Phytohormone-mediated plant resistance and predation risk act independently on the population growth and wing formation of potato aphids, Macrosiphum euphorbiae

Aphids increase production of winged individuals as a generalized response to multiple threats, including predators, competitors, and poor host plant quality. While wing formation in response to these individual threats is well documented, few investigations have evaluated whether combined threats lead to additive or non-additive outcomes. We tested the interactive effects of predation risk and plant quality on population growth and wing induction in the potato aphid, Macrosiphum euphorbiae. Plant quality was varied using phytohormonal manipulations of tomato (Solanum lycopersicum) to elevate or suppress the jasmonate and salicylate defense pathways. Predation risk was altered by exposing aphids to lethal or risk (unable to feed) individuals of the convergent lady beetle, Hippodamia convergens. Phytohormonal treatments resulted in >4-fold variation in aphid population growth and thus strongly affected plant quality; however, the percentage of winged individuals was no different across plant types. Predators similarly reduced aphid abundance, but also elicited a ~3-fold increase in wing formation, an effect that was similar in magnitude when comparing lethal with risk predators. The overall impact of plants and predators on aphids was largely additive, an outcome that was unexpected given the likelihood for interactions between these two factors and our prior results with other herbivores in this system. We discuss this discrepancy in the context of phenotypic plasticity, non-lethal predator effects, and the ecological challenges faced by wing dimorphic insects.