Phenological variation in the composition of a temperate forest leaf tie community.

Arthropod communities in an array of temperate ecosystems follow similar phenological patterns of distinct compositional turnovers during the course of a season. The arthropod community inhabiting leaf ties is no exception. Many caterpillars build leaf ties, shelters between overlapping leaves attached together with silk, which are colonized secondarily by a variety of arthropods. We created experimental leaf ties by clipping overlapping leaves together with metal clips. We censused the arthropod community within experimental ties on two host plants, American beech (Fagus grandifolia Ehrhart), and white oak (Quercus alba L.), weekly for 10 wk during the summer of 2009. Diversity measures for leaf-tying caterpillars and the entire arthropod community within ties varied little between tree species and sampling periods, but caterpillar and arthropod density per tie was significantly higher on white oak than beech and abundance increased on both tree species as the season progressed. The composition (i.e., species presence and abundance) of the leaf-tying caterpillar community and the arthropod community as a whole differed between host-tree species and sampling periods. Although the arthropod communities on American beech and white oak differed, they showed similar patterns of compositional turnover, with distinct communities in early and late summer and a transitional community midsummer.

Why do leaf-tying caterpillars abandon their leaf ties?

Leaf-tying caterpillars act as ecosystem engineers by building shelters between overlapping leaves, which are inhabited by other arthropods. Leaf-tiers have been observed to leave their ties and create new shelters (and thus additional microhabitats), but the ecological factors affecting shelter fidelity are poorly known. For this study, we explored the effects of resource limitation and occupant density on shelter fidelity and assessed the consequences of shelter abandonment. We first quantified the area of leaf material required for a caterpillar to fully develop for two of the most common leaf-tiers that feed on white oak, Quercus alba. On average, Psilocorsis spp. caterpillars consumed 21.65 ± 0.67 cm(2) leaf material to complete development. We also measured the area of natural leaf ties found in a Maryland forest, to determine the distribution of resources available to caterpillars in situ. Of 158 natural leaf ties examined, 47% were too small to sustain an average Psilocorsis spp. caterpillar for the entirety of its development. We also manipulated caterpillar densities within experimental ties on potted trees to determine the effects of cohabitants on the likelihood of a caterpillar to leave its tie. We placed 1, 2, or 4 caterpillars in ties of a standard size and monitored the caterpillars twice daily to track their movement. In ties with more than one occupant, caterpillars showed a significantly greater propensity to leave their tie, and left sooner and at a faster rate than those in ties as single occupants. To understand the consequences of leaf tie abandonment, we observed caterpillars searching a tree for a site to build a shelter in the field. This is a risky behavior, as 17% of the caterpillars observed died while searching for a shelter site. Caterpillars that successfully built a shelter traveled 110 ± 20 cm and took 28 ± 7 min to find a suitable site to build a shelter. In conclusion, leaf-tying caterpillars must frequently abandon their leaf tie due to food limitation and interactions with other caterpillars, but this is a costly behavior.