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Spring and Fall CankerwormsPage 2of 3 Eggs stay on trees throughout the winter and hatch in the spring. Egg-hatch varies from late April to mid-May depending on weather conditions. It normally coincides with the opening of elm buds. Newly hatched fall cankerworm larvae are less than 1/16 of an inch long, and spin silk threads that allow them to readily blow from tree to tree. They reach full-grown length in about four weeks. Larvae then drop to the ground to pupate in the soil before emerging as adults later in the season. Spring Cankerworm -- Spring cankerworms differ from fall cankerworms in several ways. First, the pupae in the soil do not emerge as adult moths until early spring. Emergence of the winged males and wingless females typically occurs during the third week in March in the Twin Cities area. Another difference is that eggs are deposited in loose clusters in bark cracks and under bark scales and are not easily seen. Spring and fall cankerworm eggs hatch about the same time in the spring and the larvae have similar life histories. Damage A mature, vigorous tree is able to withstand a single season of complete defoliation with little effect on tree health. Even two seasons of defoliation produce only a slowing of growth. However, limb dieback and loss of vigor can result if the same tree is completely defoliated for three or more consecutive years. Young, newly transplanted, or weakened trees are more susceptible to injury from defoliation. Therefore, a tree's age, size, vigor, and previous history of defoliation should be considered before choosing management tactics. Many people are not as concerned about the health of their tree as they are about the nuisance that cankerworms can cause. Large numbers of caterpillers spinning to the ground on silken threads, crawling or falling on picnic tables, doors, walks, and/or house siding, can create an unpleasant environment. Fortunately, if this behavior occurs, it usually lasts only about one week. Management is not effective at this stage of the cankerworm's life cycle. Cankerworms may also create a nuisance during the spring and fall as moths crawl up exterior walls of homes. Again, management is not effective nor recommended at this time. Management Chemical -- Whether the purpose is to protect trees or to eliminate the nuisance of cankerworms, management measures need to be taken when the caterpillars are relatively small and inconspicuous. The best time for chemical treatment is in the second week of feeding when the damage is minor, the caterpillars are still small (less than 1/2 inch), and leaves have fully expanded. Both the cankerworms and the early damage can be easily missed so look at your trees closely. If leaves are still growing and pesticides are applied, pesticide coverage will be reduced as the leaves continue to grow, reducing management efficacy. Cankerworms rapidly increase in size during the third and fourth weeks after egg-hatch. During this period, damage becomes extensive and very noticeable. In many cases, large portions of the leaves have been eaten and only the veins remain. Unfortunately, treatment at this time is not effective because the damage to the tree is already done, and insecticides can actually irritate the cankerworms causing them to drop from trees in larger numbers. Bacillus Several insecticides are effective for treating cankerworms when applied at the correct time including: thuringiensiscarbarylcyfluthrin; esfenvalerate; malathion (Dipel, Thuricide); (Sevin); (Malathion); permethrinBacillus thuringiensis (Eight). However, the bacterial insecticide should be used when possible. This environmentally compatible product has the advantage of being specific for the caterpillars of moths and Bacillus thuringiensis butterflies and does not harm beneficial insects, wildlife, or humans. must be applied while the cankerworms are still small (less than 1/2") to achieve good management. The following steps are suggested to achieve good cankerworm management: 1.Determine egg-hatch date by either: a.Finding fall cankerworm egg masses and observing them during early spring for signs of hatching (hatched eggs will have a hole in the center), or http://www.extension.umn.edu/distribution/horticulture/DG0876.html6/5/2009