ENVIRONMENTAL FACTORS AFFECTING RELATIVE
DISTRIBUTION OF FORAGING RED IMPORTED FIRE ANTS I
IN A SOYBEAN FIELD ON SOIL AND PLANTS'
Kathleen A. Kidd and Charles S. Apperson
Department of Entomology
North Carolina State University
Raleigh, NC 27607
Ab.<;(racl: A field investigation was conducted in Brunswick County, NC, to determine
environmental factors thnl regulat.e the distributional patterns of Solerwpsis inuicta Buren,
the red imported fire ant, foraging in a soybean field. Although S. iflvicta was observed to
forage during the day and night, foraging activity diminished at night. The impact of some
environmental factors on distribution patterns of foraging ants was assessed with standard
multiple regression analyses. Foraging activity on soil was most strongly influenced by air
and soil temperatures while foraging on soybclln plants was predominantly. related to air
temperature. More ants were found foraging on the soil than on soybean plants. The
proportion of ants foraging on t.he soil relat.ive to soybean plants did not change significantly
over each 24 h sample I>eriod.
Key Words: Solerwpsis irwicla, foraging, fire ants, distribution.
J. Agric. Emomol. 1(3); 212-218 (July 1984)
Solenopsis invicta Buren, the red imported fire ant (RIFA), is distributed across
the southeastern United States and currently inhabits ca. 93 million ha from Texas
to North Carolina (Canter 1981). A large-scale chemical control program has been
directed toward the RIFA, but the program is controversial because a thorough
evaluation of the economic importance of the fire ant had not been conducted
before the initiation of the control program.
The role of the RIFA in agricultural ecosystems has not been completely
determined. The RIFA is recognized as an important predator in cotton (McDaniel
and Sterling 1979; Sterling et al. 1979), sugarcane (Reagan et al. 1972), and
soybeans (Whitcomb et a1. 1972), but the RIFA is also associated with reduced
soybean seed yields (Adams et al. 1976, 1977, 1983; Apperson and Powell 1983;
Lofgren and Adams 1981). The mechanism of reduction has not been completely
elucidated. Mechanical interference with harvest caused by RIFA mounds accounts
for only a small portion of these losses. Smittle et al. (1983) injected soybean
plants with 32 P and subsequently recovered radioactive ants from the base of the
plants which suggests that the RIFA feeds on soybean roots. The reduction of
beneficial insects may also be a factor in the lovl"ering of seed yields since the
diversity of predator ant species in Florida soybean fields (Whitcomb et a!. 1972)
and populations of predator carabid beetles in Louisiana soybean fields (Brown
and Goyer 1982) are reduced by the RIFA; however, Sterling et a!. (1979) showed
that the RIFA has little effect on predator abundance in cotton.
The objectives of our inv.estigation were to characterize the foraging activity of
the RIFA on soybean plants and on the soil, and to assess the impact of some
1 HYMENOPTER": Formicidnc
2 Puper !\o. 1l1l20 of th~ JOllnl1ll Series of tbe N<lrlh Carolirm Agri~. Hcs. Sen.... ftaleil:h. Heceived for puhlicntion 6
September 19113; accepted 1.1 Februllry 19114.
212
KlDD and APPERSON: Foraging or Red Imported Fire Ants
213
environmental factors on distribution of foraging ants. A better understanding of
foraging activity may provide insight into the importance of the RIFA in the
soybean ngroecosystem.
MATERIALS AND METHODS
Experiments were conducted from July to September, 1982, in 8 6.1 ha farm
field near Bolivia, Brunswick Co. NC. The "Ransom" variety of soybeans (Glycine
ma.t: (Linnaeus) Merril1) was planted during the week of 12 July. Planting had been
delayed by heavy rains. Metolachlor (Dual@) and metribuzin (Lexone@) were
applied as preemergence herbicides and the field was cultivated for additional
weed control at 2, 3, and 5 wk after planting. Sampling was carried out in a 0.1 ha
plot located in a corner of the field, bordered by woods on two sides and soybeans
on t\VO sides.
RIFA Sampling Procedures
To determine periodicity of foraging and to evaluate the degree to which
foraging occurs on the soil and on soybean plants, ant activity was assessed with a
combination of direct observations and beat sheet samples.
Samples were taken along 2 m of row at 10 sites within the plot. The soil
under the soybean plants of two I-m sections of rows and the furrow in between
were carefully inspected. Soybean plants were examined, beginning at the ground
level and proceeding up the mainstem. Visual counts of RIFAs on the soil and on
plants were recorded separately. ,"Vhen the soybeans were at the full bloom stage
(Fehr and Caviness 1977), the dense foliage made thorough visual observations
difficult. After the visual counts of ants on the soil were made, the mainstem and
foliage were inspected, and the soybean plants were then vigorously shaken over a
1-01 2 beat sheet. When walking to each sample site and when placing the beat
sheet, care was taken to avoid disturbing the soybean plants. Care was also taken
that weeds were not sampled. All arthropods collected on the beat sheet. were
placed in plastic cups containing ethanol or placed on ice in a cooler. Samples
were sorted in the laboratory, and RIFAs were identified and counted. Collection
sites were chosen randomly except that no site was resampled during the same
sample period.
