Dealing with Tent Caterpillars Q & A


The following was published on the Town of Esterhazy website regarding the tent caterpillars.

Should I get rid of them and how?
Tent caterpillars are a short‐term nuisance in most locations. If you want to get rid of them, the simplest way is to remove and destroy the larvae and their nests by stripping or pruning them from branches. You can cut off the branch, bag it and leave it in the shade. Evening and early morning are the best times to prune because tent caterpillars tend to congregate in their nests at night. The caterpillars will die and you can add all the material, including the carcasses, to your compost or dispose of it. The City will also accept the material through the weekly garbage pickup, as long as it is contained in garbage bags. Burning the nests, a traditional method of control, can often result in more damage to the tree. Additionally, this practice may result in personal injury and property damage. A simpler solution is found in pruning off the tented branch.

Spraying an insecticide to kill caterpillars is another effective method. Each product has restrictions as to which plants and sites where it can legally be applied. If applying to shade and ornamental trees, the label should say it is for use on shade and ornamental trees. Please read and follow label directions.

a.    Biological insecticides containing Bt, (a bacterial product made of Bacillus thuringiensis ) are the recommended products to use for FTC control in the backyard because of their safety and the low toxicity to non-target organisms. Bt products are only toxic to caterpillars; they do not kill bees, flies, mosquitoes, etc. However, Bt products are slightly slower to act since they must be eaten by caterpillars before they take effect. Apply Bt to the leaves of host plants not to the bark or other non-edible materials. It is most effective on young (small) caterpillars.
b.    Insecticidal soaps can be sprayed directly onto caterpillars or onto plants they infest. Insecticidal soaps are insecticides made from naturally-derived fatty acids. Repeat applications may be necessary.
c.    Chemical insecticides can also be used but would normally be a second choice after Bt, due to safety considerations. Commonly used chemical insecticides contain malathion (Malathion), acephate (Orthene), carbaryl (Sevin) or methoxychlor (Methoxychlor). These products can also kill bees and other organisms, so exercise caution when using them. This product can be found at places in town like Home Hardware ( Brand names and stores are not meant to be an endorsement of a particular commercial product.)

What are they?
Tent caterpillars are the larvae of certain moths that are native to western North and Central America. They hatch from eggs in early spring when the leaves of their host plants are just unfolding. Western tent caterpillars prefer alder, apple, ash, birch, cherry, cottonwood, willow, Garry oak, domestic fruit trees, and roses. Their tents are constructed in locations that get the morning sun, which the caterpillars need to warm their bodies.

How long will they be around?
Caterpillars grow rapidly and usually complete their larval life cycle in seven to eight weeks. Around mid‐June, they will crawl away from their natal tree and seek a protected place in plants, under leaves, or on structures to attach and spin their cocoons. The adult moths emerge about 7 to 10 days later.
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 three weeks but do not hatch until the following spring.

Why are there so many some years?
Tent caterpillars follow a boom‐or‐bust population cycle and the cycle varies depending on weather and other factors.

Will they kill my trees and shrubs?
In most cases, no. Tent caterpillars defoliate (eat the leaves of) the branch their tent is on, but once they vacate the tent, the branch will usually grow new leaves. This does not affect the overall health of the tree.
Healthy ornamental trees and shrubs should survive even serious defoliation. However, trees that have been under stress (excess cold, heat, crowding, drought, flooding, etc.) may succumb and require more protection.
Where trees are crowded or stressed, the defoliation could be a life and death matter. While healthy trees will leaf out again, weak trees may die. In a natural setting, surviving trees can thrive in the absence of competition.

(Sources: District of Sannich, Washington State University.)

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