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Plant Protection - Bagworms

The bagworm, which commonly attacks junipers, arborvitae, pine, and hemlock and other conifers, is an interesting caterpillar that carries its home on its back.  The spindle-shaped bag is one to two inches long and made of extremely strong silk combined with bits of leaves and twigs from the host plant.  In this part of the country, they are most commonly seen from late spring to late summer.

In late summer, the worm pupates, or rests, and upon hatching, becomes in the case of the male, a one-inch furry black moth.  The male moth flies about in search of a female, who is wingless and legless and practically never leaves the bag.  The male mates with the female through a hole in the bottom of the bag.  The female then lays 500 to 1000 eggs in the bag and dies.

The eggs overwinter in the bag and hatch out in late spring to become worms which eat the leaves of the host plant and create their protective bags.  The worms are brownish-black with a yellowish head.

If you find a lot of bags on your tree in late summer, there is little you can do in the way of spraying, since the tough bag is such excellent protection.  At this time, the best thing to do is remove the bags by hand and destroy them by fire or burial so that the eggs do not overwinter and reinfest your trees the following spring.

If you find the bagworms when they are still small in late spring, you can spray your trees with an insecticide such as diazinon, dimethoate (Cygon, De-Fend) or Bacillus thuringensis (Biotrol, Thuricide).  Bacillus thuringensis, sometimes called Bt, is a biological insecticide used to control caterpillars.  While toxic to caterpillars, Bacillus thuringensis is non-toxic to spiders and other predators.  Inspections and sprays should be timed to control the worms before much damage is done to trees.  When using any chemical, be sure to read the label and follow directions.

If left uncontrolled, the bagworm can become a serious pest of your ornamentals.  In heavy infestations, whole forests of red cedar have been stripped of their foliage by the worm.  Treat foliage with a protective spray as soon as the first worm is seen because they grow and eat at such a rapid rate.

Birds such as woodpeckers, sapsuckers, and titmice are fond of eating the worms in their bags, so if you can encourage these birds to settle in your area, you may be able to reduce the population of bagworms by biological control.

READ AND FOLLOW ALL LABEL DIRECTIONS AS IMPROPER USE OF INSECTICIDES MAY AFFECT ANIMAL AND HUMAN HEALTH.