Christmas in July? It must be bagworms

bagworm bags

Inside each spindle-shaped bag is the larva of the leaf- and ~needle-eating bagworm ( )..

Sometimes I need to heed my own advice and spend more time scouting my garden and landscape for problems. As we all know, small problems tend to be easier to solve than big ones. This past weekend, I ran into a big problem with my Atlantic white cedar (Chamaecyparis thyoides), which was adorned with a variety of nearly identical “Christmas ornaments”. Unfortunately, I quickly realized that Christmas was more than 5 months away, and that these nature-based “ornaments” hanging from the scantily clad limbs were the handiwork of bagworms (Thyridopteryx ephemeraeformis).

Bagworms, the larval or caterpillar stage of a moth that is rarely seen, defoliate many other conifers, including juniper, arborvitae, cypress, spruce, and hemlock.  They also will consume the leaves of many other trees. In fact, these voracious buggers attack more than 120 species of woody ornamentals, including deciduous shrubs and trees such as black locust, buckeye, elm, honeylocust, maple, sycamore, and willow.

Heavy bagworm infestations can lead to branch dieback.  Sometimes trees can be killed outright, especially after having been completely defoliated over one or two seasons.

These gourmands of the insect world intrigue me, mostly because of the spindle-shaped bags constructed by the young caterpillars or larvae as they feed.  Each larva builds a bag of silk and bits of leaves and twigs from the host plant so its appearance varies from plant to plant.

Unbeknownst to me, the bagworms larvae were feasting and toting their bags for 8 to 10 weeks; they gradually enlarge their bags with every molt. When the larvae reach full size, they stop feeding and secure their one- to two-inch long bags to a twig to pupate. About a month later in September and early October, the brown furry male moths with clear wings will emerge, fly to the wingless females, and mate with them inside her bag.

Although the female bagworm is a moth, she doesn’t look like one.  She has no wings, eyes, legs, antennae, or functional mouthparts.  In fact, her soft yellowish-white body never leaves the bag.  After mating, she lays 500 to 1,000 eggs inside her bag before dying, encasing her clutch with her mummified body.  There is only one generation a year.

The best course-of-action right now is to remove the bags by hand to interrupt mating and to reduce next year’s population. These bags may be difficult to remove without damaging the twig, so pruning shears may be necessary to cut the threads. From my single 5 ½-foot tall Atlantic cedar, I picked and discarded 132 bags. Sadly, my wife or daughter expressed no interest in this task; they didn’t believe me when I told them that it was like removing ornaments from a Christmas tree.

OK, I’ve learned my lesson. Next year I’ll be on the lookout for small bagworm caterpillars emerging in late May or June. Egg hatching begins when the black locust flowers begin to fade and is complete when the Japanese tree lilacs are in full bloom. When the young larvae start feeding and building their bags, I will consider applying the bacterial insecticide, Bacillus thuringiensis var. kurstaki (Dipel® or Thuricide®) or spinosad (Ferti-Lome® Borer, Bagworm, Leafminer & Tent Caterpillar Spray or Spinosad). See “Less Toxic Insecticides” for additional controls.

This weekend I felt that I was alone in my battle against bagworms, but I’m not.  Working in concert with my efforts are birds and several ichneumonid and chalcid wasps that parasitize the larvae.  Also, low winter temperatures can damage the overwintering eggs. I just hope that next year the birds, parasites, and parasitoids that relish bagworms pick up their game so I won’t have to.

© Bob Polomski 2015

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