White satin moth caterpillars invade Pinedale
by Dawn Ballou, Pinedale Online!
July 21, 2023
Some of the aspen trees around Pinedale have been looking rather sickly lately, and upon closer inspection it might be because they are covered with a bazillion furry caterpillars. The caterpillars are about 1.5 to 2 inches long, black with whitish double dots on their backs, and tufts of red and black hairs coming out of their bodies.
The culprits are white satin moth caterpillars, according to Scott Schell, UW Extension Entomology Specialist in Laramie, Wyoming. They are mainly found in Europe, but were reported in North America in the 1920s and have spread. They were first reported in Wyoming in Uinta County in 2004.
According to Dagan Montgomery, Agriculture and Natural Resources Extension Educator in Sublette County, "White Satin Moths are a common issue in southwestern Wyoming. An invasive species from Eurasia, they first became prevalent in Wyoming in the early 2000’s. The moth itself is snow white and some would say pretty to look at, but the caterpillars they originate from can be very damaging to plant life. They are commonly found on poplars, willows and aspen, but may be found on a number of hardwoods."
Montgomery added, "After wintering in the trees, larval White Satin caterpillars emerge in early summer to begin feeding, and the damage becomes most apparent in June. In late June and through July, they cocoon themselves to emerge in late July and August as nocturnal moths. Females then lay eggs which hatch in two weeks. These young larvae seek a place to winter, where they reemerge again the next year to start the cycle over. Eggs will be in clusters and covered in a froth. White Satin caterpillars that are unchecked can completely strip leaves and defoliate trees to the point of killing them."
If caught in time, there are a few management options to protect ornamental trees, Montgomery suggested. Eggs and caterpillars can be knocked off of trees with a pressure washer. Sticky bands can be placed around trees to trap caterpillars as they climb. An insecticide containing Bacillus thuringiensis var. kurstaki (Btk) can be applied in the late spring to kill larvae before they can do too much damage. Other insecticides containing carbaryl or cyfluthrin can also be applied to foliage where larva have started feeding. These are applied by spraying on the leaves, where caterpillars ingest the treatment and later die from it. Trees that are already affected with defoliation from caterpillars and trees that are susceptible are better able to overcome or resist the problem when they are strong. Fertilizing vulnerable or affected trees in the spring and generously watering them throughout the summer and early fall (especially here in Sublette County) will help them recover and/or survive issues the following year.
White Satin Moths are not currently listed as noxious pests in Wyoming. At the height of the infestations, there can be thousands covering infested trees and crawling on surfaces as they search for food. They are not harmful to pets or humans, however there have been reports of some people getting a rash after handling the caterpillars.
For more information, contact:
Dagan Montgomery, Agriculture & Natural Resources Extension Educator
Sublette County, Wyoming
9660 US Highway 191, PO Box 579
Email: dmontgo8@uwyo.edu
Office: (307) 367-4380
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