Too Late to Spray for Bagwoms
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. But late-August sprays are often totally ineffective because the insects have usually stopped feeding. Never spray in August unless the insects are actively feeding. Handpicking is still possible if there are not so many bags it becomes impractical. Understanding the life cycle of this moth will explain why spraying now is not recommended and help plan effective control measures.
In the Midwest of the USA, and other areas with the same climate, bagworms normally finish feeding and close their bags during mid-August. After that, insecticides are ineffective because they cannot reach the pest.
Bagworms are unusual because 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 releases a sex hormone that attracts the male who does pupate and emerges as a flying moth. The male flies to the female’s bag and mates with her while she remains in the bag. After mating, the female’s 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 during the winter. Each female case normally contains 300 to 1,000 eggs. Egg hatch does not occur until the next spring, usually around the end of May.
Insecticide sprays are more likely to be effective when the bagworms are small. Even Bacillus thuringiensis (Dipel, Thuricide) can be effective on young bagworms. Another organic product, spinosad (Captain Jack’s Deadbug Brew; Fertilome Borer, Bagworm, Leafminer and Tent Caterpillar Spray; Monterey Garden Insect Spray), is very effective against both young and more mature bagworms. Other commonly used pesticides include acephate, cyfluthrin, permethrin, malathion and Sevin.
During most years, spraying about June 15 will provide good control. Don’t forget that insecticides are not the only means of control. Hand picking and destroying the bags is effective any time when the bags are large enough to be picked.
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