterns and capabilities is crucial to
dealing with current management challenges and
anticipating future ones. Spencer et al. (2005) reported
that 85–90% of males and females in maize and
soybean fields move between 4.6 and 9.1 m/day.
Western corn rootworm larval damage in maize
following soybean was observed in Croatia in 2003.
Damage was recorded along the edges and within
soybean fields, which bordered continuous maize fields
in 2002. Research carried out in Hungary (Kiss et al.
2005) and in Croatia, Hungary and Yugoslavia (Kiss
et al. 2001) in the period of 2000–2001 showed that
crop rotation in these regions does not result in high
WCR selection pressure at the levels that have been
seen in the areas with the variant in the USA Corn
Belt. Therefore, it is unlikely that the problem in
Croatia is due to the variant of the WCR. An
explanation for what has been observed in Croatia is
that WCR adults along the edge of maize fields
randomly moved several metres into the neighbouring
soybean fields and laid some of their eggs and then
moved back into their preferred egg laying site, maize.
Shaw et al. (1978) reported that non-economic damage
occurred when corn followed soybean fields with
volunteer corn as well as when corn followed no
weedy soybean fields.
Larval feeding damage in the first-year maize
following soybean in Croatia in 2003 was recorded
up to a distance of 50 m into fields previously in
soybean. FarmersÕ fields in Croatia are commonly up
to about 50 m wide. This extent of egg laying outside
of corn, even without evidence of rotation resistance,
could still reduce the value of crop rotation as a WCR
management tool. This research was carried out to
determine how far WCR adults will go into neighbouring non-maize fields to lay eggs.
401
Fig. 1. Pherocon AM trap locations in the maize,
soyabean and wheat fields, Knezˇevo, Crotia, 2003
200 m distance from the edge of the continuous maize into
neighbouring fields to the south, east and west (fig. 1). Traps
were set up in the field on 25 June. Traps were changed
weekly until 12 August, and biweekly thereafter until 16
September. In 2004, the whole field, which was previously in
maize, soybean and wheat, was planted to maize. Larval
presence on 13 June and root damage ratings in July (Iowa
State University 1–6 scale, Hill and Peters 1971) were
recorded at different field locations (in the centre and along
the edge of the maize field and 5, 10, 15, 20 and 50 m within
the bordering crops to the west and east) within the field
(fig. 2). Each observation location was replicated four times.
The second part of investigation was carried out in 2005/
2006. In both years Pioneer maize hybrid Florencia was
planted by using six-row corn planter on 15 April 2005 and
on 22 April 2006. Seed was treated with fungicide tiram and
no other treatments were applied. The number of seeds was
80 000/ha. In 2005, the test field was 120 m wide and was
divided into three sections. On two sides, maize was planted
and in the middle section soybean was planted. The WCR
adult population was monitored using PAM traps in the
middle and on the borders of the maize field and each 5 m
within the soybean field to a maximum of 40 m (fig. 3). At
each distance, four PAM traps were placed within 20 m of
each other. Traps were set up on 23 June and were changed
on weekly until 16 September. In 2006, the whole field was
planted to maize. As before, 1–6 root damage ratings were
recorded at each of the monitoring locations (fig. 4). Four
sets of 10 plants each were taken at each location for a total
of 40 plants for root damage evaluation.
Statistical analysis of the data on WCR captures in 2003
was not conducted because the investigation was preliminary
2 Materials and Methods
The first part of investigation was carried out in 2003/2004.
In both years experimental fields were prepared by using
conventional methods. Sowing of domestic hybrid Bc 408B
was carried out on 15 April 2003 and on 5 May 2004, both by
using six-row planter (model PSK, producer: OLT, Osijek,
Croatia). Seed was treated with fungicide tiram and no other
treatments were applied. The number of seeds was 80 000/ha.
The WCR adult population was monitored in a continuous
maize field in 2003. This maize field was part (200 · 200 m)
of a large field (35 ha) sown to wheat and soybean in 2003.
