Nuisance of the Week - 8/27/02

Nuisance of the Week:
Bagworms
8/27/02

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Bagworms can cause a great deal of damage during the last few weeks of feeding and gardeners may be tempted to spray for them now. However, late August sprays are totally ineffective. Understanding the life cycle of this moth will explain why and can help tremendously in planning effective control measures.

In the Kansas City area, bagworms normally finish feeding and close up their bags about mid-August. After that point, insecticides are ineffective because they cannot reach the pest. After the bag is closed, the male pupates and will eventually emerge as a flying moth. Bagworms are unusual in that they use an uncommon form of reproduction called paedogenesis in which the female larva reproduces. The female larva never pupates but produces mature sexual organs during the last larval instar. She then releases a sex hormone that attracts the male. The male then flies to the female's bag and mates with her while she remains in the bag. After mating, the females body fills with eggs. She will eventually die inside the bag and her body will become a dried, mummified "egg case" that will protect the eggs over winter. Each female case normally contains 300 to 1000 eggs. Egg hatch does not occur until the next spring, usually during the latter half of May in Kansas. Insecticide sprays are more likely to be effective if applied when the bagworms are small. Even Bacillus thuringiensis (Dipel, Thuricide) can be effective on young bagworms. Other commonly used pesticides include Orthene, cyfluthrin, permethrin, malathion and Sevin. During most years, a spray about June 15 will give good control. Don't forget that insecticides are not the only means of control. Hand picking and destroying the bags is effective any time of year the bags are large enough to be picked.

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Nuisance of the Week is brought to you in cooperation with the Kansas State University Research & Extension
Written by Ward Upham, Extension Associate