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Title: Differential impact of adults and nymphs of a generalist predator on an exotic invasive pest demonstrated by molecular gut-content analysis

item Greenstone, Matthew
item Rowley, Daniel

Submitted to: Biological Invasions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/5/2008
Publication Date: 7/7/2008
Citation: Harwood, J.D., Yoo, H., Greenstone, M.H., Rowley, D.L., O'Neil, R.J. 2008. Differential impact of adults and nymphs of a generalist predator on an exotic invasive pest demonstrated by molecular gut-content analysis. Biological Invasions.

Interpretive Summary: Most of what we know about the role of insect predators is based upon adults, either because the immatures are difficult to study or because they have simply been ignored. This is problematical, because the majority of insect predators are in fact immatures. Further, predators are the first line of defense against many exotic invasive crop pests, and in order to make the most effective use of them we need to understand the role of all predator life stages. The Asian soybean aphid invaded North America in 2000 and causes $100 million per year in soybean crop loss and costs of control. Using techniques similar to DNA fingerprinting methods in human forensic cases, we identified both immatures and adults of a major insect predator in U.S. crop fields, the insidious pirate bug, and also used these techniques to identify soybean aphid remains in the pirate bugs’ guts. We showed that the pirate bug is an important early-season predator, able to maintain significant numbers by feeding on other insects before the soybean aphid enters fields, and then slowing the buildup of aphid populations until more specialized predators arrive to attack them. This information will encourage growers to avoid using insecticides in early-season soybeans, thereby saving money, protecting the environment, and helping to conserve valuable natural enemies such as the pirate bug.

Technical Abstract: Generalist predators can regulate herbivore populations through a variety of mechanisms, but food webs are complex and defining the strength of trophic linkages can be difficult. Molecular gut-content analysis has revolutionized our understanding of these systems. Using this technology, we examined the structure of a soybean food web and identified the potential for adult and immature Orius insidiosus (Hemiptera: Anthocoridae) to suppress Aphis glycines (Hemiptera: Aphididae), and tested the hypotheses that foraging behavior would differ between life stages, but that both adults and immatures would exert significant predation pressure. We also identified the strength of trophic pathways with two additional food items: an alternative prey, Neohydatothrips variabilis (Thysanoptera: Thripidae), and an intraguild predator, Harmonia axyridis (Coleoptera: Coccinellidae). A. glycines constituted a greater proportion of the diet of immature O. insidiosus, but N. variabilis DNA was found in greater frequency in adults. However, both life stages were important early-season predators, a phenomenon predicted as having the greatest impact on herbivore population dynamics and establishment success. No adult O. insidiosus screened positive for H. axyridis DNA, but 2.5% of immature individuals contained DNA of this intraguild predator, thus indicating the existence of this trophic pathway, albeit a relatively minor one in the context of biological control. Approximately two-thirds of predators contained no detectable prey and fewer than 3% contained more than one prey item, suggesting the possibility for food limitation in the field. This research implicates O. insidiosus as a valuable natural enemy for the suppression of early-season A. glycines populations.