Scout Now for Bagworms

by T. Lemmons, Extension Educator

University of Nebraska-Lincoln Extension

Homeowner, producer, and acreage owner alike have been fighting for years against one of the more recent invading pests, the Bagworm. Given time and opportunity, this prolific insect with and insatiable appetite for green tree material, may easily destroy tree breaks, windbreaks, and ornamental groves. Bagworms prefer to feed on red cedars, but may be found on any number of tree species to include (but not limited to) pines, spruces, junipers, and many deciduous tree species to include (but not limited to) walnut, apple, elm, maples, oaks, and birches.

Most often this insect is detected in the late summer or early fall with its characteristic silk bag camouflaged with pieces of foliage, bark, and tree debris; however this is also the stage where control is most difficult.

Once the worm has built-up a suitable silk bag (and is thus easily detected), it is fully protected against most pesticides and insecticides. Once a pesticide material is detected, the worm simply retreats into the silk bag, armored against the harmful materials. In the bag stage of late summer, the only effective method of control is handpicking, a daunting task for those with large areas of trees.

In the early spring, the new generation of bagworms will emerge from the silk sack prepared the season before. At this stage, they are no bigger than a pinhead, but are highly susceptible to chemical control measures given the absence of their own protective bags. The best method for detecting emergence of the bagworm is to collect several of the bags from your trees. Next, place the bags into a Ziploc or other form of clear sack for observation. Place the clear sack in an area that consistent with the outside temperature, and wait. Bagworm emergence is triggered by degree-days – once enough heat units have been received, the sack will break open and the pinhead-sized crawlers will be observed. Now wait 7 days (to ensure that all the bags on the trees have a chance to open) and apply a chemical control agent to the tree. Usual emergence is from early to Late May, depending on daily temperatures and conditions.

Bagworms are best controlled in the early or immature stages using a myriad of chemical agents to include (but not limited to) carbaryl (Sevin), malathion, chlorpyrifos (Dursban), acephate (Orthene), and pyrethrums (just to name a few). If applying a chemical agent, please be sure to read completely and follow precisely the product label. Thoroughly cover the tree during application as larvae may be on the underside of tree branches or leaves.

For more information on Bagworms or other insects of interest, please contact your local University of Nebraska-Lincoln County Extension office.