Cankerworm
Last spring portions of southern Washington and Frederick Counties, totaling about 2,000 acres of forested area, experienced an infestation of cankerworm, leading to large areas of trees being stripped of their leaves. Cankerworms are appearing in large numbers this year.
The damage caused by these native caterpillars, which in their larval stage appear as 1- to 3-inch-long green or brown “inchworms,” or “loopers,” affects nearly all of our broadleaf trees, including oak, maple, beech, cherry, and ash. The caterpillar eggs hatch in the spring, and feed on the leaves of forest and shade-trees for six to eight weeks before burrowing into the soil to pupate.
The damage caused by these native caterpillars, which in their larval stage appear as 1- to 3-inch-long green or brown “inchworms,” or “loopers,” affects nearly all of our broadleaf trees, including oak, maple, beech, cherry, and ash. The caterpillar eggs hatch in the spring, and feed on the leaves of forest and shade-trees for six to eight weeks before burrowing into the soil to pupate.
Two kinds of cankerworm are responsible for the defoliation — spring cankerworm, and fall cankerworm. The adult moths of these two cankerworm species emerge to reproduce either in late November, or late February to early March. Maryland Department of Agriculture (MDA) entomologists observed adult spring cankerworm moths in southern Washington County on Feb. 12 this year.
Defoliation by these caterpillars has begun to occur as of May 1. It is likely that patches of forest in areas such as Keedysville and Weverton will again experience significant defoliation by cankerworm caterpillars again this spring. Historically, this native defoliator experiences population increases and collapses based on different factors, including the weather.
It is difficult to predict outbreaks, which usually dissipate two to four years after first appearing on the landscape. The last significant population of cankerworm in this area of Maryland was in the early 1990s, which followed earlier severe defoliation from the non-native gypsy moth caterpillar.
Many residents of the area may remember the initial wave of gypsy moth into our area during the late 1970s, and the most recent outbreak in 2006. The MDA does a great job of monitoring gypsy moth populations each year and guiding targeted spraying. Fortunately, the non-native gypsy moth population is down in our area, but the damage caused by the native cankerworm is often incorrectly blamed on gypsy moth.
Because the cankerworm is a native caterpillar, MDA does not coordinate or recommend targeted spraying to protect trees from defoliation. Cankerworm is part of the natural cycle in our forest ecosystem. Despite last year’s defoliation, and the risk for additional defoliation this year, healthy trees will typically put out a second set of leaves and recover completely.
However, a weakened tree might succumb to the damage caused by the cankerworms. If you are concerned about the cankerworm’s impact on high-value, ornamental, or feature trees in your yard, consider insecticide treatment by a licensed pesticide applicator. In addition, watering and fertilizing may also help keep trees healthy.
Article by Washington County Forest Conservancy District Board member
Nature Notes for 5/31/2015