Genotype and temperature influence pea aphid resistance to a fungal entomopathogen DAVID A. STACEY*,MATTHEW B. THOMAS* y, SIMON BLANFORD* y, JUDITH K. PELL z, CLARE PUGH* andMARK D. E. FELLOWES§ *NERC Centre for Population Biology, Imperial College at Silwood Park, Ascot, Berkshire, yCABI Bioscience, Silwood Park, Ascot, Berkshire, zPlant and Invertebrate Ecology Division, Rothamsted Research, Harpenden, Hertfordshire and §School of Animal and Microbial Sciences, University of Reading, Whiteknights, Reading, Berkshire, UK Abstract. The influence of temperature on life history traits of four Acyrthosiphon pisum clones was investigated, together with their resistance to one genotype of the fungal entomopathogen Erynia neoaphidis. There was no difference among aphid clones in development rate, but they did differ in fecundity. Both development rate and fecundity were influenced by temperature, but all clones showed similar responses to the changes in temperature (i.e. the interaction term was nonsignifi- cant). However, there were significant differences among clones in susceptibility to the pathogen, and this was influenced by temperature. Furthermore, the clones differed in how temperature influenced susceptibility, with susceptibility rankings changing with temperature. Two clones showed changes in susceptibility which mirrored changes in the in vitro vegetative growth rate of E. neoaphidis at different temperatures, whereas two other clones differed considerably from this expected response. Such interactions between genotype and temperature may help maintain heritable variation in aphid susceptibility to fungal pathogen attack and have implications for our understanding of disease dynamics in natural populations. This study also highlights the difficulties of drawing conclusions about the efficacy of a biological control agent when only a restricted range of pest genotypes or environmental conditions are considered. Key words. Acyrthosiphon pisum, biological control, Erynia neoaphidis, genotype by environment interaction, resistance, temperature. Introduction All organisms face attack by natural enemies. Given the enormous fitness costs of susceptibility to attack, it is not surprising that an incredibly diverse range of defences has evolved. However, when considering populations within species, there is generally heritable variation in defence traits present, whereas the a priori expectation may be that natural selection would lead to the fixation of beneficial traits. There are a number of reasons why heritable vari- ation may be maintained in such circumstances, including negative genetic correlations and genotype-by-environment interactions (Stearns, 1992). Work on pea aphids [Acyrthosiphon pisum (Harris); Hemiptera: Aphididae] has shown high levels of heritable variation in resistance to parasitoid wasps, coccinellid predators and fungal pathogens (Milner, 1982; Henter & Via, 1995; Losey & Denno, 1998; Hufbauer & Via, 1999; Ferrari etal., 2001; Hufbauer, 2002; Stacey & Fellowes, 2002). Much effort has focused on life-history trade-offs as a mechanism constraining the evolution of resistance in Drosophila-parasitoid (Fellowes & Godfray, 2000) and some other systems (Sayyed & Wright, 2001). However, similar trade-offs have not been found in pea aphid–parasitoid and Correspondence: M. D. E. Fellowes, School of Animal and Microbial Sciences, University of Reading, PO Box 228, Whiteknights, Reading, Berkshire RG6 6AJ, UK. Tel: þ44 (0)118 987 5123; fax: þ44 (0)118 931 0180; e-mail: m.fellowes@reading.ac.uk Physiological Entomology (2003) 28, 75–81 # 2003 The Royal Entomological Society 75