What can I do about the bagworms eating my trees and shrubs?

ASK THE HORT AGENT

Question What can I do about the bagworms eating my trees and shrubs?

Answer First, I need to make sure we are talking about the same critter. Bagworms are caterpillars that make a cocoon shaped like a diamond. They do not make webs which resemble large spider webs. This is the problem with using common names. The word “bagworm” may be the name a person calls any number of caterpillars. Extension agents refer to this creature by its proper Latin name, Thyridopteryx ephemeraeformis -- yeah right.

Once you have established the worm is a bagworm, you should immediately come to the realization that for now your goose is cooked. Bagworm eggs hatch toward the end of May or first week in June. These cute little worms are only 1/8 of an inch long. Instinctively, they begin to test the strength of their silk by spinning a silk thread and hanging around. Spring breezes catch the wee little worms and blow them to greener pastures. Green pastures for a baby bagworm is a long line of Leyland cypress trees.

If a bagworm is lucky enough to land on a suitable host plant, then it spins a silk cocoon. This cocoon serves as protection from predators, sunlight, pesticides, etc. Foliage from the host plant is cleverly incorporated into the silk cocoon. Bagworms were the original inventors of the ghillie suit (http://en.wikipedia.org/wiki/Ghillie_suit) .

As bagworms feed on foliage, they grow and enlarge their silk protection. Homeowners finally begin to spot these pests in August or September. By then, the feeding damage is very obvious and the caterpillars are difficult to kill. Since young caterpillars are easy to kill, launch the pesticide assault in late May or early June.

Products containing Bt (Bacillus thuringiensis), like DiPel or Worm Whipper, are not contact insecticides. They have to be eaten by young caterpillars. Simply spray the Bt on the foliage and let the baby bagworms nibble. After a couple bites, they develop a stomach ache that won’t go away. This target specific, non-contact type of pesticide bypasses the protective cocoon and is very safe to the applicator and surrounding environment. However, it does not work well on mature caterpillars.

Contact insecticides like Sevin, Malathion or Orthene are also more effective when the caterpillars are young. By the end of August or September, bagworms are nearly invincible. Handpicking or a .22 rifle will work better than insecticides (depending on your aim).

For more information about bagworms visit http://www.ces.ncsu.edu/depts/ent/notes/O&T/trees/ort081e/ort081e.htm

If you don’t have internet access, call the Extension Office at 893-7533 or email me at gpierce@harnett.org

Another name for the ghillie suit is yowie. Ironically, this is also the word most often spoken in September when homeowners realize how many bagworms are eating on their Leyland cypress tree.

Gary L. Pierce

Horticulture Extension Agent

Harnett County