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Invasion of the Bagworms

Act now or they’ll devour your narrow-leafed evergreens

Keep your eyes open and you’ll notice large sections of brown foliage in arborvitae, junipers, Leland cypress and pines. Look closely and you’ll see thousands of bagworms dangling from the branches.
    As of August 1, the bag worms’ bags were over one inch long and at a stage where they are impossible to kill even with the strongest pesticide. The fat bags contain females certain to have hundreds of eggs ready and waiting to be fertilized by a male. The spindly thin sacks are males, anxious and getting ready to go at those fat females and make whoopee. Next spring, all those eggs will hatch.
    With regard to the damage they have caused, if the entire plant has turned brown — with no signs of green foliage — it is dead and will never recover. Trees and shrubs with some surviving green may survive in part; those brown areas devoured by the bag worms will never recover. Conifers such as arborvitae, false cypress, fir, juniper, Leland cypress, pine, spruce and many others are unable to generate adventitious buds; nor do they have dormant buds that can generate new branches.
    If you have plants covered with bagworms, the best thing you can do is to cut the plant down and burn it or deliver it to the county’s recycling centers and have them grind it into chips. Do not simply cut it down and store it behind the garage. If you allow those heavily infested plants to remain in place, you are guaranteeing a generous supply of bagworms next spring to invade your other plants as well as those of your neighbors.
    I have never seen so bad an infestation of bagworms as I have this year. I have handpicked hundreds of them off my Christmas trees. Some trees are more susceptible than others.
    Often I will find only one or two on one tree but handpick a hundred or more from an adjacent tree. This is not only true for white pine but also for Douglas fir and blue spruce.
    As the growing season progresses, you will notice that the bagworms are migrating toward the ends of the branches. So when you are hand picking, look inside the plant for bagworms that are working their way out. When I handpick bagworms, I throw them on the ground to encourage the growth of the BT bacteria in my orchard. Once the bagworms are removed from the tree and thrown on the ground, they are unable to drag their bags back to the tree. Thus they starve and die.
    The only narrow-leafed evergreen that does not attract bagworms is the yew (Taxus). Several species and varieties are available, from ridged upright to spreading. Yew has rich, dark-green foliage and can be pruned or sheered into desired shapes. However, yews should not be planted were young children play because the foliage contains toxic alkaloids, which is why bagworms do not feed on them.

Ask Dr. Gouin your questions at [email protected]. All questions will appear in Bay Weekly. Please include your name and address.