Clonal variation and covariation in aphid resistance to parasitoids and a pathogen

The potential rate of evolution of resistance to natural enemies depends on the genetic variation present in the population and any trade-offs between resistance and other components of fitness. We measured clonal variation a nd covariation in pea aphids (Acyrthosiphon pisum) for resistance to two pa rasitoid species (Aphidius ervi and A. eadyi) and a fungal pathogen (Erynia neoaphidis). We found significant clonal variation in resistance to all th ree natural enemies. We tested the hypothesis that there might be trade-off s (negative covariation) in defensive ability against different natural ene mies, but found no evidence for this. All correlations in defensive ability were positive, that between the two parasitoid species significantly so. D efensive ability was not correlated with fecundity. A number of aphid clone s were completely resistant to one parasitoid (A. eadyi), but a subset of t hese failed to reproduce subsequently. We discuss the factors that might ma intain clonal variation in natural enemy resistance.