Parasitoids survive by feeding on host insects. Idiobiont parasitoids paralyze their hosts and thus are expected to target only hosts that are large enough to support their offspring's development. There can be marked variation in host quality and a long-standing challenge is to understand how changes in host food resources impact host quality, and in turn influence parasitism rates and parasitoid fitness. We know food quality generally impacts hosts' size, juvenile development, and life span. However, because traditional methods typically group all instars as a single stage, we do not know whether food quality effects on host growth and development differ across instars. As a result, there are few data enabling us to predict the impacts of food quality on hosts' life histories that may, in turn, affect their susceptibility to parasitism. We test whether food quality effects on host life history are equal throughout ontogeny to ascertain whether trait variation across instars should be considered when characterizing food quality effects on host susceptibility to parasitism and host-parasitoid interactions. We combine a cross-sectional experimental approach with a stage-structured population model to estimate instar-specific vital rates (e.g. development and mortality) in the host, Callosobruchus maculatus across a food quality gradient.
Results/Conclusions
Vital rates differ across food quality treatments within each instar; however, their effect sizes differ with instar. Hosts consuming low quality food spent 24%, 19% and 6% more time, and were 33%, 53% and 63% smaller than hosts consuming high quality food in the second, third and fourth instars, respectively. Overall, our results show that consuming poor food quality not only means slower growth for C. maculatus, but also a longer window of exposure to parasitoids. If hosts feeding on poor quality food are more vulnerable to their parasitoids, then the size of the host community will likely be impacted. Alternatively, fourth instar hosts feeding on poor food quality may not be large enough to support parasitoid offspring growth and development. Since host size is an important predictor of parasitoid success and fitness, smaller hosts exposed for longer periods may not be as susceptible to parasitoids as larger hosts exposed for shorter periods. The net effect of the response of host life history traits on parasitoids remains to be seen, but by conducting detailed instar-specific studies it is possible to layout the potential processes for how host resources scale through this type of food chain.