Tent Caterpillar

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This integrated pest management (IPM) fact sheet provides information useful for identifying and combating a particular pest while minimizing environmental impact. It was originally written by Philip Dickey of the Washington Toxics Coalition and designed by Cath Carine of CC Design for the Green Gardening Program from Seattle Public Utilities. It has been modified for use on Toxipedia.org.

Species


  • The western tent caterpillar (Malacosoma californicum) is most common in the Puget Sound area.
  • The forest tent caterpillar (Malacosoma disstria) is rarer.

Related Species


The apple ermine moth (Yponomeuta malinellus) and the fall webworm (Hyphantria cunea) spin cocoons that may be confused with those of tent caterpillars.

Host/Site


Caterpillars spin their webs on the branches of many types of trees and shrubs.

Tent caterpillar and typical tell-tale damage to leaves.

Identification


Appearance
Western tent caterpillar has checkered yellow appearance with blue dashed line down center of back, and is up to 3 inches long.

Indicators
The most visible indication of tent caterpillars is their large tents in spring. Tents are white, silky webs that cover the tips and forks of tree branches. Egg masses, visible on bare twigs in winter, are gray or brown frothy material about an inch in length, circling the twigs and hardened resembling styrofoam. Visible damage consists of chewed leaves and may include defoliated twigs and branches.

Tent Caterpillar Cocoon

Life Cycle
Eggs hatch in early spring (April or May) when new buds are emerging. Caterpillars immediately begin feeding on leaf tissue, and tents appear on the tips of branches. Caterpillars feed out on leaves during the day but return to the tent at night. They are less active and stay closer to the tent when the weather is cool or wet. After molting four times over their five to six week growing period, they stop eating. Soon they select a site and spin their cocoon. Adult moths emerge after about two weeks, mate immediately, lay eggs, and die within a few days. The eggs winter over until the following spring.



Natural Enemies


Birds in gardens help generally with all pest control. Birds that eat tent caterpillars include chickadee (family Paridae), junco (genus Junco), nuthatch (family Sittidae), and bush tit (Psaltriparus minimus). Tachinid flies (family Tachinidae) deposit visible white eggs on the caterpillar body, generating maggots that bury into the body after hatching. Other predators include braconid wasps (family Braconidae) and ground beetles (family Carabidae).

Monitoring


Monitor in the spring when small tents begin to appear near the end of branches. Observe the number of tents and how much of the tree is affected. Monitor leaf damage as caterpillars emerge. The presence of white eggs laid on caterpillars indicates predation from tachinid fly. Hunt for egg masses when pruning leafless trees.

Action Threshold


Tent caterpillar populations follow cyclical patterns of unknown duration. Pests may persist in high numbers for several years and then recede almost to zero. Most trees can stand up to 25% loss of leaves without permanent damage, and some will recover from nearly 100% defoliation in a single year. The trees will leaf out again in summer after an infestation. Several tents in a tree should not be a problem unless aesthetic standards absolutely prohibit it.

Cultural and Physical Controls


Individual tents can easily be removed by pruning in the early morning or evening when the weather is still cool and caterpillars are inside. A pole pruner is recommended to reach high branches. Immerse pruned out nests in a bucket of soapy water or seal in a plastic bag and stomp them. Individual caterpillars can be hand picked. Egg masses may be hand stripped or pruned out of plants in winter.

Biological Controls


Bacillus thuringiensis (Bt) bacteria are effective as a stomach poison if applied to leaves when caterpillars are young and feeding. Timing is critical because caterpillars must feed on Bt for this method to work. Follow label directions and thoroughly coat the foliage in the vicinity of tents. Caterpillars may appear healthy for several days after the spray but will have stopped feeding and become inert. Bt is selectively toxic to caterpillars and will also kill non-pest caterpillar species. Control the spray so that it stays off plants without caterpillar damage.

Chemical Controls


Chemical controls are not recommended and should not be necessary. The commonly used insecticides are broad spectrum chemicals that will kill beneficial insects and can harm birds.

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