Summer landscapes prone to bagworm, webworm infestations
Monday, August 9, 2021
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In Rogers County, Billy Clark stood in his pecan orchard surveying the thousands of bagworm sacks hanging from tree branches.
Bagworms and webworms are a common sight on trees and other plants in late summer, but why they are more prominent in certain years is a mystery. One is mostly harmless to host plants while the other can be quite destructive.
“I was under the impression they only attacked cedar trees, but to my alarm, that’s not the case,” Clark said.
Although he doesn’t expect much of a crop this year thanks to a spring freeze, he wonders if the bagworms will affect pecan tree yields next season.
Becky Carroll, an OSU Extension fruit and nut specialist, said many growers who annually manage their crops for an insect called the pecan nut casebearer don’t see bagworm problems. Insecticides applied in late May or early June for the pecan nut casebearer can also effectively control bagworms and webworms, but extreme weather has played a role in the recent bagworm uprising.
“The past two years, we’ve had early fall freezes and late spring freezes, so some of our producers who’ve lost their crop haven’t been spraying for pecan nut casebearer,” Carroll said. “Once you miss a couple of years, those bagworm populations start to build.”
Bagworms will feed on foliage and are active year-round, said Eric Rebek, an OSU Extension entomology specialist. Males and females mate, die and leave their eggs in the unsightly bags that hang from tree branches. The bags, made of foliage the worms have weaved together with silk produced by special glands, are used for protection and are typically seen on cedars, junipers and other evergreen trees.
This year, the bagworm population has increased to the point they are now expanding their host range to other plants, such as rose bushes and oak trees. Rebek said it is difficult to identify one factor that creates such an abundance.
“It’s something with the environment, weather or moisture related, that affects what they feed on as well as their natural enemies, such as predators, parasites and pathogens,” he said. “It’s a complex interaction that determines the ups and downs of their population cycle we see over time.”
On the other hand, webworm damage is more temporary. The red-headed and black-headed races of webworms are both present in Oklahoma and build nests on deciduous trees from early July into the fall.
“These trees have produced the photosynthate they will use for food and lose their leaves in the fall anyway,” Rebek said.
Webworm caterpillars drop from the webs and enter a stage of wandering, searching for a place to burrow underground for the winter. The following summer, they emerge as adult moths to mate and begin laying eggs in host trees.
Even though webworms are considered primarily an aesthetic problem, Rebek said pecan producers are at risk of webworm invasions. Any pest that feeds on leaves can affect overall yields and diminish pecan crop returns.
While insecticides are an effective treatment for webworms on deciduous trees, they are not necessary. Burning the insects out of tree canopies or any other kind of fire treatment is strongly discouraged.
“Prune out the webs as they start to appear or use a stick to break open the webs,” Rebek said. “This exposes the caterpillars to wasps and other arthropod predators that will feed on them.”
Bagworm management is ideal in the winter when the bags can be handpicked off trees and placed in bags for disposal offsite. For large infestations, Rebek recommends consulting an arborist or landscape professional. Also, local Extension educators can provide the most up-to-date information on reduced-risk insecticides for evergreen trees.