There may be as many as 200 to 300 eggs in a single female’s bag. Since the female bagworm cannot fly, local populations can build up to damaging levels as succeeding generations of insects emerge over the course of several years, but there is only one generation per year.
How can bagworms be controlled? Handpicking the old cocoons is the best way to control light infestations on small plants. If you find old bags on your plants, but no new caterpillars yet there may still be time to remove the bags before eggs begin to hatch. Destroy bags by burning, immersing them in kerosene or by crushing. If bags containing eggs are discarded on the ground, the larvae will still hatch and find their way back to your landscape plants.
Otherwise, plan to apply an insecticide to control this year’s small larvae in mid to late June. Bacillus thuringiensis, BT, is available at nurseries and garden centers as Dipel or Thuricide. It’s very effective at controlling the insects without damaging other beneficial insects. It also has very low toxicity to birds and mammals.
Additional low-risk insecticides can be used, such as spinosad, azadiractin and insecticidal soaps. However, these products may require repeated applications to give good control.
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Traditional insecticides labeled for bagworm control include bifenthrin, cyfluthrin and permethrin. Affected plants must be thoroughly covered with the insecticide so it is ingested by the insects as they feed.