YARD AND GARDEN: Bagworms are on the rise in June | Home & Garden

Bagworms are caterpillars that live in spindle-shaped sacs that they build to protect themselves.

Made of silk threads and bits of foliage, these bags look so much like part of the tree that you might not notice them until they’ve done a lot of damage. You’ll have to get really close to see the little guys, which are 1/4 inch long to begin with, so you’ll need keen eyesight.

In early June, bagworms hatch from eggs that have overwintered in the old sacs and begin spinning their own sacs. In June, July and August, bagworms nibble on their favorite foods: pine, spruce, arborvitae and juniper. If left unchecked, they can easily kill a tree in a matter of weeks. When evergreens aren’t handy, bagworms aren’t choosy; They also eat 128 species of plants, including deciduous trees and shrubs, but since these leaves regrow, the damage isn’t usually as severe.

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Each female pouch can produce up to 1,000 babies. Bagworms stop growing in late August or early September. By this point, the sacks are about two inches long and pesticides are no longer effective. The winged male, a small, furry black moth with clear wings, fertilizes the wingless, maggot-like, yellowish-white female, which never leaves the pouch. (Quite a femme fatale, huh?) She puts eggs in the bag where they spend the winter. There is only one generation each year (a small blessing).

When you find bags in the tree, pick them up and drop them in a bucket of soapy water. This method is only effective if you catch them before the eggs hatch from the bags in June. If there is a hole at the bottom of the bag, then thousands of baby caterpillars have already flown out of the coop and you need to move on to plan B.

Plan B: Spraying the bags with insecticides. For the greenest attack, use a biorational pesticide like Bacillus thuringiensis kurstaki, aka (BTK) and found in Dipel and Thuricide, and Spinosad (organically grown and found in products like Conserve). The biorational materials only kill the caterpillars, not beneficial insects like bees, praying mantises and butterflies. Biorational pesticides are most effective when directed against small larvae. As always with pesticides, read and follow labels to ensure safe and effective use.

Check for live bagworms (they wiggle when they eat) two weeks after application to see if you need to spray again. As the bags approach their full size, pesticides become ineffective. Hand picking is the best control measure at this point.

For more information on bagworms, visit the University of Illinois Extension Focus on Plant Problems website.

Illinois Extension leads public affairs for the University of Illinois, turning research findings into action plans that enable Illinois families, businesses, and community leaders to solve problems, make informed decisions, and adapt to change and opportunity.

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