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Posted 14 June 2010. PMN Crop News.

Scout for Bagworms Now

Source: Kansas State University Press Release.

Olathe, Kansas (May 28, 2010)--Bagworms are among the most recognized tree and shrub pests. They may also be among the least well controlled.


Time and timing are both important, according to Dennis Patton, horticulturist with Kansas State University Research and Extension.

“In Kansas, for example, bagworms typically hatch in late May or early June. That’s when you need to start inspecting closely for signs of activity,” he said. “It takes some effort, but you don’t want to apply an insecticide that isn’t needed. You won’t want to waste one, either, by spraying at the wrong time.”

Bagworms are best known for attacking such evergreens as arborvitae, pine, spruce and juniper (e.g., Eastern redcedar).

But, they also infest deciduous plants, Patton said. Their hosts can include the barberry, blackberry, box elder, cherry, clematis, elm, locust, maple, oak, peach, poplar, pyracantha, quince pear, rose, sumac, sycamore and willow. If necessary, bagworms will even feed on clover, ragweed, parsley and nightshade.

Bagworms can move from plant to plant – as they’ll demonstrate if they strip a host and need more food, he said. Even so, a plant with an old bag hanging down can deserve the closest of inspections.

“Bagworm problems start out small,” Patton explained. “But, the hatch from one old bag can be close to 1,000 new bagworms. Given a couple years to spread, the infestation could clean off a juniper’s foliage within days.”

Baby bagworms are about the size of the point on a sharpened pencil lead, he said. When they hatch out, they’re willing and able to eat foliage. But, they’re so small that both they and their damage can be impossible to see without a magnifying glass.

Each one quickly starts growing, however, and spinning a silken bag around its body. It also camouflages that bag with bits and pieces of host foliage, which turn brown.

“If you’re patient and look hard, you may begin seeing them in a week or so. They’ll be slowly but constantly moving around -- their dot of a bag on their back,” he said. “That’s when spraying can achieve best results. In turn, your plant will suffer little to no measurable damage.

“The longer you wait after that, the bigger the bagworms and their damage will be. And, the more protection their bag will provide. By late summer, a mature bag will actually repel chemicals.”

Many products on the market now are legally labeled to control bagworms. Their product name may or may not be a clue. But, K-State entomologists say the list of ingredients on the container should include one of the following active ingredients:

* (organic) Bacillus thuringiensis or neem oil.

* (synthetic) acephate, bifenthrin, carbaryl, cyfluthrin, deltamethrin, lambda-cyhalothrin, malathion, permethrin or spinosid.

“Any of them can do a good job,” Patton said. “But, you’ve got to follow label directions exactly. Your timing has to be as early in the bagworms’ life as possible. And, your coverage must be thorough.

“The bagworms may be feeding toward the outside of the plant. As often as not, though, they’re working their way into the interior branches. You’ve got to get in there, too, and apply a thorough covering of the spray mixture. Otherwise, you won’t be able to soak all of the little bags.”

Some products may recommend a follow-up spray, and that can be a good idea for heavy infestations, he said.

Plant owners might as well wait until late fall to early spring, however, if they act fairly soon to achieve good timing with a first spray. By fall, the bagworms will be wrapped up, but full-size. And, their bags will be easiest to see in deciduous plants.

“Goodness knows it’s possible to control bagworms by handpicking them and smashing or trashing the bags. People did that for generations,” Patton said. “If your pine gets very tall, though, or you’ve got lots of infested trees … or an infestation gets really heavy, handpicking can be impractical – perhaps impossible.

“After all, missing just one can be a thousand-worm mistake.”

Kathleen Ward