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You may have seen inchworms hanging from a strand of silk from trees, getting on a passing person. They’ll be here soon and here are tips for dealing with them.
“Cankerworm” is an older name used for what is commonly called inchworms, loopers, measuring worms or spanworms.
There are numerous caterpillars which show the behavior of inchworm. Cankerworm larvae feed on tree leaves from late April to mid-June. The adult females of cankerworms are wingless and emerge to lay eggs in the late fall (fall cankerworms) or early spring (spring cankerworms). In this part of Virginia, the fall cankerworm is the one that strips the trees of their leaves in the spring.
Fall cankerworms emerge as adults in late fall, often during warmer periods in February. The wingless females are a dull grey color and crawl up on tree trunks to await a winged male. The males are about one inch long, dull grey in color and often have two light, wavy stripes on the forewings. After mating the female lays a cluster of barrel shaped eggs, often encircling small branches. The eggs overwinter and hatch in late April to early May. Occasionally adults emerge in March, especially in more northern areas.
Fall cankerworm larvae grow to 3/4 to 1 inches long and are usually apple green to brownish green in color with a dark middle stripe and three narrow white lines on each side. Fall cankerworms have three pairs of fleshy prolegs at the end of the abdomen.
Upon hatching in April and May the young larvae rapidly feed on the fresh tender spring leaves of various trees. Periodic outbreaks of large numbers are especially annoying as the crowded larvae often hang from trees on a strand of silk.
By late May to early June in Virginia, the larvae have matured and they then descend to the ground on silk threads. The larvae then burrow into the ground to a depth of one to four inches, spin a silken cocoon and pupate. The pupae remain in the soil until the late fall, where they emerge as adults and then crawl up the tree to lay the next generation of eggs.
The most effective controls are directed toward trapping the wingless females. In addition, spraying the active larvae after they start feeding on the tree leaves also works with smaller, landscapetrees.
Strategy 1: Adult Trapping – Banding the trunks of susceptible trees with sticky adhesives such as tanglefoot will trap females as they crawl up the trunk to mate and lay eggs. Trapped females may remain attractive to the males which also get stuck in the sticky band. The best time to wrap a tree trunk with a sticky band is mid to late December until late February – too early and sticky bands can become saturated with falling leaves.
Strategy 2: Horticultural Oil Sprays – The 2 to 3 percent dormant horticultural oil spray is quite effective against the eggs. Be sure to thoroughly wet the trunk bark if spring cankerworms are present. Reduce the oil rate, especially on maples, if the trees seem to be active in the spring. This is best done from early December through bud break in early spring.
Strategy 3: Spray with Bacillus thuringiensis (Bt) – This biological control is quite effective against young cankerworm and looper larvae. Wait until all the eggs have hatched but spray before the larvae get to be over inch long. Like any other spray that needs to be applied to the entire tree, this can be done only on smaller trees with hand held sprayers, so that the tree can be completely sprayed. For large trees, some professional arborists or companies can be hired to spray B.t. from the ground using a hydraulic sprayer. This costs some money but may be desirable for large specimen trees that add a lot of property value, particularly large oaks, which are a favorite host of fall cankerworm.
Strategy 4: Insecticide Sprays – Most common insecticides are effective for control of cankerworms and loopers. Best results are obtained if the spray is applied after all the eggs have hatched and the larvae are still small. These can be found at most hardware, and feed and seed stores.
If you have any questions or concerns regarding fall cankerworm, please the Virginia Cooperative Extension office or Matt Coleman, forester, at the Virginia Department of Forestry office.