Insect herbivores often experience variability in the quality of their diet throughout development, such as through changes in induced defenses or switching host plants. If herbivores do not respond linearly to diet quality, then the magnitude of variation in diet will likely be important for fitness which can also affect parasitoid wasps developing within the host during feeding. We examined the effects of diet variation in plant defenses on a suite of fitness correlates of the cabbage looper Trichoplusia ni and its parasitoid wasp Copidosoma floridanum. Newly hatched, parasitized caterpillars were placed into treatments of either low (0.5 and 1.5 ng xanthotoxin/mg diet), high (0 and 2.0 ng/mg), or no variability (1.0 and 1.0 ng/mg) by manipulating the magnitude but not the mean of xanthotoxin (a plant defensive compound) between two diets. Half of the larvae were started on the low xanthotoxin diet first and the other half were started on the high diet. Larvae were switched between xanthotoxin diets every three days. A baseline growth curve for the wasps was established by placing parasitized caterpillars on each of different diets (0, 0.5, 1.0, 1.5, and 2.0 ng/mg) for the entirety of development without switching.
Results/Conclusions
The stress of parasitism combined with xanthotoxin in the diet caused high mortality of host caterpillars compared with previous studies involving unparasitized caterpillars. Mortality did not differ between variability treatments but was significantly higher on caterpillars experiencing high (1.5 or 2.0 ng/mg) concentrations of xanthotoxin during development in the growth curve replicates. Caterpillars in the low variability treatments were larger than those experiencing no variability but only if they started on the lower diet first. Larvae were generally larger with lower concentration of xanthotoxin in the starting diet for both high and low variability treatments compared to no variability. Interestingly, the likelihood of death was also higher with increasing host mass. Day six mass was also a strong indicator of wasp development time with larger mass leading to shorter development time. Larvae starting on the higher of the two diets were similar size to larvae experiencing no variability. Overall, hosts, and therefore parasitoids, were more greatly affected by the starting diet concentration than by the magnitude of variation between diets. This suggests an advantage for female parasitoids that select hosts feeding on plants with initially low defenses, even if those defenses are inducible over high constitutive defenses.