Utah Pests News Spring 2008

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Cankerworms Inching Their Way Here 



Cankerworms are in the family Geometridae, related to other “inchworms.” The females are flightless, and can lay 100 eggs or more.



In Utah and much of the West this past season, we saw localized outbreaks of cankerworm in many urban areas.  Trees such as boxelder, elm, maple, cherry, linden, and honeylocust were almost completely defoliated.  The good news is that all the trees re-foliated within six weeks.  The bad news is that cankerworm will be back this spring.

Both fall (Alsophila pometaria) and spring (Paleacrita vernata) cankerworm occur in Utah, although the fall cankerworm is most common.  The small, brown adults of both cankerworms fly when you would least expect them: the fall cankerworm in November-December (after a hard freeze), and the spring cankerworm in February-March.  The males seek out the flightless females for mating. Eggs are laid primarily on small twigs near buds.

Both species of cankerworm larvae hatch in early spring, just as the leaves break bud (around 120 DD after March 1). Initial feeding causes the leaves to appear tattered, but toward the end of the season, many host plants are completely defoliated.  Once food dwindles, they use silk threads to move to adjacent trees or to lower canopy vegetation.  They have just one generation per year, feeding for approximately 6-8 weeks before dropping to the soil to pupate.

  In Utah, the fall cankerworm (top) is most common. The larva differs from the spring cankerworm (bottom) in that it has three prolegs instead of two, and has three small stripes down its side instead of one large one. Larvae of both species can range in color from green to brown.
  There are a number of natural enemies of the cankerworm such as the “caterpillar killer” (Calosoma frigidum, left). This pest is mostly controlled by egg parasites, primarily Telenomus alsophilae, but also Trichogramma sp. (right), and Euplectrus mellipes.
Most healthy trees can tolerate one to two years of spring-time defoliation, and treatments are usually not required.  Stressed trees, however, will have a harder time recovering and may become more susceptible to other insect or disease attacks. Branch dieback and reduced growth are common symptoms.


The safest materials for suppressing cankerworm are spinosad (a fermentation product from a bacterium) and Bt (the bacterium, Bacillus thuringiensis).  These products are only effective on the youngest caterpillars (less than ½ inch in size), so early detection is critical.  Examine your trees for feeding as the leaves emerge, and/or shake branches in several locations over paper or a flat tray to look for dislodged larvae.

If you miss the early treatment and want to suppress the older caterpillars, there are several products available (pyrethroids, pyrethrin, permethrin, carbaryl, etc.).  Insecticides must be applied to taller trees by a commercial applicator.

And as is always recommended in an integrated pest management program, keep trees healthy with optimal watering, fertilization, replacing turf with mulch, and maintenance pruning.


Cankerworm populations are typically on a 5-7 year cycle, and the population will soon dwindle.  Because it is a native insect, it has evolved with several predators, parasites, and diseases.  As the cankerworm population increases, natural enemies also rise.  Predation, combined with reduced food supplies and severe weather, predisposes the cankerworm to a population crash.  Expect to see continued damage through the 2009 season, with a gradual decline over the following 2-3 years.

-Marion Murray, IPM Project Leader