Cankerworms attack boxelder in Whitewater State Park

Trees affected by cankerworm.

Spring and fall cankerworms, Paleacrita vernata and Alsophila pometaria, are two of the more common species of loopers, spanworms, and inchworms that feed on deciduous trees. The common names of these two insects indicate the respective seasons during which their eggs are laid. The females of both species are wingless. Both are indigenous to North America and occur from Nova Scotia and southern Canada to the Carolinas, Texas, Colorado, New Mexico, California, Montana, and Manitoba. They have a wide host range but are important pests primarily of elm, apple, oak, linden, beech and boxelder.

Eggs of both species hatch as soon as buds begin to open in the spring, and they occur together in mixed populations. Young larvae feed on buds and unfolding leaves. There are both green and dark larval phases of each species. The larvae often drop from the leaves on silk strands of their own making, from which they are often detached by the wind and blown considerable distances. Larvae devour all but the midrib of the leaf and often defoliate entire trees. Four to five weeks after egg hatch, larvae enter the soil to pupate, usually in early June.

The eggs of the fall cankerworm are laid in carefully aligned masses of about 100 eggs on small twigs. The spring cankerworm lays its spindle-shaped eggs in clusters of about 100 in the crevices of rough bark on larger limbs and trunk.

Cankerworms are a common early spring defoliator across much of southwestern Minnesota in most years, but not in the southeast. Populations in western Minnesota are highly variable from year to year. It is unusual to see repeated defoliations in the same location. So it is interesting to note that some 200 acres were defoliated in Whitewater State Park in Winona County during May and early June. The defoliation event will have no impact on the boxelder.