Costamagna, A. C. 2006. Do varying natural enemy assemblages impact Aphis glycines population dynamics? Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

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The soybean aphid Aphis glycines Matsumura (Hemiptera: Aphididae), was detected for the first time in North America in Wisconsin during 2000, and it is currently present in 22 states. Since its detection, it has developed outbreak populations that resulted in up to 50% yield reduction during alternating years. I conducted field studies that demonstrated the crucial role of existing generalist natural enemies in suppressing soybean aphid population growth below economic threshold levels. By using predator exclusion cages, I demonstrated that ambient levels of predators exert strong suppression on soybean aphid (3 to 20-fold reduction) in comparison with open controls and sham cages. Through work at the NSF-Long Term Ecological Research site at the Kellogg Biological Station of Michigan State University, I found that this control occurs over a broad range of agricultural production systems, ranging from conventional practices with or without tillage, to organic soybeans. This suppression occurs regardless of initial aphid infestation, with aphid populations being reduced below threshold levels at high, medium and low initial aphid densities. The strength of this suppression results in a trophic cascade, with similar soybean biomass and yield in treatments where aphids were exposed to predation as in treatments completely protected from aphids by aphid exclusion cages. Direct field observations revealed that the coccinellids Harmonia axyridis and Coccinella septempunctata combined provided most of the mortality on soybean aphid, were very effective on a per capita basis, and responded to aphid density by increasing their abundance, time within the aphid patch, and consumption rate. Exposure to predation significantly shifted the aphid’s within-plant distribution towards lower nodes on the plants, suggesting indirect effects of predation.

I also conducted studies using cages that differentially excluded large versus small predators and parasitoids, which showed that ambient levels of naturally occurring parasitoids provide only minor or no reduction of soybean aphid populations, even in treatments in which parasitoids were provided with a refuge from intraguild predation by coccinellids. Similar results were obtained in a separate study using artificial infestations with the parasitoid Lysiphlebus testaceipes. I observed direct evidence of intraguild predation on parasitoid mummies by generalist predators, in which up to 80% of mummies were killed by predators, but percentage parasitism did not differ from controls, suggesting that intraguild predation did not disrupt parasitism. Finally, I developed a mathematical model for soybean aphid population growth by fitting the data of field studies under predator exclusion conditions, using a novel approach that assumes a linear decrease in the intrinsic rate of increase of the aphids. Overall, these findings provide one of the first empirical tests of recent theoretical models in which the role of generalist predators is predicted to be an important source of insect population regulation. In addition, these results support theoretical predictions that intraguild predators with strong impacts on herbivores may not disrupt, or may even enhance biological control. From the pest management perspective, it suggests the incorporation of natural enemies as an important factor to take into account when determining soybean aphid thresholds.

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