The strong impact of food quality on host life history does not scale up to impact parasitoid efficacy

Parasitoids survive and reproduce by feeding on host insects. Idiobiont parasitoids paralyze their hosts upon attack, typically targeting only hosts large enough to support offspring development. Variation in host quality across stages of host development are known to impact parasitoid efficacy. A previous study shows food quality effects on host vital rates, growth, and development are not consistent throughout ontogeny, suggesting host food quality may cascade to impact parasitoid efficacy and life-histories. However, we do not yet know the importance of developmental variation within and across host instar stages on parasitoid efficacy and life-history.

The goal of this study was to ascertain whether trait variation within host instars is more important for parasitoid efficacy and life-histories than trait variation across host instars. Using a cross-sectional experimental approach, we exposed 5-60-day-old seed beetle hosts raised on 90%, 95%, and 100% black-eye pea artificial seeds to mated female parasitoids and measured the number of hosts parasitized and various parasitoid life-history traits.

Our results generally show the natural variation in host life-histories throughout ontogeny had a larger impact on parasitoid efficacy and parasitoid life-histories than any variation within host instar stages in response to host food quality. Specifically, parasitism rates among host instar stages differed by 10-40% across a food quality gradient, while parasitism rates within host instar stages differed by only 1-4%. Similarly, parasitoid hind tibia length varied up to 45% across host instar stages but varied less than 1% within host instar stages across a food quality gradient.