So Many Cankerworm Caterpillars

It’s been the best of springs and the worst of springs.  At least it was an interesting spring with more Lepidoptera than usual.  See the article on hackberry emperor butterflies elsewhere in this newsletter.

Earlier in the year, we saw an unprecedented outbreak of spring cankerworms in northern and northeastern Iowa.  Spotty occurrences have been reported in the past, but nothing like we’ve heard about this year.  The spring cankerworm infestation was widespread but spotty and included woodlands, state parks, and home landscapes.  The annoyance of so many caterpillars was short-lived as the caterpillars eventually moved to protected locations to pupate and wait till next spring to reappear as adult moths.

inchworm caterpillars on the side of the house
Spring cankerworms crawling on house siding.

The inquiries were all similar: cankerworms were wandering across the patio and on the house siding, or grown trees had been defoliated, or masses of caterpillars were hanging from trees on silk threads. Images at https://bugguide.net/node/view/14161  and https://www.facebook.com/groups/plantsleuths

Cankerworms are inchworms that feed on the foliage from a wide variety of trees. Spring cankerworms appear as adults in the springtime (March - April) and lay the eggs on the trees. The eggs hatch, and larvae appear at the time the elm tree buds are opening. The caterpillars feed for 4 weeks before reaching the full-grown length of about 1 inch. When fully grown, the caterpillars spin silk webbing to drop from the trees. The good news is at that point, the defoliation of the tree has stopped.  The bad news is there is no control for the migrating, fully-grown cankerworms.

Cankerworms hanging from trees or inching through your property may be annoying, but they are no longer causing any harm. The feeding damage to tree foliage is over and done. There is only one generation per year, so we will not see them again this summer once this is over.

Spraying trees, landscapes, or house siding to control cankerworms is not recommended. Treating with insecticide will provide little if any benefit and could be potentially more harmful than beneficial. Spraying after the caterpillars are grown and wandering from the host plant will kill beneficial insects without helping the trees or hurting the caterpillars.

The University of Minnesota has an excellent online article that addresses all the issues with this pest, including how uncommon it is, how it was a more extensive problem decades in the past, how the problem may repeat for a few years before disappearing, and that there is nothing to do but wait for the cankerworm migration to end. Please see https://extension.umn.edu/yard-and-garden-insects/spring-and-fall-cankerworms

Repeated defoliation will weaken shade trees, but a single outbreak and leaf loss are not significant. The interesting questions are what happened to cause the outbreak now, and what will happen next year?  Where did the cankerworms go, and why did they come back?  If this happened to you this year, mark your calendar to watch for the start of a repeat appearance next April and May.

The other interesting question is, how did so many cankerworms get here?  It’s a bit of a puzzle since the spring cankerworm female is a wingless, flightless moth who can’t travel to a new area to lay her 250 eggs.  Instead, like the gypsy moth, bagworm, and others, the newly hatched spring cankerworms disperse by “ballooning” on silk threads.  The first instar caterpillars go where the wind blows them, and where they drop from the sky could be variable, random, and highly localized.

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Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Horticulture and Home Pest News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on June 18, 2021. The information contained within may not be the most current and accurate depending on when it is accessed.