Aphids as Beneficial Insects? Using a Fire Ant - Aphid Interaction for the Sustainable Management of Insect Pests in Southern Cotton

Aphids as Beneficial Insects? Using a Fire Ant - Aphid Interaction for the Sustainable Management of Insect Pests in Southern Cotton

Summary

Fire ants are extremely aggressive generalist predators that have expanded their range across the southeastern United States.  These insects generally occur at high densities in Southern agricultural fields and consequently fire ants may provide effective control of economically important pests in a variety of cropping systems.  We proposed to investigate the effect that aphids have on the interaction between fire ants and insect pests in Southern cotton fields.  The fire ant-cotton aphid interaction may have a beneficial effect on cotton production and allow growers to predict the impact of fire ants on pests because aphids excrete a sugary liquid (honeydew) that is very attractive to foraging fire ants.  As a result, we predicted that when aphids feed on cotton plants, fire ants become more abundant on and actively defend plant parts that are particularly vulnerable to insect damage (e.g., leaves and bolls).  During the 2003 field season we manipulated aphid densities in large field enclosures and examined the impact of aphids on fire ant predation of key cotton pests.  Preliminary data from this experiment suggested that the presence of cotton aphids facilitated biological control by fire ants.  Aphids attracted fire ants onto plants which resulted in slightly lower caterpillar survival and significantly reduced caterpillar damage.  Furthermore, plant damage was negatively correlated with number of bolls produced by the cotton plant.  In the upcoming field season we plan to expand and improve this project by manipulating fire ant density to quantify the direct effect of aphids on cotton plants and determine the impact of this mutualism on various plant variables (i.e., seed cotton yield, above-/below-ground biomass, etc).  Ultimately this information will be necessary to estimate the overall impact of fire ants and aphids on insect populations in agricultural communities such as cotton.

Objectives/Performance Targets

We conducted a pilot study in 2003 in which we manipulated cotton aphid density and lepidopteran herbivory on cotton plants in large field cages and then weekly recorded fire ant abundance on plants, caterpillar survival, leaf area consumed (percent plant damage), and yield (number and mass of bolls).  We plan to improve on the 2003 field data with the following protocol for the 2004 field season.  We will manipulate the level of herbivory (low and high) by a caterpillar pest and the presence and absence of the ant-aphid mutualism in a 2 x 2 factorial design experiment.  This experiment will be conducted in 2004 in conventional cotton cultivated at the Row Crops Unit of Auburn University’s E.V. Smith Research Center in Shorter, AL.  The four treatment combinations (low herbivory, mutualism absent; low herbivory, mutualism present; high herbivory, mutualism absent; high herbivory, mutualism present) will be assigned randomly to individual 1.8 x 1.8 x 1.8-m field cages (n = 36) that are arranged in blocks.  Each cage will be erected over two adjacent rows of cotton seedlings that are approximately 20 meters from the nearest field edge.  We will apply 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.  In the field cages assigned to the ‘mutualism-present’ herbivory treatments, we will establish and maintain 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 will be added to the ‘mutualism-absent’ herbivory treatments.  The model herbivore used in this experiment will be 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, will be 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.  We will sample 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, we will clip all cotton plants in each cage at the cotyledon scar and transport them to the lab for further processing.  Leaves, stems plus petioles, and reproductive structures will be separated, dried at 60 C for three days, and weighed to determine total aboveground biomass and proportional allocation of biomass.  I will estimate 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 will use repeated-measures ANOVA to test for block and treatment effects on 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, plant height, the number of nodes, and the numbers of vegetative and reproductive branches.  We will use ANOVA to test for block and treatment effects on cage means of total plant biomass, final percent plant damage by caterpillars, final leaf area of mainstem leaves, and the three yield measures.  We will use MANOVA to test for block and treatment effects on proportional allocation of biomass.  Because aphid densities are likely to vary somewhat among the cages, we will also use ANCOVA (with aphid density as a covariate) to test for differences in plant biomass, plant damage, and yield between the two herbivory treatments (low and high) in which the mutualism was present.  This analysis will allow us to test whether aphid density and intensity of herbivory interact in their effects on cotton plant fitness.  The expected results of this experiment are that the presence of the mutualism will benefit cotton plant fitness (yield) via ant suppression of beet armyworm herbivory and that the mutualism will provide the greater benefit under high intensity of herbivory.  Specifically, we predict: 1) weekly caterpillar survival will be higher in the ‘mutualism-absent’ treatments and lower in the ‘mutualism-present’ treatments, 2) percent leaf damage will increase weekly in the ‘mutualism-absent’ treatments, with the greater increase under the ‘high’ level of herbivory, but remain low in the ‘mutualism-present’ treatments, 3) plant growth (leaf area, growth rate, number of nodes, etc.) will be greater in the ‘mutualism-present’ treatments than in the ‘mutualism-absent’ treatments, and 4) plant yield will be greater in the ‘mutualism-present’ treatments but the difference in yield between the ‘mutualism-absent’ and ‘mutualism-present’ treatments will be greater under the ‘high’ level of herbivory.

Accomplishments/Milestones

The following describes our initial data from the preliminary (2003) season:  Fire ants were significantly more abundant on cotton plants with greater numbers of cotton aphids, suggesting that ants are attracted to aphid-infested plants.  Increased fire ant abundance on plants typically reduced caterpillar survival, although these effects were non-significant.  Percent plant damage by caterpillars, however, was significantly negatively correlated with ant abundance on plants, suggesting that fire ants can suppress caterpillar herbivory.  Although neither cotton aphid density nor fire ant 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 by fire ants.  The experiment described above (see ‘Objectives/Performance Targets’) will provide a stronger test of this hypothesis and will test for an effect of intensity of herbivory.

Impacts and Contributions/Outcomes

Ultimately, cotton producers may benefit in two respects from our research.  Primarily, recognizing that aphids are not simply pests, but have beneficial attributes may be justification for reevaluating aphid management (and economic injury levels) in cotton fields.  Secondly, by encouraging higher aphid densities, outbreaks of primary cotton pests (e.g., bollworm) may occur less frequently.  Preliminary data collected during the 2003 field season provided very promising results. However, the true contribution and impact of this project will not be realized until the proposed 2004 field experiment has been completed.

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