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How to kill bagworms

Those ugly tree pests can be treated
Bagworms. Photo courtesy of the University of Delaware

Bagworms are among the most recognized tree and shrub pests. They may also be among the least well controlled.

The younger the larvae, the easier they are to control, explained Bob Bauernfeind, K-State Research and Extension entomologist. The worms’ youthful size is just one reason that’s true. At 1 millimeter long, new larvae are almost undetectable. They have lots of growing to do. Even so, they don’t chow down as soon as they hatch. Instead, the larvae spin a protective silk bag around their body -- leaving strategic holes for eating, pooping and slo-o-owly walking.

“Bagworms are all about defense,” Bauernfeind said. “This year’s crop has grown enough that if you’re patient and focused, you can see them without a magnifying glass. They’re sort of hiding in plain sight, though. They’ve already started to ‘decorate’ their gradually expanding bag with bits and pieces of foliage.

“As those ‘decorations’ dry out, the bag becomes a bristly brown. It also becomes increasingly hard to penetrate with sprays.”

Unfortunately, that’s the point at which many homeowners begin to notice bagworm damage. And, when larvae are big enough to eat that much, their protective cover is almost complete.

Hurried treatments won’t provide good control, even while larvae are small. Sprays must be thorough, from branch tip to trunk and from top to bottom, he warned. Reaching the inside, as well as the outside foliage takes time and may require refilling spray equipment.

Many labeled products will reduce bagworm numbers, Bauernfeind added. Organic options (most effective on young larvae) include Bacillus thuringiensis and spinosid. Popular chemical controls’ labels list such active ingredients as acephate, bifenthrin, carbaryl, cyfluthrin, cyhalothrin, malathion, or permethrin.

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.

 

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