Jeffrey Hahn, Asst. Extension Entomologist
There are two common species of tent caterpillars that are now active in Minnesota, eastern tent caterpillars and forest tent caterpillars. Both normally hatch closer to early May, but the cool spring weather we have experienced has slowed down their emergence and they only first started to appear closer to the middle of the month. Here is how you can distinguish between these insects.
Eastern Tent Caterpillars
This insect is easy to identify because it constructs silken webs in the fork of branches as soon as they young larvae hatch. The caterpillars feed outside of the tents on leaves during the day (as long as the weather is nice) and return to the webbing at the end of the day and during rainy weather for protection.
The caterpillars are bluish black with yellow and a white stripe running the length of the top of its body. They are also mostly smooth except for a series of hairs sticking out along the side of their bodies. They are two inches when fully grown.
Look for eastern tent caterpillars on hardwood trees, particularly fruit trees, like apple, chokecherry, crabapple, plum, and cherry. Eastern tent caterpillars are common most springs. They maintain relatively steady populations from year to year and generally do not occur in outbreak numbers.
Forest tent caterpillars
Also known as armyworms, forest tent caterpillars are familiar insects in the north and central areas of Minnesota. These caterpillars are blue and black with distinctive footprint or keyhole shaped white spots on their backs. They are mostly smooth except for hairs that stick out along the sides of their body. They grow to be two inches long when fully grown. Despite their name, they do not make conspicuous webs on trees.
Forest tent caterpillars feed on many deciduous trees, including aspen, birch, maple, crabapple, apple, ash, oak, and elm. They go through cycles of tremendously large numbers, lasting 5 to 8 years, before collapsing to such low numbers that they are not noticed. Periods of low populations lasts about 8 to 13 years. Forest tent caterpillars peaked in 2002 and their numbers have since crashed.
The Minnesota Department of Natural Resources (DNR) reports that in 2010 just over 70,000 acres were defoliated, primarily in the middle one-third of Minnesota in a crescent that extends from south of Mille Lacs Lake through St. Cloud to Wilmar and up through Detroit Lakes. There were also a few isolated areas of defoliation in Hubbard, Cass and Crow Wing Counties. Forest tent caterpillars are also found in the Twin Cities area. Their numbers are expected to increase some in 2011 compared to last year but an outbreak is not expected.
The decision to treat tent caterpillars should be made based on several criteria. First, consider what percentage of leaves have been eaten. If only a few branches are affected, the tree can tolerate that damage. Leaf feeding tends to be more a cosmetic problem and not one that threatens the health of the tree. Even if defoliation is severe, healthy, well-established trees can withstand this feeding in a given year. However, young trees are less tolerant and should be protected. Unhealthy, stressed trees should also be protected from severe defoliation.
Another important consideration is the size of the insect. Ideally these insects should be treated when they are 1/2 their full-grown size or smaller, i.e. about one inch in size. The larger they are, the closer they are to being done feeding. Because the tent caterpillars emerged later than usual, they are not as far as long as they would normally be by the beginning of June. There is still time to treat them and minimize their defoliation. However, if by the time you see them, the are close to two inches long, it is not worth treating them.
There are a variety of residual insecticides that you can use if it is desirable to protect your trees. Consider using products that have a low impact on the environment, such as Bacillus thuringiensis, spinosad, and insecticidal soap. Bacillus thuringiensis is a particularly good product if the tree is flowering since it will not harm visiting honey bees.