Crop & Pest Report


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Early Season Defoliators on Trees (05/27/21)

We’ve begun to see the spring defoliators of trees show up this year.

We’ve begun to see the spring defoliators of trees show up this year.  Both cankerworms and tent caterpillars have been seen in eastern North Dakota, though we haven’t had any reports from elsewhere across the state. 

Cankerworms damage was heavy in central North Dakota in 2019 and 2020, but only moderate in the east.  This year, cankerworms have been reported in Grand Forks County, but not elsewhere.  The damage caused by cankerworms is stressful on trees; when added to drought stress, some trees may be damaged so much that they can’t recover.  However, we’re hopeful that the worst of the outbreak occurred in 2020, at least in central North Dakota.

Both the spring cankerworm (Paleacrita vernata) and the fall cankerworm (Alsophila pometaria) cause damage during early spring.  Many common chemical insecticides are effective against young larvae, and the bio-pesticide Bt works well for especially young larvae (<10 days old).  Older larvae are harder to control.  For more detailed information about cankerworms, see NDSU Extension publication Cankerworms in North Dakota (E999).

Tent caterpillars have also begun to show up in eastern North Dakota.  Three tent caterpillars occur in North Dakota – eastern tent caterpillar (Malacosoma americanum), forest tent caterpillar (M. disstria) and the prairie tent caterpillar (M. californicum lutescens).  Both the eastern and prairie tent caterpillars focus on chokecherry and related tree species; forest tent caterpillars have a broader host range. 

Both eastern and prairie tent caterpillars build ‘tents’ – easily visible nests where they congregate and feed.  Forest tent caterpillars don’t actually build tents.

Control measures for all the tent caterpillar are the same.  For small infestations, the larvae can easily be picked off the tree and destroyed.  The bacterial-based pesticide Bacillus thuringiensis var. kurstaki (Btk) works well to control young larvae, while pyrethrins or synthetic insecticides are needed for older larvae.

Additional insecticides available to homeowners include:  acephate (Orthene), azadiracthin (Azatin), carbaryl (Sevin), esfenvalerate (Bug-B-Gon), malathion, permethrin, spinosad (Conserve), or other insecticides registered for trees.

Don’t use a blowtorch to destroy the caterpillar nest.  While effective, the tree will also be damaged.

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Joe Zeleznik

NDSU Extension Forestry Specialist

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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