Prairie Tent Caterpillar

Tent caterpillars are active in southeastern North Dakota and western Minnesota. Homeowners, arborists, and park managers should be on the lookout for tent caterpillar activity.

Prairie tent caterpillar can utilize a variety of hardwood host, though chokecherry is its preferred host. Prairie tent caterpillar is the most common tent caterpillar species in North Dakota. Praire tent caterpillar overwinters in the egg stage and larvae emerge in the spring with the flush of their host plant foliage. Larvae feed for 6 to 8 weeks and are about 2 inches long when mature. Larvae are black with a white mid-line stripe broken into dashes, and light blue lateral stripes also broken into dahses. Like eastern tent caterpillar, larvae of prairie tent caterpillar form silken tents in the forks of branches and small trees and feed on nearby foliage. Mature larvae spin cocoons in curled leaves or in leaf litter. Adult moths emerge in mid-summer. Females lay eggs near the base of the host plant in the ground. Larval damage is similar to that of eastern tent caterpillar.

Life cycle:Forest tent caterpillar larvae emerge from egg masses in early to mid-May, about the same time aspen leaves begin to open. Caterpillars feed actively on aspen and other broadleaf trees for five to six weeks. Despite its name, the forest tent caterpillar does not make a true silken tent. However, the larvae do spin an inconspicuous silken mat where caterpillars congregate on the trunk and branches.In June, older larvae become restless and move around trees and other vegetation to find food. Significant damage to nearby plants can occur at this time. Near the end of June, full-grown caterpillars wander from where they have been feeding to search for protected places to spin silky cocoons (to pupate).Full-grown caterpillars are about two inches long, mostly blue and black, with a row of white, footprint shaped markings on their backs, and many hairs along the edge of the body.Adult moths emerge from cocoons about two weeks later in mid-July.

These tan moths are nocturnal and are attracted to lights at night. Adults live for about five days. During this time they deposit 100 to 350 eggs in gray cylindrical masses surrounding small twigs. The eggs overwinter and larvae hatch from them during the next spring. There is only one generation per year.In Minnesota, the number of forest tent caterpillars changes in relatively predictable cycles. At the beginning of this cycle, forest tent caterpillars can be difficult to find. Over a period of eight to thirteen years, their populations start to increase until they reach tremendously large numbers. These outbreaks usually last for about three to four years in Minnesota, although in southern areas of the United States outbreaks have been known to last as long as nine years.

Damage:Trees that are fed upon by forest tent caterpillars are rarely killed by these insects because following complete defoliation, deciduous trees are able to produce another set of leaves during the same season. The main impact of forest tent caterpillar feeding on deciduous trees is a reduction in the rate of growth. Vigorously growing trees can tolerate up to two or even three consecutive years of heavy defoliation without suffering serious damage or mortality. If a prolonged defoliation cycle occurs, (four or more years), moderate to heavily defoliated trees may experience a reduction in growth, suffer branch dieback and could eventually be killed. A general rule of thumb is that complete defoliation can occur when an average of nineteen egg masses are found on an aspen tree that has a six inch diameter trunk at about four and half feet. When trees become stressed, such as during a drought, they are less tolerant of defoliation. A stressed tree can be injured or even killed in a much shorter time period than an unstressed tree. Protecting high value trees would be appropriate during forest tent caterpillar outbreaks when trees are under moisture stress.

Control:Simple physical procedures (mechanical control) by the homeowner can be carried out to help manage the forest tent caterpillar. One management procedure is to remove and destroy overwintering egg masses from branches of small trees before eggs start to hatch in the spring. Furthermore, caterpillars and cocoons can be brushed off houses, picnic tables, or decks with a stiff broom or brush or knocked down with a spray of water. Be careful not to crush too many caterpillars; they can smear and leave marks on some paints.

The homeowner involves chemical treatment with insecticides. Insecticides should be sprayed when caterpillars are small and easy to manage. Larger larvae are more difficult to kill and can continue to heavily defoliate trees before some insecticides take effect. An effective larval insecticide is Bacillus thuringiensis var. kurstaki (also referred to as BT), a microbial insecticide derived from a bacterium. BT offers effective control and conserves beneficial insects. Other insecticides available to homeowners that conserve beneficial insects are insecticidal soap, spinosad (Conserve), and azadirachtin (Azatin). Additional insecticides available to homeowners include carbaryl (Sevin), malathion, acephate (Orthene), and permethrin.

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