The WCR beetles were monitored by using Pherocon AM
(PAM) non-baited yellow sticky traps (Treece, Salinas, KS)
in the middle and on the borders of the maize field and up to
Fig. 2. Location of larval and root sampling, Knezˇevo,
Crotia, 2004
2007 The Authors
Journal compilation 2007 Blackwell Verlag, Berlin, J. Appl. Entomol. 131(6), 400–405 (2007)
402
Fig. 3. Pherecon AM trap positions in the maize fields,
Tovarnik, Croatia, 2005
Fig. 4. Locations of root sampling, Tovarnik, Croatia,
2006
and without replications. Based on the data on adult captures
in 2003 we decided on distances for the future investigations.
The data on larval presence and root ratings in 2004, adult
captures in 2005 and root damage ratings in 2006 were
transformed by log (x + 1) transformation and analysed by
anova with mean separation by Student–Neuman–Keuls test
(a ¼ 0.05).
Two regression analyses were done: regression analysis of
distance from maize field for the observations done in 2004
vs. average number of larvae/root; and regression analysis of
distance from maize fields in first year for the observations in
2004 and 2006 vs. root damage ratings. Pearson correlation
coefficients were calculated and classified according to
Roemer-Orphal classification (Vasilj 2000).
3 Results
The total number of WCR beetles captured on the
Pherocon AM traps located within the maize field,
and in soybean and wheat fields at different locations
J. Igrc Barčić et al.
from the edge of the maize field, is shown in the
fig. 5.
As shown in fig. 5, for the whole period of monitoring, higher average daily captures of the beetles were
recorded up to a distance of 50 m in the neighbouring
field. Because of low numbers beyond 50 m, in fig. 6
the average daily PAM trap are presented only for
centre, edge and 50 m trap locations.
The maximum daily captures of WCR adults exceeded 6 beetles/trap/day on PAM traps located in the
centre and along the edges of the maize fields and on
traps located 50 m within neighbouring fields to the
east and west. Results of the number of larvae
recorded and the root damage ratings for 2004 are
shown in table 1. The maximum daily captures of
WCR on Pherocon AM traps located at different
locations in 2005 are shown in fig. 7.
Maximal daily captures of the beetles exceeded 6
beetles/trap/day in the centres of the maize fields and
on south edges of these fields. The mean captures of
WCR adults on Pherocon AM traps at different field
locations in 2005 and the root ratings at the same sites
in 2006 are shown in the table 2.
The total captures of WCR adults in 2005 ranged
between 12 and 395.5 beetles/season. The total capture
in the centres of the maize fields and on the south edges
of these fields did not significantly differ. At all other
distances and on the north edge of the maize fields, the
total captures were significantly lower. The root
damage ratings in the right side of the maize field
and in the previous soybean field up to 10 m were
significantly higher than the root damage ratings in the
other parts of the fields; these damage ratings were
between 2.69 and 4.64.
Figures 8 and 9 show results of the regression
analysis of the distance from previous year’s maize
field vs. number of larvae and vs. root ratings.
4 Discussion
Captured beetles were concentrated in a range of 50 m
within fields neighbouring maize, whereas few beetles
were captured at 100 or more metres apart in all
three directions. These findings suggest that the future
investigations should be more concentrated up to
the distances of 50 m from the maize field. Also, the
findings indicate that the WCR movement, which was
recorded in the experimental field, was not only a result
Fig. 5. Average daily captures of western corn rootworm on Pherocon AM
traps located at different
sites, Knezˇevo, Croatia,
2003
2007 The Authors
Journal compilation 2007 Blackwell Verlag, Berlin, J. Appl. Entomol. 131(6), 400–405 (2007)
Western corn rootworm adult movement
403
Fig. 6. Daily captures of
western corn rootworm on
Pherocon AM traps located
in locations up to 50 m in
neighbouring fields,
Knezˇevo, Croatia, 2003
Table 1. The average number of larvae per sample and
the average root rating recorded at different field positions, Knezˇevo, Croatia, 2004
Position of
the sampling
Maize: centre
West: edge
East: edge
South: edge
West 5 m
East 5 m
South 5 m
West 10 m
East 10 m
South 10 m
West 15 m
East 15 m
South 15 m
West 20 m
East 20 m
South 20 m
West 50 m
LSD (P ¼ 0.05)
Number of larvae/
plant
0.7 a
0.55 ab
0.1 ab
0.45 ab
0b
0b
0.25 ab
0.1 ab
0.1 ab
0b
0.55 ab
0b
0.15 ab
0b
0b
0.35 ab
0b
0.069 t2
Table 2. The mean capture of western corn rootworm
adults (average number of beetles/trap collected in
85 days) and the average root rating recorded at different field positions, Tovarnik, Croatia, 2005 and 2006
Root damage
ratings (1–6)
4.46 a
3.93 ab
1.99 bc
–1
3.3 abc
1.85 bc
–
3.58 abc
2.12 bc
–
2.98 abc
1.66 c
–
2.4 bc
2.23 bc
–
1.6 c
1.282
Means followed by same letter are not significantly different
according to Student–Newman–Keuls test (P ¼ 0.05).
