Released: August 28, 2007

Webworms Attacking Deciduous Trees on High Plains

MANHATTAN, Kan. – The growing season’s second generation of fall webworms and mimosa webworms is creating eyesores in High Plains trees.

The fall webworms start by spinning webs on or around branch tips. But, as they continue eating and growing, they also keep increasing the size of their web and the leaf loss going on inside.

Except for the expanding ugliness factor, that’s not necessarily a problem, according to Bob Bauernfeind, entomologist with Kansas State University Research and Extension.

Killing the web-protected larvae with an insecticide is usually an overreaction – one that’s also difficult to do well, he said. Removing infested branches is simply pruning that’s unneeded and can disfigure the tree.

“Control measures are critical only if fall webworms are feeding on trees that are incapable of handling very much leaf loss. That usually means when you find obvious-looking webs on small, young trees or any trees already weakened by other stresses,” Bauernfeind said. “Healthy, well-established trees typically bounce back the next spring with no noticeable effects.”

K-State researchers have found that redheaded fall webworms – the most noticeable late-season web spinners – tend to prefer birch, flowering crabapple, hickory, pecan, sweet gum, sycamore and walnut trees. Their blackheaded kin – which make smaller, flimsier webs – are likely to be in elm, mulberry and Osage-orange trees.

But, both races like redbuds and the occasional pin oak. In a pinch, fall webworms seem able to substitute almost any deciduous tree. Nationwide, their known hosts now total 90 tree species.

The mimosa webworm probably prefers dining on mimosa trees. From Kansas on north, however, its two annual generations mostly feed on honeylocust trees.

“These larvae aren’t as obvious as the fall webworms,” Bauernfeind said. “For one thing, mimosa webworms are smaller. Like the fall webworms, they feed while enclosed in their webbing. However, the mimosas enclose their web in dead foliage. Often, all you can really see is sections of stuck-together, tannish-brown locust leaves.”

Entire tree canopies can be brown by the time mimosa webworms finish feeding in late fall, he said. Although startling to see, this destruction also has little negative effect for established trees. By late fall, the trees have nearly completed their year’s photosynthesis, so can easily produce new leaves the following spring.

“If you’ve got fall webworms, however, you do need to monitor trees that could be at risk,” Bauernfeind said. “You’ll want to eliminate the infestations – preferably while the worms are still small and not eating very much.”

Simply removing their web masses by hand will stop fall webworms in their tracks.

“If you hate touching webs and wooly worms, use a bristly toilet brush or a pole with a nail driven through one end. Swirl and twirl it until you collect and can dispose of the whole mess,” he said.

If even this approach is too disgusting, however, just tearing the webs open can help. That exposes the fall webworms to their many natural predators, Bauernfeind said.

Several properly labeled, easily available insecticides can kill the worms. But, treating an entire tree is rarely warranted, he said.

“If possible, you need to push the sprayer wand through each web, so you can deliver the insecticide directly onto the worms inside,” Bauernfeind said. “Although it’s a slower approach, you also can get rid of webworms by treating the foliage adjacent to their web – the leaves they’ll be expanding to include next.

“As with physically removing the web masses, however, spraying them is only possible if you can reach them. You just can’t do much about web masses in high places.”

More information about horticultural insect pests is available at any county or district K-State Research and Extension office, as well as on the Web at http://www.ksre.ksu.edu/Topic6.asp.

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K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.

Story by:
Kathleen Ward
kward@oznet.ksu.edu
K-State Research& Extension News