Why do leaf-tying caterpillars abandon their leaf ties?

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.

High densities of leaf‐tiers in open habitats are explained by host plant architecture

“…Importantly, caterpillars abandon their shelters long before the resource is exhausted: the damage per shelter that was left by a caterpillar was on average 21% of the area of leaves from which it was composed. This result is in line with findings by Sliwinski and Sigmon (2013) who observed similar behaviour in two other species of leaf-tying caterpillars and concluded that the abandonment of shelters by solitary caterpillars can be driven by factors other than a lack of food. Plant defences are known to force herbivores to leave their feeding sites, leading to dispersion of damage within a plant (Edwards & Wratten, 1983).…”

“…In line with this hypothesis, we suggest that abandonment of shelters by leaf‐tiers well before the resources are exhausted is an adaptation that decreases the risk of parasitism. However, long trips in search of a site suitable for shelter construction, which may take as long as an half an hour (Sliwinski & Sigmon, , and our observations), are associated with an increased risk of mortality from visually oriented predators (Bergelson & Lawton, ; Loader & Damman, ) and other risks. For example, dropping from a plant during travel caused 17% mortality in two Psilocorsis species (Sliwinski & Sigmon, ).…”

“…The number of new shelters built during the 10 days preceding pupation could be as high as three. Several other species of leaf‐tying moths also build multiple shelters during their lives (Waldstein et al , ; Ide, ; Sliwinski & Sigmon, ), and the construction of a new shelter (at realistic numbers during a caterpillar lifespan) was found to have no physiological costs (Weiss, ). Importantly, caterpillars abandon their shelters long before the resource is exhausted: the damage per shelter that was left by a caterpillar was on average 21% of the area of leaves from which it was composed.…”

Between predators and parasitoids: Complex interactions among shelter traits, predation and parasitism in a shelter‐building caterpillar community

“…2), whose continued presence often had to be assessed using feeding damage or changes to the shelter structure, rather than direct observation of the caterpillar. The disappearance of some caterpillars may also be more complicated than simple predation, as caterpillars can fall from their plants while seeking new shelter sites (Loeffler, 1996; Sliwinski & Sigmon, 2013). Although these caterpillars are not removed from the plant by predators, they can be killed by terrestrial predators before they can return to the plant (Sliwinski & Sigmon, 2013).…”

“…The disappearance of some caterpillars may also be more complicated than simple predation, as caterpillars can fall from their plants while seeking new shelter sites (Loeffler, 1996; Sliwinski & Sigmon, 2013). Although these caterpillars are not removed from the plant by predators, they can be killed by terrestrial predators before they can return to the plant (Sliwinski & Sigmon, 2013). Despite these complications, shelter‐building caterpillars offer an excellent opportunity to directly measure predation in the field.…”