When reports started coming in about webworm attacks in Kansas soybeans , Sunflower State growers saw startling damage. And if they didn’t get the worms sprayed quickly, web-of-doom critters’ defoliation reached 60% or even 100%.
“If growers see mid-summer defoliation from webworms, the plants can bounce back. But yields might be reduced by at least half if plants are at least 60% defoliated,” says Jeff Whitworth, Kansas State University Extension  entomologist.
Whitworth and growers over parts of Kansas saw damage that was “pretty dynamic” when the webworms surprised them in July and into August. He says treated fields saw gratifying regrowth. But untreated fields were close to finished.
“We had never seen anything like that with webworm infestations,” says Whitworth. “Fields were pretty well defoliated. It surprised me that they came back as well as they did.”
After seeing the rare outbreak, growers will be better prepared for them in 2010, says Whitworth. “If the infestation is from four to 12 worms per row-foot, it’s a serious situation,” he says. “So close scouting is essential.”
Iowa State University (ISU) Extension  entomologists say that larvae are greenish or occasionally brownish orange. There are six black spots (tubercles) on each segment and a stiff hair extending from each tubercle.
They usually spin webs over the leaves they’re feeding on, and larvae quickly crawl backwards and may escape from the webbing when disturbed. It takes three to five weeks for the larvae to complete their development.
When larvae have exhausted their food supply, they often move en masse into an adjacent crop, similar to armyworms. ISU says they pupate in the soil and this stage of development takes one to three weeks.
“These worms may have two to three generations per year in Kansas (and the Midwest) and will get 1 in. long before they are mature enough to stop feeding and pupate,” says Whitworth.
“Remember, soybeans have an amazing capacity to compensate for early season defoliation, since they usually produce excess leaves anyway. Therefore, defoliation often looks worse than the actual impact later on yield. Soybean plants often compensate for stand losses with additional growth.”
They normally feed on host plants like weeds or alfalfa, which can also see webworm big damage. Peak damage occurs July through August with multiple generations possible.
“When growers started spraying for weeds last summer, the webworms looked for a new host,” says Whitworth. “They attacked soybeans in much of southeast Kansas and into west-central parts of the state.
Phil Mulder, head of the Oklahoma State University Department of Entomology , says webworms weren’t as bad in Oklahoma in 2009, but growers should still be aware of their ability to strike and cause quick damage.
“Webworms usually come in fairly late and damage is minimal, but not always,” says Mulder. “Before blooming, soybean plants can withstand about 35% foliage loss without seeing a major loss in yields, but if webworms infest plants that are in the pod-forming stage, plants can withstand no more than a 15-20% foliage loss. After full pod, it goes back to 35-40% foliage loss.”
Whitworth says that if defoliation is 40% or less, yield loss should be minimal. “But if its 60%, the yield loss may be significant,” he says. “The plants may survive with an 80-100% defoliation. But virtually all yields will be lost.”
Growers should take treatment action any time there is defoliation by webworms or other insects. “Soybeans are pretty tolerant, but webworms can do a lot of damage in just a few days,” says Mulder. “Similar to many caterpillar pests, webworms will consume 80% of the forage they eat during the larval stage when they reach the larger size.”
Whitworth notes that since webworms are not usually a field-wide problem, “we don’t have treatment guidelines.” However, there are sufficient pyrethroids and organophosphate insecticides to handle the problem.
“Growers have a lot of flexibility in treating for webworms,” says Whitworth. “From synthetic pyrethroids to a combination of products with pyrethroids and traditional organophosphates, growers have a good selection available.”
Insecticides that should provide control include: Baythroid XL, 1.6-2.8 oz./acre; Sevin, 1-1.5 lbs./acre; Cobalt, 13-26 oz./acre; Delta Gold, 1.5-1.9 oz./acre; Proaxis, 3.2-3.84 oz./acre; lambda-cyhalothrin products, 0.02-0.03 lb./acre; methyl parathion, 1 lb./acre; lambda-cyhalothrin + thiamethoxam, 3.5-4.5 oz./acre; permethrin, 0.1-0.2 lb./acre; Mustang MAX EC, 2.8-4 oz./acre; and Hero, 4 to 10.3 oz./acre. (Additional insecticides may also be available. Go to http://tinyurl.com/CSDWebworms .)
Aerial application may be required because plants may be too large to allow for some ground rigs. “Many growers who discovered the infestations were able to get their fields aerially sprayed the same night,” says Whitworth.