Rana Sarfraz: Colourful tent caterpillars may become nasty pests
During late April and May of 2001 and 2002, central Kentucky lost one-third of its foal crops to an epidemic of equine abortions with estimated economic losses of over US$500 million. These abortions in horses were linked to tent caterpillars.
Tent caterpillars can pose a serious threat to your home landscaping, reduce value of your property, and create an overwhelming nuisance to people. They hatch in early spring when the leaves of their host plants are just unfolding. The three most common species in Canada are the forest tent, the eastern tent, and the western tent caterpillar.
Forest tent caterpillars are pale blue with black, and have a series of white spots on the back. Eastern tent caterpillars are brownish-black with a light stripe down their backs. Blue spots and brown-yellow lines are found along the sides of their bodies. Western tent caterpillars are reddish-brown on top and pale underneath, and have a row of blue spots on their backs, with orange spots interspersed in between.
The common caterpillars around your homes are probably western tent caterpillars. They prefer alder, apple, ash, birch, cherry, cottonwood, willow, Garry oak, domestic fruit trees, and roses. They construct their tents in locations that get morning sun, which the caterpillars need to warm their bodies.
In the spring, the eggs hatch into young caterpillar larvae that remain together as a family to form a silk tent. Caterpillars grow rapidly and usually complete their larval life cycle in six to eight weeks.
In June to early July, the caterpillars crawl away from their natal tree and seek a protected place in plants, under leaves, or on structures to attach and spin their cocoons. After about two weeks, the moths emerge, mate and lay 50 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.
Tent caterpillars follow a boom‐or‐bust population cycle, and the cycle varies depending on weather, disease, and other factors. Judy Myers and her associates at UBC have been investigating the mechanisms of these cyclic populations for over 40 years. Outbreaks last two or more years and occur at intervals of six to 16 years. During outbreaks, the caterpillars become so abundant that they are capable of completely defoliating tens of thousands of acres of forest.
Tent caterpillars defoliate 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 (cold, heat, drought, flooding, crowding, et cetera) may succumb and require more protection.
These measures can provide homeowners some relief:
- In the summer, look for tough, yellow-to-white cocoons on tree trunks, fences, debris, and sheltered areas. In the fall, look for shiny, dark brown saddle-like bands of eggs around small branches of host trees. Destroy egg masses and cocoons. A dormant oil spray may be used on trees in late winter to smother the eggs before they hatch in early spring.
- In the spring, remove and destroy the larvae and their nests (tents) by stripping or pruning them from branches. They do not bite or sting. Cut off the branch, bag it, and leave it in the shade. Evening and early morning are the best times to prune because 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.
- Do not burn the nests as it can often result in more damage to the tree. Additionally, this practice may result in personal injury and property damage.
- Microbial insecticides that contain the bacterium Bacillus thuringiensis (Btk) effectively control tent caterpillars. After ingestion of vegetation treated with Btk, caterpillars cease to feed and die within one week. Products containing this bacterium are generally non-toxic to mammals, birds and fish.
- For current chemical control options, consult your local pest control representative. If the tent is within reach, break it open with a stick and direct the insecticide into it. Spraying is most effective in the evening when the caterpillars return to their tent.
Rana Sarfraz is an entomologist who is currently working at the University of British Columbia.