Samples were taken every 4 h for a tolal of seven sampling intervals during
each 24-h period. The 24-h samples were taken on five different dates with beat
sheets used on the last three dates. Sampling was carried out at various stages of
plant growth as classified by Fehr and Caviness (1977). Samples were taken at
stages V6 (fifth trifoliate developed, 9 August), R1 (beginning bloom, 19 August),
R2 (full bloom, 26 August), R3 (beginning pod, 1 September), and R5 (beginning
seed, 28 September) to determine if distributional patterns of foraging ants
changed with the phenology of the soybean plants.
A separate experiment was conducted to examine the distribution of foraging
RIFAs on soybean plants. Cotton wicks saturated with a 10% sucrose solution
were attached to plants at heights of 15, 30, and 45 em in an area of the soybean
field where the RrF A was active. Sucrose has been shown to be a highly
acceptable food (Ricks and Vinson 1970) to which the RIFA will recruit workers
(Wilson 1962). Thirty plants of uniform height were chosen for each height
regimen and a total of 90 plants was baited. The baits were placed on the plants
214
J. Agric. Entomol. Vol. I, No.3 (1984)
in the morning at ca. 1000 h and the plants were examined at the end of 1 and 4.5
h and the numbers of ants present and wicks occupied were recorded. The soil,
within a 6·cm radius of each plant stem, also was examined and the presence and
numbers of ants noted.
Abiotic and Edaphic Parameters
Measurements of air and soil temperatures, relative humidity, and soil moisture
were taken during each sample period for correlation with RIF A foraging activity.
An Extech@ Model 1200 digital thermometer was used to measure air and soil
temperatures. Readings were taken at each sample sileo Air temperatures were
measured ca. 3 cm above the soil surface and soil temperatures were taken by
pushing the probe just under the surface. Relative humidity readings were taken
with a Bacharach@ sling psychrometer. A single reading was taken during each
sampling interval. Soil samples for moisture determination were taken from the
soil surface with a Hoffer sampler at each observation site. Soil samples were
weighed to the nearest 0.1 g in the field after collection, then brought back to the
laboratory and dried to constant weights. Values for soil moisture were calculated
on a dry weight basis.
Statistical Analyses
For correlation with temperature and moisture data. the number of ants
counted on soil or plants at each time interval was transformed to a percentage of
the total number of ants counted on the soil or the plants during the 24 h of each
sample date. The percentage of ants was used as the dependent variable in
statistical analyses. Data were analyzed with the stepwise regression procedure of
the Statistical Analysis System (SAS) (Helwig and Council 1979). The stepwise
regression procedure was used to find the best correlation (P :S 0.1) of the
percentage of ants with temperature, moisture. and time variables, and interactions
of all variables. The multiple correlation coefficient (R 2) was used as a measure of
the predictability of the statistical models developed by the stepwise regression
analyses.
To determine the proportion of ants foraging on the soil and on the plants over
the 24-h observation period, the numbers of ants counted on the soil and the
plants were combined, and the percentage of ants on the soil and on the plants
was calculated for each time interval. The data were grouped in 2·h time intervals
and means for the five sample dates were calculated by time interval. The
percentages of ants on the soil and on thc plants during cach 2-h period were
analyzed with an analysis of variance test using the general linear models
procedure of SAS to determine if there was a significant change in the proportion
of ants on the soil and on the plants over the 24 h. The -mean percentage of the
total ants observed foraging the soil and on the plants for each sample date was
analyzed with the Duncan's multiple range test procedure of SAS to determine jf
the proportion of ants on the plants changed significantly with the phenology of
the soybean plants.
RESULTS AND DISCUSSION
Peak foraging activity by the HIF A occurred between 1000 hand 1200 h
(EST). The RIFA was observed to forage during the day and night with a decrease
in activity at night (Table 1, Fig. 1). Approximately 2 . 3 and 5 - 7 % of the total
KIDD and APPERSON: Foraging of Red Imported Fire Ants
215
Table 1. Red imported fire ant foraging activity on plants and soil during 24.h
periods in a soybean field in Brunswick Co., NC.
