Forest Tent Caterpillar
The Forest Tent Caterpillars are dark colored with bluish markings along their sides, and a row of footprint shaped white spots across their top side. They can reach a size of up to 2 inches. Outbreaks of this caterpillar usually last for three to four years, then subside. Populations of caterpillars across the state are proving to be less cyclical, and lingering for longer periods of time. These caterpillars prefer to feed on the leaves of birch, aspen, oaks, and lindens/basswood, but will feed on many other broadleaf trees. These caterpillars do not make tents, but do form a mat on a trunk or branch where they congregate, which is typically seen on the upper part of the tree. When trees are defoliated, older larvae move between trees and shrubs, across lawns, driveways or patios in search of another food source. Larvae pupate 5-6 weeks after hatching and feeding, usually in June. They pupate in a folded leaf, bark crevice or other sheltered area. Moths emerge 10 days later and live only a few days. They are buff colored and are about an inch in size and are often attracted to lights. Moths then lay eggs on the upper crown branches of trees encircling entire twigs as a glossy brown mass overwintering and hatching again in the spring.
Eastern Tent Caterpillar
Reports of the Eastern Tent Caterpillar have been coming in throughout the area as well. These are the caterpillars that create the webbed tents seen in the crotches of trees in spring. The eastern tent caterpillar hatches at the time buds begin to open, usually mid-March-April. After hatching, the caterpillars spin tents in trees to protect themselves from birds and other predators and from temperature extremes that occur at night or on cool or rainy days. Caterpillars emerge from their tents in warm, sunny weather to feed on leaves of trees such as apple, cherry, crabapple, plum and chokecherry trees, as well as many other deciduous trees. These caterpillars are black with a white stripe down their back and brown or yellow stripes along their sides. Larvae feed for 4-6 weeks anywhere from April to June depending on their emergence date. They then pupate in a cocoon about 1 inch long made of silk and located in protected areas, and may be attached to other objects like trees, garages or other structures. A reddish-brown adult moth emerges 3 weeks later to lay eggs. Eggs are laid in a band of shiny varnish-like material that overwinter on small tree branches and again hatch in the spring.
Tent caterpillars are more of a nuisance than a serious pest of trees. Damage is usually cosmetic because of the unsightly tents in trees. Also, trees appear ragged from caterpillars feeding until they can replace lost leaves. Caterpillars are considered a nuisance as they are found crawling over plants, walkways and roads. Tent caterpillars rarely kill healthy, mature landscape plants. While caterpillars can nearly defoliate a tree, the tree will usually recover and put out a new crop of leaves.
Control of caterpillars
Both of these insects can be controlled without the use of insecticides. Removal and destruction of egg masses from ornamental and fruit trees during winter and removing tents in early spring, can avoid the use of chemicals and defoliation of trees. In early spring, the small webs can be removed by hand. Larger webs can be removed with a stick or broom or a heavy spray of water. At the point of pupation when caterpillars have spun their cocoon, they can also be destroyed. Put the caterpillars in a pail of soapy water or crush, bury or burn them if it is permitted. Do not try to burn the web in the tree because you will cause more damage to the tree than the caterpillars would.
It is not effective to spray caterpillars with an insecticide when they are inside of the webbing. The webbing serves as protection for the caterpillars, and sprays do not penetrate through the webbing. It is also not effective to spray for caterpillars if you have not seen any activity or feeding from them. Wait until a warm sunny day when the caterpillars leave the tent to feed on the leaves and apply the product to the foliage. As caterpillars grow, it becomes more difficult to treat for them. In large wooded areas, treatment may need to be done aerially, by a licensed applicator.
In times of heavy infestation, focus your treatments to young trees, as well as to ornamental and fruit trees. Larger deciduous trees will recover after feeding, even if feeding has occurred over several consecutive years. Bacillus thuringiensis var. kurstai (BT) is a bacterial insecticide that is very low toxicity and is effective if applied when the caterpillars are less than one inch in length; it can be found as Dipel, Thuricide or as Bonide Bacillus Thurengiensis. Other insecticide products that are effective against tent caterpillars include spinosad, acephate (Orthene), carbaryl (Sevin), permethrein (Eight), cyfluthrin, esfenvalerate and malathion.
If the tree is flowering, you are strongly encouraged to use a lower impact insecticide such as Bacillus thuringiensis or spinosad, neither of which harm pollinating insects that are active around flowering trees. As always, use insecticides with care. Read the label before you purchase the product and again before you use it. Follow all directions and safety precautions.By Janelle Daberkow, University of Minnesota Extension