Forest tent caterpillars
Forest tent caterpillars (also known as armyworms) are common throughout the northern two thirds of Minnesota. They feed on many different trees and shrubs as well as other plants. In northern Minnesota, forest tent caterpillars particularly feed on birch and aspen while in central Minnesota (including the Twin Cities), they prefer oak and linden. Forest tent caterpillars primarily eat blueberry leaves when they are marching across the ground looking for new trees hosts. They generally move in large groups, and a relatively small number of caterpillars can quickly defoliate a blueberry plant.
Forest tent caterpillars are blue and black, with a series of white footprint shaped spots on their back. Mature caterpillars are about two inches long.
Forest tent caterpillars have one generation per year. The caterpillars first emerge in early to mid-May and mature five to six weeks later. Blueberries are most likely to be attacked in early to mid-June when forest tent caterpillars climb down the trees. Forest tent caterpillar populations are cyclical, with peaks occurring every seven to fourteen years.
Defoliated trees and shrubs rarely die, because the plant still has time to form new leaves before fall. When they defoliate blueberry plants, they destroy the crop for two years. The current season's crop is lost because there are no leaves to support the fruit, and the following season's crop will be ruined, because the plant will be making leaves in July and August instead of forming flower buds for the following year.
When populations are low, no treatment is necessary. In years with high populations, plants should be protected with an insecticide. Low impact insecticides include Bacillus thuringensis which should be applied before the caterpillars reach blueberry plants and spinosad, which will kill the caterpillars on contact. There are many conventional options, including permithren, bifenthrin and carbaryl. Insecticides should always be applied as soon as the first caterpillars reach the blueberry plants. When using insecticides, always read and follow the label directions.