I had a call from a woman in distress about thinning foliage on a row of arborvitae. She described some little “pine cones” hanging on the trees tree that occasionally wiggled. When pulled apart, she found a caterpillar and a lot of silky webbing holding the “pine cone” together.
What she had discovered was bagworm, an insect that had until recent years was not been able to live north of Indianapolis. They have now adapted to our warmer climate and can survive northern Indiana winters.
She obviously had these creatures in her trees for several years, thinking the “pine cones” were a part of the tree. These insects lay eggs inside those pine cone shaped bags. The adult females die inside the bag after laying eggs. Those eggs overwinter in those bags. The number of babies that can hatch from a single bag is unimaginable. A single bag can have as many as 1000 eggs inside, and a singe tree might have a hundred or more bags dangling from the branches. That’s a lot of babies, hidden in plain sight.
Bagworms feed on a number of different plants in the landscape, but they thrive on junipers, arborvitae, spruce and pine. I have seen them on maple, oak, walnut, apple, peach, cherry and many ornamental trees. As the young bagworms feed, the protective bags grow in size, becoming more destructive, more noticeable and harder to control. It is important to get them under control early, before they take over the tree.
Bagworm eggs hatch in late May or early June, which makes this the ideal time to eradicate them. Soon after hatching, a worm about the size of a pencil lead starts eating the foliage. As the worm grows it spins a silken bag around its body, camouflaged with bits and pieces of the host plant’s foliage. By late summer the bag may be several inches long and become impossible to control.
Control after hatching is ideal, due to the fact that they are small and no measurable plant damage has occurred. There are numerous products on the market to control bagworms ranging from cyfluthrin, permethrin, malathion, Orthene, or an organic product called Bacillus thuringiensis that are labeled for use on trees.
Thorough spray coverage of the whole planting is important, especially in windbreaks or screening plantings of arborvitae. The bagworms can be feeding on the outside of the plant, or deep inside the shrub and well hidden. In large plantings or with tall trees, you may need to hire a licensed professional to do the job.
As summer progresses, the bags become larger and control is more difficult. A mature bag with the silk inside will repel the spray solution, making the spray job useless. By the arrival of fall, the only method of control is to hand pick and remove the developing bags.
Do not be caught by surprise late in the season when the damage is done. Take a few minutes now to walk through the landscape inspecting the plants closely for the signs of bagworms. This will help head off big problems and save you time and money in the long run.
By the way, bagworms are NOT related to the critters that create the webbing in our trees in the fall. Webs in the fall are created by another critter called a fall webworm. Webworm damage may be ugly and very visible, but since the defoliation occurs late in the summer, webworm damage does little harm to the trees. Fall webworm damage is more cosmetic than harmful. So do not bother spraying fall webworms.
Certainly, do not burn the webs out! The flames will definitely injure the branch much more than the webworm feeding.
By Jeff Burbrink, Extension Educator – Ag & Natural Resources, Purdue Extension LaGrange County
