Relative impacts of natural enemy taxa on soybean aphidpopulation regulation

Costamagna, A. C. and D. A. Landis

Presented at the All Scientist Meeting (2004-10-08 )

Soybean aphid (Aphis glycines Matsumura) is an invasive insect pest from Asia, that was first detected in North America in 2000. Existing natural enemy communities known to contribute to A. glycines population regulation in Michigan soybeans include almost exclusively predators, and there are relatively few records of parasitoids, most of them occurring in the highly diverse context of the KBS-LTER Biodiversity Plots, Hickory Corners, Michigan.  During 2004 we conducted a field experiment to evaluate the joint impact of predators and parasitoids on A. glycines populations and to examine the role of predators in limiting parasitoid impacts via intraguild predation (IGP).  The experiment was performed using natural parasitoid populations in the KBS-LTER site.  Using tomato cage frames covered in mesh we differentially excluded all natural enemies (< 1 mm mesh, Aphid treatment) or only large predators (2 mm mesh, Mesh treatment), thus allowing natural parasitism to occur.  We also included un-caged plants with and without aphid manipulation (Open and Field treatments), plants enclosed with a sham cage (Sham treatment) as a cage control, and plants caged without aphids (Plant treatment). Soybean aphid populations were regulated throughout the season by the assemblage of natural enemies, resulting in a trophic cascade that increased plant height. Small size predators and parasitoids did not prevent aphid peak populations (Mesh treatment), but delayed them for approximately two weeks in comparison with predator exclusion treatments, and this also resulted in a trophic cascade that increased plant height. We observed evidence of IGP by large predators, reaching approximately 30% of mummies eaten in some cases. Parasitism was higher within cages providing refuge from IGP by large predators, but did not reach high levels and appears to not be responsible for aphid population reduction.

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