Cankerworms and other Loopers

Problem type: Insect

Name of problem: Cankerworms and other Loopers

Plant name(s): All deciduous plants

Symptoms / Characteristics:
Holes are made in the leaf, starting off small and enlarging as the feeding larvae grow. Eventually the entire leaf will be eaten with only the leaf veins remaining. Leaves can appear tattered and the entire tree may be defoliated.

There are many cankerworms, inchworms or loopers that attack deciduous trees. The spring and fall cankerworms, linden looper and the elm spanworm are among the most common found and can cause severe damage. The larvae of this type of insect are distinguished from other defoliators by the way they travel. To crawl, they loop their bodies by moving their rear portion close to their front, then stretch out again. This has given rise to the many names they are called such as inchworms, loopers, measuring worms and spanworms. In most species, larvae can move from tree to tree by hanging on a silk thread suspended from a leaf or branch and being blown by the wind.

Larvae of the spring and fall cankerworm may be both green and dark in color. Other loopers have a wide variation in color, size and pattern. Spring and fall cankerworms are generally very similar. The fall cankerworm lays its eggs in the fall in bands around twigs. The spring cankerworm lays its eggs in the spring in clusters in crevices of bark. Eggs of both species hatch as soon as buds begin to open in the spring. Larvae then feed for about 4 weeks until early June, then drop on silk threads to the soil to pupate. Adults are moths, with the females being wingless and must crawl up the tree to lay eggs. The spring cankerworm and the elm spanworm remain in the soil over the winter as pupae. As the ground thaws in the spring, adults emerge and females being wingless, then crawl up the tree to lay their eggs. There is one generation per year.

Cankerworms and other more important species appear in cycles. They are present in large numbers for 2-3 years, causing severe damage to plants, then their numbers seem to suddenly drop and disappear for 5-8 years, then reappear.

Control / Preventions:
Sticky bands or Tanglefoot® can be placed around tree trunks to trap wingless females and prevent them from climbing the tree to mate and lay eggs. A contact insecticide can be applied once leaves are fully expanded. Chemicals applied on rapidly growing leaves will not provide effective control. During severe infestations, spot spraying clusters can provide good control. Most common insecticides can be used to control cankerworms and other loopers. Spraying is most effective once all eggs have hatched and larvae are still small.

Another effective control against cankerworms is the use of a naturally occurring bacteria, Bacillus thuringiensis (Bt) that has been formulated for commercial use. This biological control can be applied once all eggs have hatched but larvae have not grown over an inch long.