Post by Mayda Nathan
For shy, cryptic animals that are – at their biggest –an inch long, leaf-tying caterpillars play an outsize role in the tree canopies where they live. Dr. Elisha Sigmon’s recent seminar highlighted the importance of the leaf and silk structures these caterpillars make, both for the caterpillars themselves and for hundreds of other arthropod species that live in forest trees in the Mid-Atlantic region.
By fastening together overlapping leaves with silk, caterpillars create a shelter that protects them from predators and harsh climates. Dr. Sigmon’s work has shown that opportunistic insects and spiders quickly find these shelters and move in – even if the original caterpillar is still an occupant. On trees with leaf ties, arthropod abundance and richness can be 3 to 10 times higher than on trees without leaf ties.
Just how much leaf ties boost arthropod abundance depends on the host tree species and on the time of year. When she created artificial leaf ties on white oak and beech trees, Dr. Sigmon found that although arthropod diversity within the leaf ties was similar across tree species, arthropod abundance was much higher on white oaks. As the experiment continued into late summer, insects and spiders continued to colonize the leaf ties, boosting arthropod abundance further.
Other animals don’t randomly colonize leaf ties, however. After creating artificial leaf ties with and without resident caterpillars, Dr. Sigmon concluded that the composition of arthropods found in the leaf ties depended on the presence of a caterpillar. Specifically, the relative number of herbivores and predators was larger in leaf ties with caterpillars than in those without. What’s more, when she introduced “intruder” caterpillars to leaf ties that already contained a resident, Dr. Sigmon observed caterpillar battles – shoving, hitting, and drumming on the leaf surface.
These miniature bouts are a rare documentation of direct interference competition between herbivorous insects, and they point to the leaf ties as an important resource for caterpillars and other arboreal arthropods in Mid-Atlantic forests. Beavers and corals are usually what come to mind when we consider the term “ecosystem engineer”, but the tiny leaf-tying caterpillars may well deserve that label, too.
A link to Dr. Sigmon's most recent paper, via BioOne:
Mayda Nathan is a PhD student studying the consequences of species range shifts on plant-insect interactions. She currently works with insect pollinators of the black mangrove, which is expanding its range in northern Florida.