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Cankerworms – Time to Begin Scouting (04/30/20)

Populations of cankerworms, spring defoliators of deciduous trees, have increased over the past two years causing damage throughout much of the state.

Populations of cankerworms, spring defoliators of deciduous trees, have increased over the past two years causing damage throughout much of the state.  We’re expecting another bad year in 2020.  The insects feed on a number of tree species, including boxelder, elm, ash and linden. 

The spring cankerworm (Paleacrita vernata) and the fall cankerworm (Alsophila pometaria) are the insect pests causing the problem.  Although they differ in their life cycles, both species’ larvae (caterpillars) emerge at about the same time; therefore, they’re managed together. 

Larvae are susceptible to insecticides when they’re young/small, though we usually don’t notice the defoliation in the tree canopy until it’s too late.  Therefore, scouting for these pests is critical in order to properly time our treatments.  Fortunately, a mathematical model describing the insects’ growth has been developed.  Utilizing a base-50 Growing Degree Day (GDD) model, the insects are susceptible to insecticides between 148 and 280 GDD.  The ND Agricultural Weather Network (NDAWN – https://ndawn.ndsu.nodak.edu/) uses a base-50 GDD model for corn, which we can utilize for cankerworm management.  Notice how variable the number of GDDs for cankerworms are across the state.  Scouting should begin in the southwestern part of the state, while in the northeast, it will be several days until the insects begin to emerge.

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A number of chemical insecticides are effective in controlling cankerworms when the larvae are young (see below).  In the past, sticky bands were often put on the outside of tree trunks with the hope of controlling the female adult moths, since they cannot fly and crawl up the trunks to lay eggs.  Unfortunately, sticky bands aren’t effective at controlling cankerworms, but they can serve as a detection aide.  The bacterial-based pesticide Bacillus thuringiensis var. kurstaki (Btk) is effective at controlling cankerworms, especially when larvae are small about ½-inch long, about 10 days after egg hatch. This biopesticide is highly recommended since it is safer to wildlife (e.g. birds that may eat the larvae) and beneficial insects that provide ‘natural’ control.

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For more detailed information about cankerworms, see the new NDSU Extension publication Cankerworms in North Dakota (E999).

Disclaimer:  Mention of any product does not imply an endorsement of one product versus another nor discrimination against any product not mentioned by NDSU Extension or the authors.

 

Joe Zeleznik

NDSU Extension Forestry Specialist

 

Janet J. Knodel

Extension Entomologist

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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