It’s Raining Lappet Moths

The penultimate insect ecology excursion during May session at Au Sable was to Deadman’s Hill by the Jordan River in Elmira, Michigan. Together with Dr, Hoekman and my fellow students I walked a 3.1-mile trail through upland forest and riparian habitat. Early on we encountered a groundwater spring and took the opportunity to collect insects. There was an abundance of Equisetum, Clintonia borealis, Polygala paucifolia and young maple seedlings in the vicinity. After a few minutes of exploration, we headed uphill and into the forest. It was noticeably young, which was not surprising given the logging history of the land. Maple, aspen, and beech trees dominated, with the occasional shagbark hickory making an appearance. Throughout the hike we had noticed what appeared to be the larvae of some species in the family Lasiocampidae, which are commonly called “lappet moths” or “tent caterpillars”. These particular caterpillars however, did not seem to have built any tents, which struck us as odd. As we continued along the trail, their numbers increased dramatically.

Some follow-up research revealed that these insects were Forest Tent Caterpillars (Malacosoma disstria). Apparently, this divergent behavior is diagnostic of the species; unlike their close relatives Malacosoma americanum and Malacosoma pluviale, M. disstria use silk sparingly, and so are not often found encased in tents with conspecifics (Sullivan & Wellington, 2012). A side-effect of this, one I can attest to, is that the caterpillars tend to lose their grip easily and only a thin strand of silk catches their fall (Fitzgerald, 2008). At least three individuals fell on me and my field log, and Dr. Hoekman commented that he had removed some that had landed on his back and shoulders. 

An aggregation of M. disstria on a twig in the understory. Photo taken by author.

Population fluxes of M. disstria have been monitored, recorded, and investigated for decades because it is one of most notable insect in North America to experience periodic outbreaks naturally (Schmidt, 2001). In Michigan, these population explosions typically occur every seven to twenty years and last for two to three years (Elsner, 2010; Fouch & Duke, 2011). In 2010 a writer for the Michigan State University Extension stated that “pockets of heavy infestation and defoliation have been reported from Grand Traverse, Leelanau, Benzie, Kalkaska and Crawford counties” and it had been “over 20 years since the last significant outbreak of forest tent caterpillar in the Grand Traverse Bay region” at that time (Elsner, 2010). Whether or not what I observed at Deadman’s Hill was a significant outbreak is uncertain, but what I do know is that M. disstria were on the ground, shrubbery, trees, and on me.

While this species may seem to be a pest and nuisance, both to the forest and humans, it plays an important role in hardwood forest ecosystems. At high population densities forest tent caterpillars can cause the death of trees by defoliation (Evenden, Mori, Sjostrom, & Roland, 2015). Loss of canopy cover or entire trees can create light gaps which foster growth of seedlings, shrubs, and forbs on the forest floor, and the death and decomposition of older or weaker trees contributes to the cycle of nutrients (Rozendaal & Kobe, 2016). Not to mention, the caterpillars are a convenient food source for many other organisms. I even witnessed a few normally nocturnal ground beetles taking advantage of the abundance of such easy prey.

An individual M. disstria ascending the trunk of a tree toward canopy foliage. Photo taken by the author. 

As previously mentioned, the frass and silk of M. disstria caterpillars is problematic for people living in areas where this species occurs, especially during peak years. Some target tree species are also economically important (Schmidt, 2001). In some instances, therefore, various methods are used to control the caterpillars or adult moths including B.t. pesticides, insecticidal soaps, and pheromone-baited traps (Fouch & Duke, 2011; Evenden, Mori, Sjostrom, & Roland, 2015; Schmidt, 2001). Contrary to popular belief, the caterpillars do not cause tremendous long-term damage to the forests because if trees are healthy they are capable of replacing the foliage which was lost after M. disstria conclude their larval stage in midsummer (Fouch & Duke, 2011). Observing this phenomenon was a unique experience, and learning about the ecological impact of M. disstria, both positive and negative, was illuminating.

Elsner, D. (2010). Forest tent caterpillar in northern Lower Peninsula. Retrieved from http://msue.anr.msu.edu/news/forest_tent_caterpillar_in_northern_lower_peninsula

Evenden, M. L., Mori, B. a., Sjostrom, K. D., & Roland, J. (2015). Forest tent caterpillar, Malacosoma disstria (Lepidoptera: Lasiocampidae), mate-finding behavior is greatest at intermediate population densities: implications for interpretation of moth capture in pheromone-baited traps. Frontiers in Ecology and Evolution, 3(July), 78. http://doi.org/10.3389/fevo.2015.00078

Fitzgerald, T. D. (2008). Use of pheromone mimic to cause the disintegration and collapse of colonies of tent caterpillars (Malacosoma spp.). Journal of Applied Entomology, 132, 451-460. doi:10.1111/j.1439-0418.2008.01286.x

Fouch, S., & Duke, E. (2011). Forest tent caterpillars can cause significant defoliation on landscape trees. Retrieved from http://msue.anr.msu.edu/news/forest_tent_caterpillars_can_cause_significant_defoliation_on_landscape_tre

Schmidt, B. C. (2001). Ecology of low-density forest tent caterpillar (Malacosoma disstria) populations: Pheromone monitoring methodology and the role of parasitoid alternative hosts (Order No. MQ69588). Available from ProQuest Dissertations & Theses Global. (304739640). Retrieved from https://login.proxy.olivet.edu/login?url=https://search-proquest-com.proxy.olivet.edu/docview/304739640?accountid=12974

Sullivan, C., & Wellington, W. (1953). The Light Reactions of Larvae of the Tent Caterpillars, Malacosoma disstria Hbn., M. americanum (Fab.), and M. pluviale (Dyar). (Lepidoptera: Lasiocampidae). The Canadian Entomologist,85(8), 297-310. doi:10.4039/Ent85297-8

Rozendaal, D. M. A., & Kobe, R. K. (2016). A forest tent caterpillar outbreak increased resource levels and seedling growth in a northern hardwood forest. PLoS ONE, 11(11), 1–15. http://doi.org/10.1371/journal.pone.0167139