Tent Caterpillars Starting to Munch on Tree Leaves

One of our most common native tree leaf eating caterpillars has begun to emerge and munch their way through certain trees’ canopies.  The Eastern Tent Caterpillar is an early-season insect whose egg masses overwinter on tree branches of preferred species.  That way, when they emerge in early to mid-May, the tiny caterpillars don’t have to search for food.

Eastern Tent Caterpillar gets its name from the caterpillar’s habit of forming silken tents in which they spend their non-eating time, staying relatively safe from predators.  Family groups stay together, with from 150 to as many as 400 caterpillars hatching from the black, shiny egg masses.  They venture from their family group tents to eat, or to move to another branch, if they’ve eaten all the leaves on the one which they were inhabiting.

Just after hatching, the tents are as small as a conical ping pong ball.  The cool weather this weekend will keep them from growing quickly, so they may stay small longer than normal this year.  As the caterpillars eat, they grow, and the tents get bigger.  They can reach nearly the size of a volleyball in total caterpillar mass if a couple family groups get together into a colony.

As mentioned, this is a native insect and it feeds mostly on native deciduous, broadleaf trees.  Their favorites include Northern Black Cherry, Northern Pin Cherry, chokecherry, and fruit/ornamental trees such as apple, crabapple, plums, cherries, and any other flowering Prunus tree.  One good thing about their damage is that it is done early in the year, which allows affected trees to grow new leaves to replace the eaten ones and still collect enough energy to go into dormancy correctly and survive winter.

However, damage on fruit or landscape trees will cause problems, most notably lack of fruit set and decreased fruit size/quality due to the lack of leaf area.  Aesthetically, most people just simply don’t like seeing the web tents in their trees, whether landscape trees or native woodlot trees.  There are control options that can work relatively well, especially in smaller trees that you want to protect.

Physical control is the cheapest, and often the easiest, manner of controlling them.  The shiny egg masses are laid toward branch ends, usually on branches that are about 1/8 to 3/16 inches in diameter, and they completely encase the branch.  At a glance, it looks like a part of the branch, except for the shiny appearance, as they are tightly bound to the bark.  With a practiced eye, you can find most of the egg masses when you are doing your normal fruit tree pruning in later winter and early spring and simply prune those branch tips off and burn/bury the offending egg masses.

Once they hatch, hand-to-hand combat ensues, as the most efficient way to kill small tents is to simply squish them – most people prefer having gloves on to do this.  Even larger tents can simply be stripped off the branch, put on the ground, and stepped on completely.  The immediate revenge factor of this method motivates some to prefer this method.

Fire or torches are NOT valid options.  It causes significant damage to the tree, wastes fuel, and has the potential to start a wildfire that you do not want to have to try to deal with or explain.   There are many different insecticidal options to consider if dealing with large numbers of caterpillars.  Early season control can be achieved with the caterpillar-targeted Bacillus thuringiensis var kurstakii product.  Btk is a bacteria that produces a physical toxin inside specific insects, thus the reason you need to make sure you have a caterpillar version, not just any Bt product.  This product won’t affect pollinators, birds, or any insects other than relatively small caterpillars.

Larger caterpillars need different insecticides for control, but pretty much any insecticide labeled for use on fruit trees or shade trees (whichever one you are spraying) should work fairly well against these caterpillars.  Make sure to thoroughly read and follow the label and follow any precautions listed.  However, remember that insecticide use will impact pollinators if the trees are blooming, so either wait or only spray the leaves directly in the vicinity of tents, which is a good way to minimize how much product you need to use, anyway.

There are a few other caterpillars which you may think are Eastern Tent, most notably gypsy moth and forest tent caterpillar, as they are out at the same general time as Eastern Tent, but neither of these species form tents in the canopies.  Forest tent will spin a home, but it is not a tent and is found on trunks.  Fall webworm is another moth species whose young spin tent homes in branches while feeding, but they usually do not emerge until late June or early July in our area.

One place to learn more about this insect is by viewing Wisconsin Extension publication A2933 Eastern Tent Caterpillar, found at: https://cdn.shopify.com/s/files/1/0145/8808/4272/files/A2933.pdf  You are also able to contact UW-Madison, Division of Extension Marinette County Horticulture Agent Scott Reuss at 715-732-7510 or e-mail to scott.reuss@wisc.edu with any questions about this insect, or any other plant or plant pest question you may have.