What Is That … Tent Caterpillar?

Waddells

We were enjoying the nice weather and being at the cottage late in May when we noticed we were asking ourselves a lot more wildlife questions than usual, questions that turned out to be related.  Some of our questions were … what is devouring the leaves on the deciduous shrubs and trees around the cottage; what is causing all the small granules and leaf bits to fall out of the trees onto our cottage deck and vehicles; what is rappelling en masse on threads of silk out of our maple and oak trees; and, what is being squashed on the cottage road to such an extent the road has turned greenish-brown.  Based on our photographs and research we discovered the short answer to these questions was tent caterpillars.  The long answer, however, is a bit more complicated since we have two species of tent caterpillar at White Lake and throughout this part of Ontario, the eastern tent caterpillar and the forest tent caterpillar.  It turns out that this year’s devastation around our cottage was the result of an outbreak of forest tent caterpillars.

Most of us are all familiar with the silky tent-like webs eastern tent caterpillars spin in the crotch of two or more branches of deciduous shrubs and trees.

Eastern tent caterpillars, which grow to a length of two to three centimetres, spin these communal tents for protection from natural enemies and to keep warm on colder days.  The caterpillars hatch in early spring and feed voraciously for four to six weeks.  They forage outside the nest, defoliating host and adjacent plants (preferring apple, cherry and hawthorn but also devouring dozens of species of other woody plants) and return to the nests to digest their food and spend the night.  These hairy caterpillars are brownish black with blue spots and brown-yellow lines along their sides.  As this picture shows, a continuous white line runs along the middle of their back, distinguishing them from their close relatives, the forest tent caterpillars.  (Ha ha, yes there is a forest tent caterpillar cuddled in there.)

Once the eastern tent caterpillars mature they cover themselves in yellow-powdery silk cocoons to pupate with the adult moths emerging about ten days later.  Within 24 hours the moths mate and the females lay their eggs in host shrubs and trees ready to hatch the next spring.  Adult eastern tent caterpillars are hairy looking brown-coloured moths that are 1.5 to 2.4 centimetres.

On the other hand, the forest tent caterpillar, while being gregarious and having a life cycle similar to the eastern tent caterpillar, does not spin a communal tent-like web.  Newly-hatched forest tent caterpillars tend to be slightly smaller than their eastern relatives and can be observed in early spring roosting side-by-side on trunks of trees.  Individuals follow silk trails to forage, sometimes rappelling on silk threads from high in trees, preferring to devour basswood, maple, oak, birch and poplar.  Forest tent caterpillars also defoliate many other species of deciduous trees.  These rappelling threads were right beside our cottage.

The hairy forest tent caterpillars are coloured pale blue with black and are often mistaken for the eastern tent caterpillar.  They are most easily distinguished by the white ‘foot prints’ or ‘penguins’ running down the middle of their back as shown in this photograph taken on the railing of our cottage deck.  It was difficult to get a picture of a single caterpillar, yuk!

Mature forest tent caterpillars pupate, spinning less powdery silk cocoons (than the eastern species) in the folded leaves of food plants.  The adults are hairy looking yellowish to rusty-brown moths that are 1.7 to 2.1 centimetres.

Periodic outbreaks of either species occur every 10 to 15 years, depending on environmental and biological factors, and such outbreaks can last for two or more years.  Since they are native to North America, populations are usually controlled, but not during outbreaks, by insect parasites and natural predators including rodents, frogs, and birds.  While most birds find these hairy caterpillars unpalatable, some local birds including the illusive cuckoo and the Baltimore oriole devour the caterpillars and one report documented that blue jays, chickadees and nuthatches eat tent caterpillars as well, including the caterpillar, the pupae and the adult moth.  Yeah to that!  In addition, it appears that some of the numerous caterpillars that fell into the lake were eaten by fish.  This oriole was photographed near the cottage just moments after we saw it devouring a tent caterpillar.  Look how happy it seemed!

The caterpillars are such voracious eating machines that during outbreaks one can observe deciduous trees losing their green leaves in real time, leaving only the veins of the leaves, and hear a constant shower of caterpillar feces and pieces of leaves falling from the sky.  Many of  the masses of caterpillars crossing roads in search of food are squashed by passing vehicles and pedestrians causing surfaces to become green-brown with caterpillar guts.  While this year’s outbreak at White Lake seemed severe to us, on June 10, 1898 the Ottawa Journal reported an outbreak so severe in the Stittsville area that “two freight trains were forced to stop for a short time being unable to proceed due to the slippery conditions of the rails caused by the mangling of the bodies of the caterpillars”.

Fortunately, there is only one generation of both species of tent caterpillars each year and this occurs in early spring.  While trees can be completely defoliated during outbreaks, this seldom kills healthy trees which can grow new leaves later in the summer.  There is also a short term benefit for bird watchers.   Birds, such as this scarlet tanager, are much easier to spot when they land or perch in defoliated trees or shrubs.

We relied on D. Wagner’s Princeton Filed Guides, Caterpillars of Eastern North America and D. Beadle & S Leckie’s Peterson Field Guide to Moths of Northeastern North America in preparing this article.  The Internet also has plenty of information and articles on tent caterpillars.