Bagworms in Your Arborvitae

If your arborvitae, junipers or other trees and shrubs appear to be losing their foliage, you may have bagworms feeding on the foliage.  Bagworms are caterpillars that live inside a spindle-shaped bag in which they construct to protect themselves against birds and other enemies.  These bags are constructed of silken threads and bits of foliage that goes unnoticed until extensive damage has occurred.

If only a few small trees or shrubs are infested, removing or clipping the bags from the plant by hand may afford satisfactory control.  After removing the bagworms, put the bagworms in a bucket of soapy water to kill them.

When many small bagworms are infesting the plant, an insecticide may be needed to prevent serious damage.  The best time to apply an insecticide is while the larvae are still small (less than 1/2-inch long).  Small larvae are more vulnerable to insecticides, and inflict less damage.  If the bagworms are ¾ inch long or longer, a conventional insecticide probably will provide better results.

For homeowners, conventional insecticides such as Sevin and malathion, or the microbial insecticide Bacillus thuringiensis (BT) provide satisfactory results.  The BT products have very low mammalian toxicities, but are only effective against younger larvae.  Foliage should be thoroughly wetted with the insecticide spray in order to achieve thorough coverage.  Trade names are used as examples.  No endorsement is intended, nor criticism implied of similar products not named.  Always read and follow directions on the label.

Control Measures - from Ohioline factsheet HGY-2149-90

Bagworms are difficult to control because they are often unnoticed until mature. Mature larvae will often pupate early if they detect pesticides on the plant foliage. Though there are a few known parasites and predators, they are often not adequate in urban habitats.

Option 1-Cultural Control-Mechanical Hand Picking If the bags are few in number and easily reached they may be picked off the plant and squashed. This can be done easily in the late fall when deciduous foliage has been dropped or the bits of plant material on the bags turn brown and can be easily located on evergreens. Be sure to cut the attachment silk band so that the branch will not be girdled in the future.

Option 2-Biological Control-Use the Bacterial Spray Bt The bacterium, Bacillus thuringiensis (Bt), is effective against bagworms if it is used against young larvae. Applications should be made at the end of June after all the eggs have hatched and the larvae are through ballooning.

Option 3-Chemical Control-Insecticide Sprays Stomach insecticides are very useful for control of bagworms. Remember that the plant foliage is to be thoroughly covered because the larvae are protected from contact by being in the silk bag. Again, early sprays against young larvae are more effective than later applications. Products registered for bagworm control are: acephate (Orthene), bendiocarb (Ficam, Turcam)(*), bifenthrin (Talstar), carbaryl (Sevin), chlorpyrifos (Dursban), cyfluthrin (Tempo)(*), diazinon, dimethoate (Cygon), fluvalinate (Mavrik)(*), malathion, nicotine sulfate, pyrethrum, permethrin (Pounce)(*), rotenone and trichlorfon (Dylox, Proxol)(*).

Option 4-Chemical Control-Timed Sprays Using Degree-Day Emergence Using a base temperature of 14.4 degrees C (57.9 degrees F) the median first emergence is 380 DD base 14.4 degrees C (716 DD degrees F) and the median last emergence is 572 DD base 14.4 degrees C (1062 DD degrees F). Allow for an extra week of ballooning after the last emergence mark has been reached before applying Bt or an insecticide. A less precise degree-day model uses a 50 degrees F base which calculates last emergence around 900 DD units.

For more information regarding identification and control - Ohioline's information at http://ohioline.osu.edu/hyg-fact/2000/2149.html

Picture from University of Kentucky College of Agriculture - http://www.uky.edu/Ag/Entomology/entfacts/trees/ef440.htm