Can you help identify these Caterpillars
Sat, May 23, 2009 at 8:33 PM
Dear Bugman,
I recently found a silk nest filled with these black-brown caterpillars. They have rusty hair covering their bodies and have distinctive white and orange markings. The orange markings in a triangular shape and there are 2 to each segment of their bodies. the white markings are found along their sides as well as down the top of their bodies between the orange markings. Several people have said they could be Painted Ladies, yet they really don’t resemble any pictures I have found.
Thank you for your help
Penny
Kelowna British Columbia, Canada

Western Tent Caterpillars in Captivity

Dear Penny,
We regret to inform you that these are not Painted Lady Caterpillars, nor any other lovely butterfly, but rather, they are Western Tent Caterpillars, Malacosoma californicum, which can be viewed on BugGuide.  The larvae are social and gregarious feeders that construct silken nests for protection when not feeding.  Here is what the Washington State University Biology and Control of Tent Caterpillars website indicates:  “The western tent caterpillar ( Malacosoma californicum pluviale Dyar) is often the most numerous in western Washington. Its orange and black markings are familiar to many people. This species spins tents on the tips of branches. The eggs hatch in early spring just as the new buds break in April or May. The young larvae begin feeding in groups. The larvae of both species molt (shed their skins) four times during their 5- to 6-week growing period.
As the caterpillars mature, they begin to feed in small groups or singly. Just before they spin their cocoons in mid-June, they crawl about looking for a protected place in plants or on structures to attach their cocoons. The adult moths emerge in approximately 7 to 10 days. The moths are stout-bodied and light brown. They often fly in clusters around street or porch lights on summer evenings. After the moths mate, the females lay 100 to 350 eggs in a froth-covered band around small twigs or branches of host trees. The eggs mature in 3 weeks but do not hatch until the following spring.
Tent caterpillars are primarily a nuisance. They do not transmit diseases to humans, do not bite, and are not poisonous. During years when large numbers of these caterpillars hatch, they can cause slippery roads and walks when they leave the trees.
Benefits of a caterpillar outbreak can be numerous in a natural setting. While caterpillars are distasteful to most birds, some birds feed on them. When alders and other trees are defoliated, the shrubs and trees below receive increased sunlight, giving some of them a boost in growth. The eaten leaves pass through the caterpillar’s body and emerge as little pellets which can break down easily, returning nutrients to the forest floor. Pupae provide nutritious meals for small mammals, and moths are eaten by birds and bats.
Where trees are crowded or stressed, the defoliation could be a life and death matter. Weak trees may die; healthy trees will leaf out again. In a natural setting, surviving trees can prosper in the absence of competition.
Healthy ornamental trees and shrubs should survive even serious defoliation. Trees which have been under stress (excess cold, heat, crowding, drought, flooding, etc.) may succumb and require more protection.
Natural Enemies
Tent caterpillars have numerous enemies. One is a tachinid fly which parasitizes the larvae by depositing white eggs on the caterpillar’s body. When the egg hatches, a small maggot burrows into the caterpillar and begins feeding. Tent caterpillars are also subject to a virus disease called wilt. While such natural enemies will reduce the number of tent caterpillars eventually, this process is gradual and may take 2 or more years. During that time, the affected trees may suffer such severe damage, that they will not recover. ”

Western Tent Caterpillars