1
The data are missing.
2
LSD is reported in transformed data units.
of trivial flight of the beetles. Trivial flights are
conducted at short distances (Coats et al. 1986). The
sustained flights are conducted by females (Godfrey
and Turpin 1983; Coats et al. 1986) at long distances
Position of the sampling
Maize, left side, edge left
Maize, left side, centre
Maize, left side, edge right
Soybean 5 m (from left)
Soybean 10 m (from left)
Soybean 15 m (from left)
Soybean centre
Soybean 15 (from right)
Soybean 10 m (from right)
Soybean 5 m (from right)
Maize, right side, edge left
Maize, right side, centre
Maize, right side, edge right
LSD (P ¼ 0.05)
Mean adult
capture in 2005
Root damage
ratings (1–6)
in 2006
105.05 bcd
395.5 a
221.75 ab
62.25 cde
39.25 efg
48.5 efg
20.75 fg
12.0 g
26.5 efg
43.0 def
148.0 abc
319.0 ab
205.0 ab
0.327 t1
3.93 a
2.13 c
1.73 c
1.94 c
1.79 c
1.69 c
1.98 c
1.77 c
4.64 a
2.69 b
3.85 a
3.01 b
2.98 b
0.057 t1
Means followed by same letter are not significantly different
according to Student–Newman–Keuls test (P ¼ 0.05).
1
LSD is reported in transformed data units.
and at high altitudes (Witkowski et al. 1975). Pherocon AM traps were placed 1 m above the ground and
this location was not very suitable to capture the
beetles, which were flying at higher levels. Coats et al.
(1986) reported that sustained flights were not made
after females were 9 days old.
Fig. 7. Maximal daily captures of western corn rootworm on Pherocon AM
traps located at different
field positions, Tovarnik,
Croatia, 2005
2007 The Authors
Journal compilation 2007 Blackwell Verlag, Berlin, J. Appl. Entomol. 131(6), 400–405 (2007)
404
Fig. 8. Regression analysis of distance from previous
year maize field vs. number of larvae, Knezˇevo, Croatia,
2004
Fig. 9. Regression analysis of distance from 2003 to
2005 maize fields vs. root ratings taken in 2004 and 2006
In the Croatian conditions, the emergence of the
beetles is completed by 10 July (Dobrinčić, 2001) and all
sustained flights would be completed in the first 20 days
of July. The period of maximal daily captures in 2003
(fig. 6) was between 25 June and 23 July at all PAM trap
locations except on traps located 50 m within neighbouring fields to the east. On these traps the maximal
capture was recorded between 23 July and 12 August.
The described flight dynamic was the result of extremely
hot and dry climatic conditions in the period from May–
August. WCR beetles emerged very early, in mid-June,
and because of the high temperatures and low moisture,
the WCR flight ceased in the second half of August. The
oviposition period was short and this was reflected in the
relatively low root damage levels in following year and
the low number of larvae/plant in 2004.
