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What Are Those Worms Dropping From the Trees?

By Chris Williams on May 4, 2015.

inchworm-grub-on-twigI can’t get my kids to play in the backyard because of little green worms that are dropping out of the trees and landing on them and on everything else.

Now I’m remembering that these worms and their silk threads were all over decks and sidewalks last spring, too. What are they? R.N., Hampton, NH

You are probably talking about spring cankerworms, although they could be fall cankerworms. Pretty confusing. Both of these caterpillars are the immature form of moths and, despite the names, they both make their appearance in spring (mid-May in Massachusetts and New Hampshire). The names, spring and fall cankerworm, refer to the season in which the adult moths are present. The larvae are less than an inch long and range from light green to brown.

Cankerworms Feed On a Wide Range of Trees

The larvae of cankerworms hatch from eggs in spring and feed on the newly emerging, young leaves of many different types of hardwood trees, including beech, ash, oak, elm, hickory, apple, cherry, maple, and linden. Their feeding results initially in a tattered appearance to the leaves. Although cankerworms make their appearance every spring, they rarely cause significant damage to the trees that they feed on, and most of the time we don’t even notice them. Some years, however, there is a severe, noticeable outbreak resulting in lots of browned-out trees. A heavy infestation of cankerworms can completely defoliate a tree, and will sometimes kill a tree that is already stressed from drought.

The caterpillars produce silk threads that they use to float from tree to tree, or to drop to the ground. We’re not aware of all the cankerworm larvae busily feeding high up in the trees, but when they are hanging in front of our faces or climbing up our arm, we take notice. They can be especially annoying in mid to late June when they are all dropping to the ground to pupate. Large populations in trees can also produce an annoying rain of cankerworm poop on those below!

There’s a Reason They’re Also Called Inchworms

Cankerworms are also called inchworms, spanworms, measuring worms, or loopers. Like other caterpillars, they have fleshy prolegs at the rear of their bodies. Unlike other caterpillars, spring cankerworms have only two pairs of prolegs which accounts for their strange gait. Since a large section of their bodies are legless, when they move, the rear prolegs have to move forward on their own to catch up with the front legs, resulting in a humped-up, rather amusing form of locomotion.

The adult male cankerworm moths are rather drab looking. The females of both spring and fall cankerworm moths don’t look like moths at all since they are wingless and fuzzy. They climb tree trunks to mate and deposit their eggs. In heavy infestations, sticky bands can be placed around the trunks to intercept the female moths.

Fortunately, we don’t have to put up with cankerworm larvae for very long. They feed for 3 to 4 weeks in spring and then drop to the ground to pupate by late June. Control of the larvae is difficult since entire trees must be treated and timing is important. In most cases and in most years, control is not warranted.

Photo credit: Cyron / Source / CC BY

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