All the dirt on inchworms

Posted on Wednesday, May 8, 2013 at 10:06 am

By Stephanie Romelczyk

Have you noticed an abundance of inchworms this year? Maybe a few have fallen in your hair as you walked under large shade trees?  Or maybe you got tangled up in their silk threads?
Spring cankerworm and fall cankerworm are the culprits! Although two different species of caterpillar, they look similar and share similar life cycles. Cankerworms are also known as inchworms, measuring worms, or loopers. Every few years, populations of cankerworms boom (like this year!).
Cankerworm eggs hatch in early spring and caterpillars feed for five to six weeks, usually in May.  The caterpillars are voracious feeders, sometimes completely defoliating trees of common deciduous, shade, and orchard trees such as oaks and apples. After the larvae have finished eating the leaves, only major veins of older leaves remain. Although the damage appears severe, older trees can tolerate high levels of defoliation once in a while. If severe defoliation from caterpillars or other causes occur two or three years in a row, trees may be weakened or die.
Depending on the species, caterpillars are ½ to 1 inch long and have two or three sets of prologs.  The absence of legs in the middle section of the body gives the characteristic appearance of looping as the insects walk. Many color forms are present including pale green to brown, blackish, reddish-green, yellow-brown, and blue-black.
A key weak spot in the cankerworm life cycle is that female moths cannot fly! Instead, adult females of both species must climb up tree trunks to lay their eggs on twigs.  Spring cankerworm females lay eggs in the spring, while the fall cankerworm females lay eggs in the fall and winter.  For both species there is only one generation per year.
How can we control cankerworms? Although there are a number of products labeled for use on cankerworms in the home landscape, trying to spray insecticides into large, mature trees is not recommended. However, insecticides can be used successfully on small trees and shrubs where defoliation has been severe.
Keep in mind that populations are normally kept in check by natural predators like birds, beetles, and parasitic wasps. A booming population of caterpillars means more food for these animals.
Remember that weak spot in the lifecycle I mentioned?  We can take advantage of the fact that female moths climb up tree trunks to lay eggs by using sticky bands to trap adults before laying eggs.  Traps need to be installed in early fall for the fall cankerworm and in late spring for the spring cankerworm. Sticky bands can be used by wrapping duct tape, burlap, or a similar product around the trunk of the tree about chest high and covering it with a sticky substance such as TanglefootTM.  Do not apply adhesive substances directly to tree bark.  Be sure to use products according to the label.  Remove the trap from the tree after moth activity has ceased.
Cankerworms dropping from overhead trees definitely carry the gross factor.  Luckily we only deal with large population outbreaks every now and then.  For more information about cankerworms, contact me at 493-8924.

Stephanie Romelczyk is the Agriculture and Natural Resources Agent for Virginia Cooperative Extension in Westmoreland County.

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