Seasonal variation of defensive symbionts of the pea aphid, Acyrthosiphon pisum

Insects have developed many strategies to defend against natural enemies which can vary over space and time. The pea aphid, Acyrthosiphon pisum, harbors two facultative bacterial symbionts, Hamiltonella defensa and Regiella insecticola that defend the aphid against the parasitoid wasp Aphidius ervi (when in association with a bacteriophage) and the fungal pathogen Erynia neoaphidis, respectively. Levels of resistance to these two natural enemies vary seasonally and between host plants. In controlled studies, defense against A. ervi larval development conferred by bacteriaphage of H. defensa is reduced under heat stress and successful infection by E. neoaphidis is temperature-dependent. Given these results, it is likely that temperature is an important determinant of successful parasitism and pathogen infection in the field. The objective of this study was to correlate parasitism and infection rates with symbiont frequency, host plant and within canopy temperature. Pea aphids, predators, parasitoids and temperature data were collected in alfalfa and clover fields tri-weekly from mid-May to late-September in southeastern, PA and upstate, NY. Pea aphids collected in the field were either screened for symbionts or reared in the laboratory or field to determine symbiont frequency and rates of parasitism and infection. We expect symbiont frequency to positively correlate with wasp density and pathogen infection rates and negatively correlate with increased temperature. Future research aims to investigate the impact that biotic and abiotic factors have in shaping symbiont prevalence and the mechanisms underlying how these symbionts affect their aphid hosts under natural field conditions.