Mean no.- and %-t (± S.E.) of ants foraging on
Plants
Time (EST)
0600­
0800
1000­
1200
1400­
1600
1800­
2000
2200­
2400
0200­
0400
Soil
n
Mean
%
Mean
%
6
12.3 ± 2.2
30.8 ± 6.1
41.3± 19.1
69.2 ± 6.1
7
36.2 ± 5.3
29.8 ± 3.4
88.7 ± 16.5
70.1 ± 3.4
6
20.7±4.2
32.5 ± 5.6
55.7 ± 22.2
67.5 ± 5.6
5
12.2 ± 2.6
32.1 ± 8.2
33.6 ± 14.1
67.9 ± 8.2
5
11.6
± 4.8
24.8 ± 8.1
32.6 ± 14.6
75.1 ± 8.1
5
16.4 ± 9.7
33.0 ± 6.8
24.2± 10.1
67.0 + 6.8
- I'rrcrnlagf'1 do nol COrn!SIJoOnd 10 melln numhers bl'<:"nuse thr IlrrCO!ntllG'n "'-ere calculolrd by IIIlmlllr dun and Ihrn
1I,·craged.
t Si/:nificunt II' ::so 0.051 difrerences by IInllly~il or ,·tlnlll":" lelill ...· ere nul round.
number of ants were observed foraging at night. on plants and soil, respectively
(Fig. I).
Tables 2 and 3 present multiple regression analyses of RIFA activity on the
soil and soybean plants. The best predictive model (R 2 = 0.189) of foraging
activity on the soil showed a linear re!ationshil> between air and soil temperatures
and RIF A foraging activity (Table 2). The interaction of several parameters (soil
temperature X soil moisture, air temperature X soil moisture) also were found to
influence foraging activity. The best predictive model (R 2 = 0.503) of foraging
activity on plants is shown in Table 3. Air temperature, the most significant
variable, showed a linear relationship with foraging activity. The interactions of
soil moisture X humidity, soil temperature X humidity, soil moisture X time of
day the samples were taken, and humidity X time also were strongly related to
RIFA foraging activity. Generally, foraging on t.he soil was most strongly influenced
by soil moisture and temperature, and foraging on soybean plants was innuenced
by air temperature, humidity, and soil temperature. The models of foraging
activity are significant (P 5 0.05), but their predictability is low, indicating that
there are other factors which influence foraging activity.
A higher percentage of anls (mean ± S.E.) was found on the soil (69.2 ± 2.8)
than on plants (30.8 ± 2.8). The percent of the total ants observed foraging on the
soil and plants during each 2-h sample period did not change significantly (P ~ 0.05)
over the diel (Table I). Although significant (P ~ 0.05) differences were found
between the five sample dates, the percentage of the total ants foraging on plants
or soil over each 24-h sample period could not be related to the phenology of the
soybean plants.
Foraging on the soybean plants was confined to the lower 20 cm of the
lTIainstem. Ants were rarely seen on the foliage and low numbers were found on
the beat sheet. When sugar·saturated wicks were attached to soybean plants in an
area where the RIFA was observed to be actively foraging, RIFAs were not found
J. Agric. Entomol. Vol. 1, No.3 (1984)
216
2.
CO
Z
..
o
SOil
•
PLANTS
20
CO
a:
0
~
<Il
.... ~
z.. 0
,.
<Il
~o
1
:'!z
0"
~~
......
1
'0
1
0
z~
1
w ..
lUi
w
.
.
z
5
w
::;:
I-NIGHT-l
0
I
I
I
1000
1200
600
800
T
1
0
1400
1600
"6.l
f
I I I
'800
2000
2200
2400
200
400
TIME
Fig. 1. Periodicity of foraging of red imported fire ants on soil and plants in a
soybean field in Brunswick Co., NC (Vertical bars = SE).
Table 2. Multiple regression analyses of effects of some abiotic variables on red
imported fire ant foraging activity on soiL
Model - Activity on Soil
Error
Corrected total
Air temperature (XI)
Soil temperature (X z )
Soil temperature X
Soil moisture (X 3)
Air temperature X
Soil moisture (X.1)
Soil moisture X
Time (X,)
Time of day (X.J
EQUATION:
y ~ -2.26
+
17.24X I
Analysis of variance
Probability
DF
F
>F
R2 Value
6
27
33
1
1
4.30
0.0036
0.489
14.38
13.05
0.0008
0.00 [2
1
9.56
0.0046
1
9.34
0.0050
3.86
3.23
0.0599
0.0835
17.38X 2
+
1.05X;\ -
0.99X 4 - Q001X,
+
0~1X.
KJDD and APPERSON: Foraging or Red Imported Fire Anls
2] 7
Table 3. Multiple regression analyses of effects of some abiotic variables on red
imported fire ant foraging activity on plants.
