Fall webworm (Hyphantria cunea) is a generalist moth species whose larvae have been found to feed on >100 plant species. In Colorado, fall webworm larvae feed on ~17 different host plants of varying nutritional quality. Studies on other insects have shown that some species can compensate for feeding on poor-quality hosts early in their development by switching to a higher-quality host later in life. We tested whether fall webworm larvae are capable of compensatory growth by rearing them on hosts that varied in quality and by switching larvae from low-quality to higher-quality hosts midway through development. We used chokecherry (Prunus virginiana) as a high-quality host, narrowleaf cottonwood (Populus angustifolia) and alder (Alnus tenuifolia) as medium-quality hosts, and crabapple (Malus sp.) as a low-quality host. Using a split-brood design, we reared larvae either entirely on one of the four host plants or switched them part way through their development from alder to chokecherry or narrowleaf cottonwood or from crab apple to chokecherry or narrowleaf cottonwood. We recorded survival, development time and pupal weight as measures of larval fitness to determine whether larvae are capable of compensatory growth.
Results/Conclusions
We found significant differences in survival among hosts. Larvae survived most often on chokecherry, followed by narrowleaf cottonwood and alder, and least often on crabapple. Pupal weight, a predictor of lifetime fitness for Lepidoptera, was greatest on chokecherry. We found that larvae switched from crabapple to chokecherry compensated for their previous poor host plant and achieved a final pupal weight that did not differ from larvae reared entirely on chokecherry. In contrast, larvae that switched from alder to narrowleaf cottonwood had pupal weights significantly lower than those of larvae reared entirely on either host plant. Short development time confers higher fitness for larvae in the wild because it minimizes exposure to predators; we found that larvae reared on chokecherry had the shortest development time. Larvae switched from alder and crabapple to chokecherry did not differ in development time from larvae reared entirely on chokecherry, indicating that they were able to compensate for feeding on their previous poor-quality host plant. Our results suggest that compensatory growth is possible for fall webworm, but depends greatly on the host plants involved in the switch.