Purdue Extension Service

Vanderburgh County, Indiana

Bagworms Ready to Hatch Out

By Larry Caplan, Extension Horticulture Educator, Vanderburgh County, IN

For the Evansville Courier and Press, June 6, 2008

Bagworms are caterpillars that live inside spindle-shaped bags which they construct to protect themselves against birds and other enemies. These bags, composed of silken threads and bits of foliage, look so much like a part of the tree that they may go unnoticed until extensive damage has occurred.

Early in June, the insects hatch from eggs which wintered in the old bags attached to tree branches. As soon as the young worms appear, they start to spin bags and continue to enlarge these as they feed and grow. The caterpillars crawl part way out of the bags to feed. If disturbed, they retreat safely inside, and it is almost impossible to pull them out.

The female bagworm never leaves her bag, and dies soon after she fills her bag with eggs in the fall. Each female bag can produce over 1,000 bagworms. To get to new, uninfested plants, the baby bagworms will spin a long thread of silk which allows it to float on the wind to other sites. This is the way the baby spiders in "Charlotte's Web" left the farm at the end of the story.

Bagworms can feed on most plants, but are most commonly found on needled evergreens. If not controlled early, they can strip most of the foliage off your shrub before you even know they're there.

Several options exist for bagworm control. Many folks will pull off the overwintering bags before the new generation hatches out. The bags can be buried in the garden or thrown in the trash. If you have low numbers of bagworms or a small shrub, you can probably remove most of them fairly easily. However, because the bags blend in so well, you're bound to miss a few.

Several parasitic wasps do a good job at keeping bagworm populations under control. The adult wasps feed on pollen, and can be attracted to your landscape by planting lots of flowers around the shrubs you are trying to protect. After feeding, the female wasps will look for caterpillars to lay their eggs into. You'll know this is working if you see bags in mid-summer that have a tiny round hole in the side - this is the exit hole of a fully developed wasp.

The safest product to spray on your evergreens is B.t. (Bacillus thuringiensis, sold as Dipel or Biotrol). This is a bacteria that only kills caterpillars, so it is very safe for you, nesting birds, and other wildlife. B.t. works best on young caterpillars, so you need to apply within 3 or 4 weeks after they hatch.

By the time July rolls around, if you still have live bagworms, you'll need to use one of the commercial insecticides. Products labeled for bagworms include Malathion, acephate (Orthene), permethrin, or carbaryl (Sevin). Be sure to read and follow all label directions.

For more information on bagworms or other pest control problems, contact the Purdue Extension Service at (812) 435-5287.