1.The pea aphid, Acyrthosiphon pisum, maintains extreme variation in resistance to its most common parasitoid wasp enemy, Aphidius ervi, which is sourced from two known mechanisms: protective bacterial symbionts, most commonly Hamiltonella defensa, or endogenously encoded defenses. We have recently found that individual aphids may employ each defense individually, occasionally both defenses together, or neither. 2.In field populations, Hamiltonella-infected aphids are found at low to moderate frequencies and while less is known about the frequency of resistant genotypes, they show up less often than susceptible genotypes in field collections. To better understand these patterns, we sought to compare the strengths and costs of both types of defense, individually and together, in order to elucidate the selective pressures that maintain multi-modal defense mechanisms or that may favor one over the other. 3.We experimentally infected five aphid genotypes (two lowly and three highly resistant), each with two symbiont strains, Hamiltonella-APSE8 (moderate protection) and Hamiltonella-APSE3 (high protection). This resulted in three sublines per genotype: uninfected, +APSE8, and +APSE3. Each of the fifteen total sublines was first subjected to a parasitism assay to determine its resistance phenotype and in a second experiment a subset were chosen to compare fitness (fecundity and survivorship) in presence and absence of parasitism. 4.In susceptible aphid genotypes, parasitized sublines infected with Hamiltonella generally showed increased protection with direct fitness benefits, but clear infection costs to fitness in the absence of parasitism. In resistant genotypes, Hamiltonella infection rarely conferred additional protection, often further reduced fecundity and survivorship when enemy challenged, and resulted in constitutive fitness costs in the absence of parasitism. We also identified strong aphid-genotype X symbiont-strain interactions, such that the best defensive strategy against parasitoids varied for each aphid genotype; one performed best with no protective symbionts, the others with particular strains of Hamiltonella. 5.This surprising variability in outcomes helps explain why Hamiltonella infection frequencies are often intermediate and do not strongly track parasitism frequencies in field populations. We also find that variation in endogenous traits, such as resistance, among host genotypes may offer redundancy and generally limit the invasion potential of mutualistic microbes in insects.

Parasitism and Fitness Assay DataTab 1 "Parasitism Assays": columns show variables that were recorded or measured and rows represent a single replicate of 20 parasitized aphids. Survival, mummification, and dual mortality were recorded 10 days after initial parasitism. Tab 2 "Fecundity and Survivorship": Fecundity and survivorship data for pea aphids that were recorded at 3-day-intervals for 24 total days. Tab 3 "Sublethal effects": average cumulative fecundity of only surviving aphids from tab 2.