Analysis of variance
F
Probability
>F
Rt Value
5
28
33
5.68
0.0010
0.503
I
25.01
0.0001
9.84
0.0040
8.65
0.0065
8.43
7.81
0.0071
0.0093
DF
Model - Activity on Plants
Error
Corrected total
Air temperature (X ,)
Soil temperature X
Humidity (X,)
Soil moisture X
Time (X,,)
Soil moisture X
Humidity (x.,)
Humidity X Time (X;,)
EQUATION:
Y = -13.87
+
2.079X 1
0.019X,
O.OOIX"
+
0.016X,
+
O.OOOIX,
on the sucrose baits at the 45 em height and were only found on three wicks at the
30 em level 4.5 h after the baits were placed on the plants (Table 4). Ants were
never found on the plants above the level of the sugar wicks. We conclude that the
RIFA forages primarily on the surface of the soil.
Table 4. Occurence and abundance of red imported frre ants on soil and sucrose
baits attached to soybean plants:
Total No. of ants
Occurence of ants
Height of bait (em)
Baits
Soil
Bails
Soil
3
44
66
I Hour Observation
15
30
.5
15
30
45
2
0
0
15
15
17
4.5 Hour Observation
20
8
15
3
18
0
.. Thin)' llllllll~ wt'rp b"i'''d a1 I'lich ht'lllhi rl'll impn .
0
0
166
16
0
76
57
31
53
J. Agric. Entomol. Vol. 1, No.3 (1984)
218
ACKNOWLEDGMENTS
Drs. Emmett Lampert, J. R. Bradley, Jr., and Blance Haning reviewed the manuscript
and made numerous useful suggestions for its improvement
REFERENCES CITED
Adams, C. T., W. A. Banks. C. S. Lofgren, B. J. Smittle, and D. P. Harlan. 1983. Impact of
the red imported fire ant, Solertopsis irwicta (Hymenoptera: Fonnicidae) in the growth and
yicld of soybeans. J. Econ. Entomo!. 76: 1129-1132.
Adams. C. T., J. K. Plumley, W. A. Banks, and C. S. Lofgren. 1977. Impact of the red
imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidac), on harvest of
soybeans in North Carolina. J. Elisha Mitchell Soc. 93: 150·152.
Adams, C. T., J. K. Plumley, C. S. Lofgren, and W. A. Banks. 1976. Economic importance of
the red imported fire ant, Solenopsis inuicta Buren. I. Preliminary investigations of impact
on soybean harvest J. Ga. Enwmo!. Soc. II: 165-169.
Apperson, C. S., and E. E. Powell. 1983. Correlation of the red imported fire ant
(Hymenoptera: Formicidae) with reduced soybean yields in North Carolina. J. Econ.
Entomo!' 76: 259-263.
Brown, D. W., and R. A. Goyer. 1982. Effects of a predator complex on lepidopterous
defoliators of soybean. Environ. Entomol. II: 385·389.
Canter, L. W. 1981. Cooperative imported fire ant IH'Ogram: final programmatic environmental
impact statement. USDA, APHIS-ADM-81-01-F. 240 pp.
Fehr. W. R., and C. E. Caviness. 1977. Stages of soybean development. S!>ecial Report 80.
Coop. Ext Servo Iowa State Un i\'. Ames, IA. 11 pp.
Helwig, J. T., and K. A. Council (cd.). 1979. SAS User's Guide. SAS Institute Inc. Raleigh,
NC 494 pp.
Lofgren, C. S., and C. T. Adams' 1981. Reduced yield of soybeans in fields infested with the
red imported fire ant, Solellopsis illuic/a Buren. Pia. Entomol. 64: 199-202.
McDaniel, S. G., and W. L. Stcrling. 1979. Predator determination and efficiency 011 Heliothis
uirescens eggs in cott.on using 32 P. Environ. EnLomol. 8: 1083-1087.
Reagan, T. E., G. Coburn, and S. D. Hensley. 1972. Effects of mirex on the arthropod
fauna of a Louisiana sugarcane field. Environ. Entomo!' 1: 588-591.
Ricks, L. L, and S. B. Vinson. 1970. Feeding acceptability of certain insects and various
water-soluble compounds to two varieties of imported fire ant. J. Econ. Entomol.
63: 145-148.
Smittle, B. J., C, T. Adams, and C. S. Lofgren. 1983. Red imported fire ants: detection
of feeding on com, okra and soybeans with radioisotopes. J. Ga. Entomol. Soc. 18:
78-82.
Sterling, W. L., D. Jones, and D. A. Dean. 1979. Failure of the red imported fire ant Lo
reduce entomophngous insect and spider abundance in a cotton agroecosystem.
Environ. Entomol. 8: 976-981.
Whitcomb, W. H., H. A. Denmark, A. P. Bhatknr, and G. L. Greene. L972. Preliminary
studies on the ants of Florida soybean fields. Fla. EntomoL 55: 129·142.
Wilson, E. O. 1962. Chemical communication among workers of the fire ant, Solenopsis
saeuissima (Smith). I. The organization of mass· foraging. J. Anim. Behav. 10: 134­
147.