Trade-offs in host choice by an herbivorous insect based on parasitism and host plant quality

Background/Question/Methods

Natural enemies and host plant quality have both been invoked to explain the evolution of host choice in herbivorous insects. In this study, we quantified both selection pressures for a polyphagous moth species, fall webworm (Hyphantria cunea Drury). The gregarious larvae spin extensive webs for protection and thermoregulation. Fall webworm have been recorded on dozens of plant families worldwide. In Colorado, we have observed and collected the larvae from ~17 host species. As part of an observational study, in 2010 and 2011 we collected wild larvae from multiple field sites in Colorado, returned them to the lab to finish their development and reared out any parasitoids in order to quantify host plant quality and parasitism pressure for each host plant species.  In 2011, we selected four host plants of high and low host plant quality, based on results from our observational study in 2010 for a manipulative split-brood experiment in the lab. Our two high-quality host plants were narrowleaf cottonwood (Populus angustifolia) and chokecherry (Prunus virginiana) and the two low-quality hosts were alder (Alnus tenuifolia) and crabapple (Malus sp.). We recorded development time, pupal weight, and feeding efficiency as measures of larval fitness on the different host plants.

Results/Conclusions

In our observational field experiment, parasitism rate differed significantly among host plants and was consistent between years. For the four host plants used in the manipulative split-brood experiment, parasitism was highest on narrowleaf cottonwood and chokecherry and lowest on alder and crabapple.  As a measure of herbivore performance, pupal weights were higher on narrowleaf cottonwood and chokecherry than on alder and crabapple, indicating that narrowleaf cottonwood and chokecherry are superior hosts when enemies are absent.  Similarly, development time was shorter on narrowleaf cottonwood and chokecherry compared to alder and crabapple. The positive relationship between mortality from parasitism and host plant quality suggests a fitness trade-off in which low-quality host plants may provide a refuge from natural enemies.