Individuals can be identified by the series of keyhole shaped spots that run down their backs. Among the caterpillars commonly known as loopers or inchworms, the fall cankerworm, spring cankerworm, linden looper and a few other species are injurious to ornamental trees. May. Fall and spring refers to when adults emerge to lay eggs. Cankerworm outbreaks often involve mixed populations of these pests. Such outbreaks commonly cause localized defoliation of elm, apple, oak, linden, and beech trees, . . .
Many insect pests go through cycles of abundance.
Outbreaks, which commonly last two to three seasons, are generally followed by several years of low populations. The buck moth caterpillar is a good example. Most of the time, this is a rare insect, but during outbreaks, large numbers can cause serious defoliation of oak trees. Many people know the beautiful morning cloak butterfly, but do not recognize its larval form, commonly known as the spiny elm caterpillar. It is an occasional pest of elm and willow.