Jeffrey Hahn, University of Minnesota Asst. Extension Entomologist
A couple of caterpillars were noticed recently. Eastern tent caterpillar is a common insect on apple, crab apple, cherry, and other fruit trees. They have a dark colored, hairy body with a yellow stripe down their back and grow to almost 2 inches in length. They overwinter as eggs on branches and emerge in the early spring. They construct webbing in the forks of branches which is where they rest at night and during cloudy, rainy days.
Cankerworms have also just emerged recently. A type of inchworm, they are yellowish green with a smooth body and grow up to 1 inch long. Cankerworms skeletonize leaves, i.e. they feed between the major veins. When they first start to attack leaves, this damage will begin as small oval holes between veins. As the caterpillars become larger, entire areas between the veins are consumed. Cankerworms feed on a variety of trees, including apple, linden, elm, ash, and hackberry.
Photo 1: Eastern tent caterpillars on Prunus. Michelle Grabowski
Mature, healthy, well established trees can tolerate caterpillar feeding, even when it is severe. Young trees or those that are stressed are less able to tolerate feeding and should be protected from caterpillars. A non-chemical method to eliminate eastern tent caterpillars is to wait until they have retreated to their webbing and then pull the webbing out along with the caterpillars. You can then destroy them by crushing or burning them. You can also treat the leaves with a variety of insecticides. Less toxic choices include Bacillus thuringiensis, insecticidal soap, and spinosad.
Photo 2: Cankerworms on linden. Jeff Hahn