Abstract
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 cm2 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.
Highlights
Leaf-tying caterpillars are essential components of forest ecosystems due to their role as physical ecosystem engineers—organisms that create or modify their habitat and in turn affect the community (Jones, Lawton & Shachak, 1997)
This study explored the role that food limitation, competitive interactions, and risks of leaving a leaf tie play in shelter abandonment through a series of experiments: (1) We determined the amount of food resources consumed by the most common leaf-tying caterpillars on white oak to complete development; (2) we compared these resource requirements to the size of naturally occurring leaf ties to determine if food limitation is common; (3) we manipulated caterpillar densities within experimental leaf ties to determine the effects of interactions between caterpillars on their propensity to abandon their ties; and (4) we observed caterpillars searching for a site to build a leaf tie and documented their behaviors
When controlling for sex and development time, the average amount of leaf material consumed did not significantly differ between caterpillar species (F1,44 = 0.28, p = 0.5945), though P. cryptolechiella ate an average of 8.5% more leaf material than P. quercicella (Fig. 2A)
Summary
Leaf-tying caterpillars are essential components of forest ecosystems due to their role as physical ecosystem engineers—organisms that create or modify their habitat and in turn affect the community (Jones, Lawton & Shachak, 1997). Many shelter-building caterpillars have been observed to physically compete over their shelters by using territorial vibratory signals or physically aggressive behavior, at times pushing one another to either gain access to or defend leaf shelters (Berenbaum, Green & Zangerl, 1993; Yack, Smith & Weatherhead, 2001; Scott et al, 2010; E Sigmon, unpublished data) This suggests that competition over existing shelters is common and that shelters, or sites to build shelters, are a limiting resource. Previous studies have shown that new, unoccupied ties and damaged, occupied ties are attractive to colonizing herbivores (Lill, 2004; Lill & Marquis, 2004) This is despite the fact that caterpillars that are reared in groups within a single shelter achieve a lower pupal mass than those reared individually (Lill et al, 2007). Leaf-tiers may be further limited by available sites for creating leaf ties (Marquis & Lill, 2010), forcing them to colonize occupied leaf ties if there are no suitable sites for building new ties; this is especially likely later in the season when most suitable sites have been used by previous generations of herbivores
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