The Greener View: Bagworms

Jeff Rugg on Aug 7, 2024

Q: My red maple has lost a lot of leaves and seems to be growing pine cones. What could be the cause?

A: The "pine cones" are probably bagworms. They can be a serious pest. Bagworm caterpillars create protective bags from bits of leaves from the plant they are living on, so a maple, elm or linden bag will look different than a spruce, arborvitae or juniper bag, and the bags do look like pine cones. The bags become visible as the caterpillars defoliate the tree.

Adult female bagworms are wingless moths. A female lays 500 to 1,000 eggs in the bag she creates around herself from leaves of the plant she was born on. They hatch in May or June. The caterpillars will eat lots of leaves, and as they grow, they will enlarge their bag. They haul the bag around with them like a sleeping bag, and whenever they are disturbed, they duck back into the bag for protection.

The caterpillars keep eating and growing until August or September. The wingless females stay in their bags, and the males develop wings and flies around. After mating, the males die, and the females fill their sacks with eggs and then die. Since the females don't move very far, the infestations spread slowly from plant to plant. Caterpillars move from plant to plant by crawling and by sticking to animals who move them to a nearby plant. The tiny caterpillars can also release a strand of webbing that allows them to float in the wind to other plants.

The bags prevent the caterpillars, females and eggs from being killed by most insecticides. The best, safest and most pesticide-free time to kill off an entire population of bagworms is from fall through winter until early spring while the eggs are still in the bag. The 1- to 2-inch long bags remain hanging on the tree. Many of them may be empty bags from previous years or empty male bags. Pull them all off and stomp on them, burn them or put them in a bucket filled with soapy water.

If nothing is done until the caterpillars hatch and are crawling on the tree, then the best and safest pesticide product to use is one that includes the bacteria Bacillus thuringiensis. This will kill young caterpillars, but as they mature and create large protective bags during the summer, handpicking again becomes the best way to kill them. A systemic insecticide applied to the tree early in the season will kill the bagworms as they eat the leaves.

For the first year or two, the bagworm population may not be high enough to cause enough damage to be noticed. But at some point, there will be enough of them on the tree to devour all of the leaves. If there are not too many this year, be sure to pull off all the bags so that there won't be any next year.

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Email questions to Jeff Rugg at info@greenerview.com. To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.