Unwelcome Campers – the Western Tent Caterpillar
There has been lots of activity by the western tent caterpillar this year, especially in the northern islands (Whidbey and San Juans). While the insect’s signature “tents” are unsightly and a little disconcerting, these native insects are a natural part of our Pacific Northwest forests.
According to Washington State Department of Natural Resources entomologist, Karen Ripley, Western tent caterpillar is a native insect that has a 3-year-long population outbreak about every 9 years. It’s been about 8-9 years since the last outbreak on Whidbey Island.
Tent caterpillars feed on the foliage of red alder, cottonwood, fruit trees and many other broadleaf trees and shrubs. They hatch from an egg mass around May when new foliage is emerging on trees. The tents are usually the most conspicuous sign that an outbreak is present. They are highly visible, dark masses in the branches of trees and persist even after the caterpillars that inhabit mature into moths.
The caterpillars are a nuisance. They make trees look shabby and their tents are smelly. But the good news is that caterpillars will soon stop feeding and disappear for the year. The loss of foliage is rarely significant, in the long-term and trees will produce a new crop of leaves once the pests are gone .
WSU Extension entomologist Sharon Collman reminds us that the Western tent caterpillar has a beneficial role in nature. Defoliated trees allow light, water and nutrients (from all the digested leafy material that the caterpillars leave behind) to reach the forest floor.
Quick fixes for caterpillar infestation include pruning out tents, if you can easily reach them. It may be possible to work your fingers under the silken caterpillar pad to slowly pull it down (the caterpillars will hang on). Then you can wrap the pad hand over hand until silk and caterpillars are all pulled off, place it in a paper bag, freeze it and toss it in the compost. Collman cautions, “DO NOT use torches to flame them. Handmade torches of string and rags, burn through the string and the rag unravels. This has caused roof and dry grass fires and burned people severely.”
You may be able to find and destroy egg masses later in the year, which can help to decrease the future population.
Parasitic flies and wasps and viruses are really the most effective means of control and will kill large numbers of the caterpillars, eventually bringing the outbreak to an end (but remember, it takes about 3 years for an outbreak to run its course).
Although readily available, garden pesticides will kill caterpillars, but spraying pesticides doesn’t change the tent caterpillar outbreak that significantly, and the spray can be unhealthy for humans. Also, when caterpillars are sprayed, they die but remain hanging in the trees on their silken pads; tents remain intact, as well.
If you would like more information on tent caterpillars check out:
“Biology and Control of Tent Caterpillars” – WSU Extension
“Western Tent Caterpillar” – USDA Forest Service Forest Insect and Diseases Leaflet (FIDL) #119
This story was adapted from an article in the WSU Extension Puget Sound Forest Stewardship E-Newsletter, June 2012, Volume 5, No. 4
Urban and community Forestry in Washington State 