Changes in within-plant quality mediate predator impacts on aphids

Besides inflicting direct mortality by feeding, predators can also have important negative, indirect effects on prey population dynamics. The strength of indirect effects depends on their impact on prey population growing rates. In the soybean aphid, Aphis glycines, our previous research showed a significant shift in within-plant distribution where aphids relocated to lower portions of the soybean plant when predators were present. Lower plant canopy consists of older leaves that result in reduced aphid growth rates. Therefore, we hypothesize that predators will have significant, indirect impacts on soybean aphid growth mediated by plant quality. To test this hypothesis, we used a combination of partial and total exclusion cages in field experiments: 1) bottom predator exclusion (aphids protected by cages on the lower plant canopy), 2) top exclusion (aphids protected on the upper plant), 3) total exclusion cage (all aphids protected), and 4) open control (all aphids exposed to predation). Top-exclusion showed intermediate aphid densities between total and bottom exclusion treatments, suggesting that most of the aphid population growth takes place in the upper part of the plant. We observed an overall lower population growth rate in the lower part of the plant as predicted, with bottom exclusion cages showing the same growth as open controls. These results suggest that predators directly affect aphid populations by consumption, but also have an important indirect impact on aphid population growth rates since aphids relocate in the lower plant canopy that is a lower quality resource.