Forest Tent Caterpillar Infestation in Northern Minnesota
Many have seen an increased number of forest tent caterpillar (Malacosoma) outbreaks in Minnesota. While their damage may not kill the trees whose leaves they ingest, the species can be considered a nuisance for its negative impacts on tree growth, defoliation of leaves, and additional concerns.
Biology and Identification of Tent Caterpillars
These caterpillars are native to most of the United States and Canada. They are related to silk moths and reproduce once a year in the fall. Their eggs lay dormant in the winter on tree branches until they hatch in early spring as larvae. The larvae stage takes between 6-8 weeks, followed by a two week cocoon period.
Forest tent caterpillar larva
Forest tent caterpillars may be up to two inches long with a row of footprint or keyhole markings on their back. Their bodies can be striped with blue, black, and brown. They resemble a grass-eating caterpillar known as the armyworm, but forest tent caterpillars have many hairs along the edge of the body. The caterpillars are safe to the touch and do not bite.
Tent caterpillars are social animals that create a large silken mat on the branches of deciduous trees. These silken mats allow them to live in an expanding colony for shelter, warmth, cooperative foraging, and protection (Fitzgerald). These mats are an identifying feature of tent caterpillars.
Forest caterpillar mats as photographed by Troy Salzer
Defoliation of Trees
Forest tent caterpillars eat the leaves of birch, poplar, aspen, and other deciduous trees. The caterpillars are known for the widespread loss of leaves on trees through their heavy vegetation. The defoliation of trees can slow down the growth rate, but it generally does not kill deciduous trees. During drought or after years of defoliation, trees may be killed by the damage.
Partial defoliation of tree leaves
Populations of forest tent caterpillars cycle up and down with predator populations and rates of reproduction. After a heavy infestation year, we may see a decrease in forest tent caterpillar populations. Infestations are cyclical and have a ranging return interval between 9-16 years.
Potential population control factors vary. Allowing natural predators, large gray flies, to prey on forest tent outbreaks will let the infestation run its course. Many years of infestation may cause plant damage, so additional methods can be employed to manage tent caterpillar populations.
Removing and destroying egg masses before they hatch in the spring can reduce the amount of forest tent caterpillars on trees. Egg masses can be removed by pruning twigs containing webs or spraying webs with high-pressure water. Removal with a scoop shovel of outbreak sites may also be effective.
Forest tent caterpillar egg mass
Pesticides may be used to manage tent caterpillars before further damage, but they are only effective when larvae are small (1 inch or less). MN Extension recommends using a spot treatment of Bacillus thuringiensis (BT), or another pesticide that conserves beneficial insects.
When run over by cars, forest tent caterpillars can make road surfaces slippery, which increases the risk of automobile accidents. Tent caterpillars may also create colonies on the side of buildings, which may be undesirable. Colonies can be removed by spraying nests off of buildings with water.
Forest tent caterpillars can create large, silk mats where they live in large colonies. These population infestations have several impacts on the environment, including loss of leaves and potential danger to roadways. Allowing predators to naturally react to the infestation may not prevent tree damage, so other control methods may be applied.
Brown, W. “Tent Caterpillars”. Texas A&M Agrilife Extension.
Fitzgerald T.D. (2008) Tent Caterpillars, Malacosoma spp. (Lepidoptera: Lasiocampidae). In: Capinera J.L. (eds) Encyclopedia of Entomology. Springer, Dordrecht.
Hahn, J “Forest tent caterpillars”. University of Minnesota Extension. 2018