Final Report for GS03-023
Understanding the effects of fire ants on crop production is crucial because fire ants are extremely abundant on farms in Alabama and other southern states and continue to spread into California and up the east coast. Fire ants have strong ecological effects because they reach extremely high densities and are voracious predators that consume large numbers of other arthropods. Some effects of fire ants may be beneficial to crop production because they may attack and suppress plant pests not currently controlled by natural enemies (predators and parasites). Some effects of fire ants, however, are likely to harm crop production because they may suppress populations of natural enemies that currently control economically serious pests. Although fire ants have been studied for many years, it is currently difficult to predict in which systems or under what conditions the effects of fire ants are likely to benefit or harm crop production. This project will take a novel experimental and modeling approach to understand and predict the effects of fire ants on crop production. Our most recent work suggests that the effects of fire ants on crop production may be predictable. We have found that fire ants have more intense and pervasive effects on pests when aphids are abundant. Fire ants and aphids form a mutualistic relationship in which fire ants vigorously protect aphids from predators and competitors (other pests) and, in exchange, aphids supply fire ants with a sugar-rich excretion called honeydew. Because cotton aphids cause very little damage to cotton plants, cotton aphids may result in increased cotton yield when they stimulate fire ant predation of more serious pests like caterpillars. We propose to test this hypothesis by developing and validating a quantitative model that will predict the effect of aphid – fire ant mutualisms on cotton yield. This model will be easily adapted to other agricultural as well as naturally-occurring systems to predict the effect of ant – aphid mutualisms on crop yield/plant fitness.
The goal of this project is to develop and test a model that accurately predicts the effects of fire ants on crop yield. Understanding the effects of fire ants in agricultural systems is critical because fire ants infest virtually all crops in Alabama and other southern states and are continuing to spread westward into California and northward along the east coast. Fire ants have strong ecological effects in agricultural systems because they reach extremely high densities and are voracious predators that consume large numbers of other arthropods. Some effects of fire ants may be beneficial to crop production because they may attack and suppress plant pests that are not currently controlled by natural enemies (predators and parasites). Some effects of fire ants, however, are likely to harm crop production because they may devastate populations of natural enemies that currently control economically serious pests. Although fire ants have been intensively studied for many years, we do not currently have the knowledge to predict in which systems or under what conditions the effects of fire ants are likely to benefit agriculture and in which systems or under what conditions the effects of fire ants are likely to harm agriculture. This project will take a novel experimental and modeling approach to understand and predict the effects of fire ants.
Test the hypothesis that the aphid-fire ant mutualism results in decreased damage to cotton from caterpillars and increased yield.
I tested these hypotheses in a 2 x 2 factorial experiment in which I manipulated the level of herbivory (low and high) by a caterpillar pest and the presence and absence of the RIFA-cotton aphid mutualism. The four treatment combinations (low herbivory, mutualism absent; low herbivory, mutualism present; high herbivory, mutualism absent; high herbivory, mutualism present) were assigned randomly to individual 1.8 x 1.8 x 1.8-m field cages (n = 36) arranged in blocks. Each cage was erected over two adjacent rows of cotton seedlings that are approximately 20 meters from the nearest field edge. I removed all seedlings from one row in order to provide space in which to work inside the cages and I thined the remaining row to five evenly spaced seedlings, each marked uniquely with aluminum plant tags.
I applied the treatments during the reproductive stage of cotton plant growth (onset of flowering to cut-out), when cotton plant yield is most threatened by cotton aphid and caterpillar herbivory (Matthews 1994). In the field cages assigned to the ‘mutualism-present’ herbivory treatments, I established and maintained aphid densities of approximately 50 aphids per leaf using cotton aphids collected from the field and from laboratory colonies at Auburn University. This aphid density represents the lower boundary of the action threshold for cotton aphids during the reproductive stage of cotton (50 to 100 aphids per leaf on the fifth mainstem leaf from the top of the plant). No cotton aphids were added to the ‘mutualism-absent’ herbivory treatments. The model herbivore used in this experiment were the beet armyworm (Spodoptera exigua), a common defoliating pest of cotton that also feeds on cotton squares (flower buds) and bolls. First-instar caterpillars, hatched in the lab from commercially bought eggs, were transferred weekly to cotton plants in the field cages at a rate of 10 and 30 caterpillars per plant in the ‘low’ and ‘high’ herbivory treatments, respectively. The action threshold for beet armyworms is approximately 0.5 larvae per plant. Beet armyworm caterpillar mortality is so high during the first instar, however, that first-instar caterpillars must be applied to plants at much greater rates to ensure that some will survive in the ‘mutualism-absent’ treatments.