The average number of larvae/plant (table 1) was
quite low which could be explained by spatial dispersion of larvae and the small sample size (five roots in
the line). Bergman et al. (1983) showed that WCR
larvae exhibit aggregating dispersion patterns what
could be the result of different factors and could result
in high variability in number of larvae/root. On the
east edge and at all observed distances eastward, lower
root damage ratings were recorded (between 1.66 and
2.23). The differences in root ratings between east and
west directions were not significant and are likely
influenced by the prevailing winds. The primary
direction of wind is from north-east to south-west,
which corresponds with somewhat higher root damage
ratings in west samples. This statement needs more
investigation.
J. Igrc Barčić et al.
Maximal daily captures (fig. 7) at all sites located
within north-west side (maize left side, as identified in
table 2 and figs 7–9) were recorded between 25 July
and 12 August. Maximal daily captures at all other
sites were recorded between 12 August and 26 August.
The north-west side (maize left side, as identified in
figs 8 and 9) of the trial was flooded for 2 weeks in
March, which likely affected egg survival and later on
resulted in lower root damage ratings in the centre and
on the right edge of this north-west area (table 2).
After regression analysis of the distance from 2003
maize field vs. number of larvae in 2004 the medium
negative correlation (r ¼ )0.29) was established
(fig. 8). The larval presence was established in the first
half of June when some larvae were too small to be
observed by visual searching. Together with small
sample size this resulted in low number of larvae which
were found in roots.
Regression analysis of the distance from maize fields
in 2003 and 2005 vs. root ratings taken in 2004 and
2006 gave us more consistent results. Medium negative
correlation (r ¼ )0.39) between those two variables
was determined (fig. 9). From the fig. 9 it could be seen
that at the distance of 20 m an average root rating of
2.5 is possible. The maximal WCR counts on Pherocon
AM traps in 2003 and 2005 were somewhat above the
economic threshold level (ETC), this lead to root
damage ratings in the continuous maize fields in 2004
and 2006 that were also somewhat above the ETC
(3.5). The root ratings in the continuous maize fields in
2004 were 4.46 in the middle part and 3.93 at the edge,
while in 2006 we recorded root ratings between 2.13
and 3.85. If the adult population in Ôdonor maize fieldÕ
had been higher, the observed root damage ratings in
neighbouring fields would have been higher as well.
The future investigations must determine adult capture
rates on Pherocon AM traps in Ôdonor fieldsÕ that
forecast possible economic root injury in neighbouring
fields in following year. Other possibility could be to
determine the adult capture rates in the non-maize
rotated crop where their egg laying would lead to
damage in first year maize.
5 Conclusions
Along the edges of the maize fields approximately the
same concentration of WCR adults was recorded as in
the centres of those fields. A significant number of
WCR adults was recorded up to distance of 50 m
within neighbouring fields. No difference among soybean and wheat as neighbouring crops on WCR adult
movement into those fields was found. A significant
root damage caused by WCR larvae in first-year maize
following soybean and wheat up to a distance of 20 m
into those fields was noted. So-called Ôedge effectÕ for
WCR egg laying can happen in surrounding fields,
within distances of 20 m along maize field borders. The
possible influence of prevailing winds on this edge
effect phenomenon needs to be studied. Most farmersÕ
fields in Croatia are up to approximately 50 m wide.
As an edge effect for WCR egg laying can reach
approximately 20 m into fields neighbouring maize
2007 The Authors
Journal compilation 2007 Blackwell Verlag, Berlin, J. Appl. Entomol. 131(6), 400–405 (2007)
Western corn rootworm adult movement
fields, our research results indicate that it is possible to
see WCR larval damage in rotated fields without those
WCR’s being the variant form. The main question for
future research is to determine adult population level
in Ôdonor fieldÕ at which economic damage in neighbouring fields could occur or to determine adult
population level in the non-maize crop which will be
planted by maize.
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Author’s address: Doc.dr.sc. Renata Bažok (corresponding
author), Faculty of Agriculture, Department for Agricultural
Zoology, Svetošimunska 25, Zagreb 10000, Croatia. E-mail:
rbazok@agr.hr
2007 The Authors
Journal compilation 2007 Blackwell Verlag, Berlin, J. Appl. Entomol. 131(6), 400–405 (2007)