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Tent Caterpillars are for the Birds

June 28, 2011
By Kay Charter, Saving Birds Through Habitat & Eric Ellis, CRA Biologist, Spring 2011 Catalyst

It’s amazing how many aspects of the natural world are unknown, even to the experts. A case in point is the answer to the question, “Do birds eat tent caterpillars?” Some of us have either heard or read opinions by both bird and bug people that nothing eats these caterpillars. Upon deeper investigation it is apparent that tent caterpillars, as annoying as they can be, play an important role in the Northern Michigan environment for birds and other wildlife.


There are two species of springtime tent caterpillars in northern Michigan: eastern tent caterpillars (Malacosoma americanum) and forest tent caterpillars (Malacosoma disstria), ETC and FTC for short. These insects are native to the area and have coexisted with our forests for millennia. Populations of both species fluctuate with ETC fairly common every year and FTC displaying boom and bust outbreaks that last 2-3 years every decade or so. Eastern tent caterpillars build the unsightly silken “tents” in late spring, often on wild black cherry trees. Forest tent caterpillars do not have highly visible tents but when there is an outbreak up to 4 million of them can cram into a single forested acre. Both species have just one generation per year leaving time for affected trees to grow new leaves for the season. The vast majority of defoliated trees will recover with little lasting effects other than a year or two of less growth.

The sight of these caterpillars denuding trees and crawling everywhere (even into buildings) during large outbreaks, cause both of these fuzzy insects to be thought of as pests. It is hard to argue that they do not have negative impacts. That said, the negative impacts are mainly the nuisance they cause around dwellings. The ecological impacts of these insects are surprisingly positive.

As with many other Lepidoptera species, these caterpillars are frequently parasitized or eaten by a huge number of species as are the adult moths. During an outbreak years ago Kay was on a bird hike with well-known birder Tom Ford when someone asked whether any birds will eat these pesky insects. Tom answered that severely declining populations of black-billed and yellow-billed cuckoos in particular are crazy about them. According to research conducted at the Minnesota Agricultural Experiment Station 60 birds species have been documented to eat tent caterpillars including cuckoos, orioles, jays, chickadees and nuthatches. This same study also found that tent caterpillar outbreaks are controlled by native predators and parasites including 127 insect parasites, 28 insect predators, frogs, mice, bats, reptiles, squirrels, skunks, and bears. By sifting through one day’s worth of bear poop (fun job) researchers found that a single bear on average ate around 25,000 caterpillars in a one day. What people find unsightly and annoying for a few months some years is a welcome gift of easy food for many species of wildlife and the species that in turn feed on them.

For years, Kay and her husband have put mealworms out during the nesting season. They have purchased as many as 25,000 mealworms as supplemental food for nesting birds to feed their young. During the 2009 – 2010 widespread outbreaks of both species of tent caterpillars there were no takers for their offerings. Those two years were, moreover, periods of great nesting success, especially for the Neotropical species.

It is important to note that many methods used to control tent caterpillar populations kill indiscriminately and reduce the food supply for many wildlife species, especially nesting birds. Chemical controls are costly, kill non target species (Monarch Butterflies, native pollinators, etc.), can potentially degrade water quality and kill some natural predators/parasites of tent caterpillars potentially extending the outbreak.

Although most people we know are frustrated, irritated or concerned over the appearance of tent caterpillars, we welcome their infrequent arrival. It will be a good thing for the birds and other wildlife we are trying to help.


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