I sampled all five cotton plants in each cage weekly to record the number of aphids per leaf, the number of ants and caterpillars on plants, percent leaf damage by caterpillars, leaf area of mainstem leaves, the number of squares, flowers, and bolls (open and unopened), plant height, the number of nodes, and the numbers of vegetative and reproductive branches. At the end of the reproductive stage (cut-out), or when the plants reach the tops of the cages, I cliped all cotton plants in each cage at the cotyledon scar and transported them to the lab for further processing. Leaves, stems plus petioles, and reproductive structures were separated, dried at 60 C for three days, and weighed to determine total aboveground biomass and proportional allocation of biomass. I estimated plant fitness as seed cotton yield (cotton seeds plus attached lint), cotton lint yield (lint separated from the seeds), and cottonseed yield (seeds separated from the lint).
We conducted a study in 2003 in which we manipulated cotton aphid density and lepidopteran herbivory on cotton plants in large field cages and then weekly recorded RIFA abundance on plants, caterpillar survival, leaf area consumed (percent plant damage), and yield (number and mass of bolls). RIFA were significantly more abundant on cotton plants with greater numbers of cotton aphids. Increased RIFA abundance on plants typically reduced caterpillar survival, but never significantly. On some sampling dates, however, percent plant damage by caterpillars was significantly negatively correlated with RIFA abundance on plants, suggesting that RIFA can suppress caterpillar herbivory. Although neither cotton aphid density nor RIFA density was significantly correlated with either yield measure, the mean number of cotton bolls was significantly greater on plants with reduced percent plant damage. These results suggest that the presence of cotton aphids may facilitate biological control of lepidopteran larvae by RIFA.
Educational & Outreach Activities
Kaplan, I. and M.D. Eubanks. In revision. Keystone interactions in arthropod food webs: Aphids alter the community-wide impact of fire ants. Ecology (Provisionally accepted with major revision).
Styrsky, J.D. and M.D. Eubanks. Consequences of a fire ant – cotton aphid mutualism for cotton plant fitness. National meeting of the Entomological Society of America, Cincinnati, Ohio, October 2003.
Cooper, L.B., M.D. Eubanks, and J.F. Murphy. Potential effects of a fire ant – aphid mutualism on the spread of an aphid – vectored virus. National meeting of the Entomological Society of America, Cincinnati, Ohio, October 2003.
Kaplan, I. and M.D. Eubanks. Mechanisms underling variation in the impact of a keystone predator: Effects of an ant-homopteran mutualism on the numerical response and per-capita effects of ants. National meeting of the Ecological Society of America, Savannah, Ga, August 2003.
Harvey, C.T. and M.D. Eubanks. The effect of habitat complexity on interactions within a terrestrial food web. National meeting of the Ecological Society of America, Savannah, Ga, August 2003.
Styrsky, J.D. and M.D. Eubanks. An investigation of factors that influence the status of the red imported fire ant as a keystone predator. National meeting of the Ecological Society of America, Savannah, Ga, August 2003.
The fire ant invasion of North America is having enormous effects on agricultural production, dramatically altering the management of hundreds of thousands of acres of farmland. Relatively few studies, however, have examined the effects of fire ants on crop production in any detail and we know relatively little about the costs and benefits of fire ants to agriculture. What little we have learned to date, however, suggests that the effects of fire ants vary spatially and temporally and that under some circumstances fire ants may act as beneficial insects that suppress populations of economically important pests, but that under other conditions fire ants may disrupt effective biological control of plant pests, resulting in larger populations of pests. The results of this study will be applicable to all agricultural crops that are infested by fire ants or other dominant ants and will allow farmers to predict the net effect of ants on crop yield. This work will also produce results of broad interest to community ecologists who study naturally-occurring systems that contain ants and aphids (ants and aphids are integral components of almost all terrestrial ecosystems).
The results of this study indicate that relatively large aphid populations can lead to economically important levels of pest suppression in fire ant infested areas.
The results of this work are being incorporated in the latest information provided to farmers about fire ants in agricultural systems. An IPM program that incorporates the effects of the fire ant – aphid mutualism on crop yield is being developed.
Areas needing additional study
The effects of fire ant – aphid mutualisms on other crops besides cotton needs to be studied. This is especially important in many vegetable crops where aphids are major vectors of plant viruses. In these crops the mutualism may have a negative effect on yield because larger aphid populations may result in increase virus spread.