Tritrophic Interactions of Crops, Pests, and Predators: Linking Ecological Theory with Agricultural Applications

NON-TECHNICAL SUMMARY: Insects are ubiquitous constraints on agricultural production systems in virtually all crops. Their feeding damage injures plant leaves, roots, stems, flowers, and fruits, thus limiting marketable crop yield and reducing economic gains. Indiana potato growers, for example, consistently rank insects such as Colorado potato beetle as the primary factor affecting crop growth, compared with weeds, pathogens, and other potential pests. Managing insect outbreaks has historically entailed widespread and frequent application of chemical insecticides on cropland, an approach that is generally effective but comes with an array of environmental, social, and human health costs. While insecticides remain at the forefront of insect pest management tactics currently employed in U.S. agriculture, calls for more judicious use of chemicals and development of alternative, biologically-based control strategies are ever-increasing. This demand is reflected in the adoption and growth of integrated pest management (IPM), the passage of new legislature restricting pesticide use (e.g., 1996 Food Quality Protection Act, the phase out of methyl bromide), and the explosion of the organic food movement over the past two decades. More recently, the demand for local foods has erupted in popularity, in large part because of food safety concerns among consumers. Two of the most effective means of "natural" pest control are: (1) facilitating the action of natural enemies (i.e., predators, parasites, and pathogens) of pests; and (2) exploiting innate plant defenses that repel, poison, or otherwise deter pests. Naturally enemies are considered beneficial insects, such as ladybugs that kill and eat aphids, and it is estimated that these predators contribute $4.5 billion annually in pest suppression services in the U.S. alone. Despite the magnitude of this staggering value, enemy impact on pest abundance is highly variable and in many cases pests escape from control. Similarly, plants vary enormously in their inherent susceptibility to pest damage and it is unclear which plant traits contribute to this variation. Thus, we continually struggle with how to enhance the efficacy of predators and plant defenses on herbivorous insects in agriculture. This project aims to improve upon our knowledge in this realm by studying the ecology of natural enemies, plant defenses, and their interaction. Ultimately, the goal of this research is to establish multipronged strategies that squeeze pests between what they eat (plants) and what eats them (predators), resulting in effective, safe, and ecologically-minded pest management systems.

<P>

APPROACH: Objective 1: Non-consumptive predator effects. I plan to use crop-pest-beneficial insect assemblages to test the non-consumptive impact of biocontrol agents on pests and indirectly on crop damage and yield. Experimentally manipulating and thus quantifying non-consumptive effects requires exposing herbivores to predators, but without those individuals being consumed. I will measure the behavioral responses of pests to predators. This will be tested using insect herbivores on tomato, including the guild of leaf-chewing lepidopteran insects (e.g., Manduca sexta), as well as sap-feeding herbivores (e.g., Macrosiphum euphorbiae). Predaceous stinkbugs, Podisus maculiventris, are abundant predators in agroecosystems throughout the eastern U.S. and feed on many of the pests listed above. I plan to study P. maculiventris and other common predators in the Midwest that are known to contribute to pest suppression in crops such as Nabis sp., Orius insidiosus, and Harmonia axyridis. The combination of crop, pest, and predator will be studied using lab, greenhouse, and field experiments. Lab trials will be established for behavioral assays whereby pest behavior (i.e., time spent eating, resting, moving, defensive postures) will be quantified in observational arenas with and without natural enemies added. Greenhouse and field experiments will be used to set-up caged trials in which the herbivore and either lethal or risk predators are added. The above plant-insect systems and methodology will be used to test the contribution of non-consumptive effects to the net impact of predators on prey and plants. Objective 2: Herbivore-induced plant volatiles. The strength of predator impact on prey is undoubtedly facilitated by the active role that plants play in guiding enemies to herbivores via volatile signaling. As a result, plant volatiles may be manipulated to attract natural enemies to crops in an effort to enhance suppression of pests. I intend to deploy synthetically produced herbivore-induced plant volatiles (HIPVs) which show the greatest promise for immediate application potential. My first goal is to identify which, among many candidate compounds, are most attractive to predators and parasitoids, and determine whether these compounds are species-specific. This will be accomplished by testing a range of volatile chemicals that are known to be emitted from herbivore-injured plants (e.g., methyl salicylate, linalool, cis-jasmone). I plan to use field and laboratory assays to measure enemy responses. In the field I will deploy traps containing synthetic versions of each compound in potatoes and other vegetable plots and use sticky cards to capture attracted insects. Predators and parasitoids will be identified to species. In the lab, preference of key insect predators for various compounds will be quantified using y-tube olfactometers and electroantennogram (EAG) assays. The field responses of generalist predators will then be compared with lab behavioral trials to determine the correspondence (or lack thereof) between field and lab environments.