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In managing bagworms, timing is everything

By Al Cooke
Posted Thursday, April 14, 2011

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Pittsboro, NC - In managing bagworms, timing is everything. No matter what product you select, there are certain times of year when no product is effective. That's one of the side effects of product oriented solutions. Effective control of bagworms is enhanced if we understand the insect's life cycle and time our efforts to coincide with vulnerable periods.

Right now, the bags on the trees are either empty (previously occupied by a male) or contain about 500 - 1,000 eggs (previously occupied by a female). Since insecticides will not penetrate the bag or cocoon, it's futile to use those products now. Hand removal to the extent possible is a better use of time. But don't risk life and limb on a ladder that is not stable or if you are not comfortable up there. You may find a pole pruner helpful. The good news is that they are also not doing any damage now.

Sometime about May, the eggs will start to hatch. The hatchlings will leave the cocoon suspended by a delicate silken thread and land where the wind blows them, usually on the same plant or one nearby. The young insect will begin to feed on the plant and spin a protective cocoon, which it will decorate and camouflage with pieces of the host plant. When first noticed, these cocoons may be only about ½ to 1 inch long, pointed on both ends and larger in the middle.

Once the insects hatch and start feeding there is a vulnerable window of opportunity for the human to interfere with use of products. It's probably the best use of your time to actually wait until late May to early June to allow most of the eggs to hatch so you can get them all at once. In that young stage, they are incredibly vulnerable to fairly safe, and sometimes organic, insecticides such as BiobitHP, DiPel, Foray (or other brands of Bacillus thuringiensis kurstaki, a.k.a. B.t.) or Conserve SC (Spinosad). Follow the directions on the label for best results. In any case the product must be put in areas where the insects are feeding. If it's high up in a plant, you'll still need to come up with a safe method to access them or hire someone to do it.

Later in the summer, the insects are larger, have done more damage, are not as hungry, and are more difficult to kill. Gradually the more benign products are less effective, and your fallback products include Orthene, Talstar, and Sevin. Eventually the caterpillars will pupate and insecticides are no longer effective. In late summer the males, now a moth, leave to find a female and mate. The female never leaves the cocoon beyond sticking her head out to feed and acquire decorations for her "bag." After mating, she lays eggs and dies. The cycle is complete for the year and will repeat itself.

Few plants can tolerate the damage for more than about three years. For pictures and a more complete summary, see the Insect Note here.

Incidentally, some folks refer to eastern tent caterpillars as bagworms - different insect, different strategy. But if you are plagued by visible tent-like webbing in the branch angles of trees, you may be interested in the insect note here.

Al Cooke is a Chatham County Extension Agent, Horticulture

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In managing bagworms, timing is everything

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