TRAVERSE CITY — Where are the Tent Caterpillars?
Generally speaking, people don’t become inquisitive about a lack of caterpillars attacking their trees, so I’m not surprised that no one has asked me where the Tent Caterpillars are this year.
However, I’m incredibly interested in why this is so. This is actually the third year in a row that the populations of the Eastern Tent Caterpillar (Malacosoma americanum, ETC for short) have been remarkably low in northern Michigan. Area residents may recall the recent outbreak of Forest Tent Caterpillar (FTC), a close relative, which denuded thousands of acres of forest hardwoods in 2009 and 2010.
The FTC typically drops to undetectable levels for several years after a big outbreak subsides. The ETC goes through population swings as well, but the highs are not as spectacular and the lows are usually still quite noticeable, especially due to the very obvious webs the caterpillars construct in small trees and shrubs.
Natural enemies and weather-related impacts are likely to be the reasons for the extremely low numbers of ETC in recent years. A number of different parasitoid insects and disease organisms cause significant mortality of ETC in the caterpillar stage. These biological agents are typically able to regulate the ETC population at low to moderate levels for many years in a row. Most ETC are killed each year, but enough survive to perpetuate the species, and the continuing presence of ETC allows the natural enemies to maintain their population. If the natural enemies were too efficient at killing ETC, they could be the cause of their own demise. The populations of ETC and its natural enemies tend to come to an equilibrium point, both survive, and we see some tents each spring.
Weather impacts can disrupt the equilibrium by either favoring or harming the ETC or its natural enemies. ETC is a very early spring insect, with egg hatch occurring as early as April in some years. The young caterpillars are small and frail, so there can be significant mortality from spring frosts, either by the direct killing of the caterpillars or from starvation if the cold weather kills or delays the growth of leaves on their host plants.
I asked four other Michigan State University staff who have observed ETC in the state for many years for their opinions on the probable cause of the very low populations of ETC, and the consensus is that weather conditions in 2012 and 2013 were extremely hard on the young caterpillars. In 2012 the warm weather in March brought on egg development and hatch way ahead of the normal pace, putting the caterpillars into great peril when freezing temperatures returned in April. Survivors had to deal with a shortage of food because the host plant leaves were also killed by frosts. This year the very cool and wet weather trends delayed the development of leaves on host trees, likely causing significant starvation of caterpillars.
Now, back to the delicate balance of the ETC and its natural enemies. If ETC populations are extremely low, it may lead to a crash of the populations of its natural enemies.
That could lead to an upswing in ETC in coming years until the equilibrium can be reestablished.
I’ll be watching in 2014!