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

213 KlDD and APPERSON: Foraging or Red Imported Fire Ants

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 RrFA 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 RIFA 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 HIFA 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

215 KIDD and APPERSON: Foraging of Red Imported Fire Ants

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 Soil Time (EST) n Mean % Mean % 0600­

0800 6 12.3 ± 2.2 30.8 ± 6.1 41.3± 19.1 69.2 ± 6.1 1000­

1200 7 36.2 ± 5.3 29.8 ± 3.4 88.7 ± 16.5 70.1 ± 3.4 1400­

1600 6 20.7±4.2 32.5 ± 5.6 55.7 ± 22.2 67.5 ± 5.6 1800­

2000 5 12.2 ± 2.6 32.1 ± 8.2 33.6 ± 14.1 67.9 ± 8.2 2200­

2400 5 11.6 ± 4.8 24.8 ± 8.1 32.6 ± 14.6 75.1 ± 8.1 0200­

0400 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 RIFA 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

216 J. Agric. Entomol. Vol. 1, No.3 (1984)

2. o SOil

• PLANTS

CO 20Z

CO.. a: 0 ~

<Il .... ~ z- ,. .. 0

<Il ~o 1:'!z 0" I-NIGHT-l1~~ '0 1

0

...... 0 z~ w ..

TlUi 0w

.. I 1 "6

f.. 1 I .l z

5 w ::;:

I I II 600 1000 1400 '800 2200 200 800 1200 1600 2000 2400 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 (Xz )

Soil temperature X Soil moisture (X3)

Air temperature X Soil moisture (X.1)

Soil moisture X Time (X,)

Time of day (X.J EQUATION:

y ~ -2.26 + 17.24X I

DF 6

27 33

1 1

1

1

Analysis of variance F

4.30

14.38 13.05

9.56

9.34

3.86 3.23

Probability >F R2 Value

0.0036 0.489

0.0008 0.00 [2

0.0046

0.0050

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 Probability DF F >F Rt Value

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

5 28 33

I

0.019X,

5.68 0.0010 0.503

25.01 0.0001

9.84 0.0040

8.65 0.0065

8.43 0.0071 7.81 0.0093

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 antsOccurence of ants

Baits Soil Bails SoilHeight of bait (em) I Hour Observation

3 442 1515 0 15 0 6630 0 17 0 76.5

4.5 Hour Observation 20 166 5715 8

3 15 16 3130 0 18 0 5345

.. Thin)' llllllll~ wt'rp b"i'''d a1 I'lich ht'lllhi rl'll impn.

218 J. Agric. Entomol. Vol. 1, No.3 (1984)

ACKNOWLEDGMENTS

Drs. Emmett Lampert, J. R. Bradley, Jr., and Blance Haning reviewed the manuscript and made numerous useful suggestions for its improvement

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