Look for forest tent caterpillar
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
beginning to receive reports of very heavy forest tent caterpillar
activity in the northwest Lower Peninsula. Larvae of this insect
periodically build to very high numbers in northern hardwoods, oak and
aspen forests in Michigan and can cause complete defoliation for one or
more years. Trees that are weakened from drought or competition from
other trees can die if defoliated two or three years in a row. Forest
tent catepillar are easily identified by the keyhole-shaped pattern
along their backs: http://www.na.fs.fed.us/spfo/pubs/fidls/ftc/tentcat.htm
Note that this is not the same insect that builds webbed nests in the branches of small cherry and other fruit trees this time of year. These are made by the eastern tent caterpillar. Eastern tent catepillars can be an eyesore, but cause little tree damage. While forest tent catepillar sometimes make webbing mats on trunks of trees when populations are heavy, they do not form “tents” in the tree foliage like eastern tent catepillar.