Crop & Pest Report


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Worms in my Trees (06/20/19)

Calls on tent caterpillars, defoliation and webbing on trees are coming into my office from the Red River Valley area from Fargo up to Grand Forks.

Calls on tent caterpillars, defoliation and webbing on trees are coming into my office from the Red River Valley area from Fargo up to Grand Forks. I’ve updated my previous 2016 article on forest tent caterpillars (Malacosoma disstria) and Eastern tent caterpillars (Malacosoma americanum). Larvae (caterpillars) are feeding on tree foliage this time of year. The eastern tent caterpillars make the webbed tents in the forks of tree branches, which are used as shelters and resting places. People consider the webbed tents unsightly in trees. The forest tent caterpillar does not make any webbed tents, but they wander around in masses of larvae and crawl over trees, picnic tables, patios, lawns, etc., which people consider extremely objectionable. Fortunately, they do not bite. Large numbers of forest tent caterpillars crushed on roads causes the roadway surfaces to become greasy and slippery. They infest many trees hosts: ash, aspen, basswood, birch, chokecherry, cottonwood, elm, maple, oak, pin cherry, poplar, and other hardwoods.

There is one generation per year for either species. Both overwinter as eggs. Larvae hatch in early spring. For the forest tent caterpillar, larvae are easily identified by the keyhole shaped spots along their backs and broad bluish lateral bands. For the eastern tent caterpillar, larvae are black and somewhat hairy with a whitish-yellow stripe down the middle of the back, narrow broken orange-colored subdorsal stripes, and lateral white and blue markings. In five to six weeks, the larvae pass through five larval instars and are about 2 inches long when mature. Then, they form silken cocoons to pupate. Adult moths will emerge from cocoons during early summer (late June or early July).

Damage: Defoliation is caused by larvae of both species. Light defoliation has little effect on tree health. Two or more years of moderate-to-severe defoliation by forest tent caterpillar is necessary to affect radial growth and cause branch and twig mortality. When populations of eastern tent caterpillars are high, whole trees can become covered with webbing and defoliated.

Pest Management: Bt (or Bacillus thuringiensis var. kurstaki; Dipel, Thuricide), a natural occurring soil bacterium, works well to control young caterpillars and conserves beneficial insects. Other 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. Always read, understand and follow the insecticide label directions.

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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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