Ecological consequences of shelter sharing by leaf-tying caterpillars

A wide variety of insect herbivores construct and inhabit leaf shelters (ties, rolls, folds, and webs). Shelter construction can lead to a high rate of secondary occupation by other arthropods, including other species of constructors. The consequences for the inhabitants of secondarily occupying these shelters are currently unknown. In this study, we conducted field experiments to examine the fitness consequences (survival and attack by natural enemies) for caterpillars that (i) occupy a shelter with conspecifics vs. occur singly; and (ii) establish a new shelter vs. colonize a pre-existing one. In addition, we conducted factorial laboratory experiments to test the hypothesis that caterpillars sharing shelters with conspecifics might have reduced construction costs (a potential benefit of shelter-sharing or secondary occupation). Larvae of Psilocorsis quercicella Clemens (Lepidoptera: Oecophoridae) placed in white oak [ Quercus alba L. (Fagaceae)] leaf ties alone or in groups of three had equal likelihood of survival from natural enemies. This same caterpillar species, however, had a higher disappearance rate when placed in pre-existing leaf ties than when placed in newly formed ones, suggesting a potential cost of secondary colonization. A similar experiment with a closely related species [ Psilocorsis cryptolechiella (Chambers)], however, failed to detect a cost of secondarily occupying shelters made on beech, Fagus grandifolia Ehrh. (Fagaceae). In the laboratory experiment, we found no evidence of shelter-sharing benefits; rather larvae reared in shelters in groups of three had lower pupal mass (and thus lower potential fecundity) than larvae reared singly, suggesting a cost of shelter sharing. Moreover, groups of larvae forced to repeatedly construct new shelters tended to have reduced survival relative to the other treatment, suggesting that energetic constraints are more likely to reduce fitness when larvae cohabit shelters. Taken together, these results indicate that the common phenomenon of shelter sharing by leaf-tying caterpillars has either neutral or negative effects for the occupants. The fact that these leaf-tying caterpillars actually share shelters may simply reflect limited availability of oviposition sites.