As we move June, it’s time to watch for bagworms. The tan, oblong bags from last year’s bagworms are usually quite noticeable right now. The females can lay between 500-1000 eggs inside a bag so populations can quickly build to large numbers within a year or two on infested plants. They will feed on over 130 different tree and shrub species but are most damaging on spruce and arborvitae trees since these plants rarely push out new growth when defoliated. Eggs begin to hatch when catalpa trees are in full bloom.
When a few bags are found on a plant, they can be removed and placed in soapy water or thrown in the trash. But do not just throw them on the ground as they can hatch and still feed on plants. When many bagworms are present a number of soft and traditional pesticides can be used to reduce their numbers. Softer pesticides are most effective when new bagworms are less than ¾ inch long. However, pesticide treatments are only effective while the caterpillars inside the bags are still feeding. By late summer, the adults have laid their eggs inside the bags and died, so pesticides treatments are no longer effective.
Fortunately, the cold temps this past winter are just on the cusp of what will kill overwintering eggs. With a little luck, the number of bagworms in the landscape this year will be reduced.