OSU to soybean producers: Identify defoliators before you spray

Friday, September 16, 2016

Caterpillars are feeding on Oklahoma soybeans, but producers need to identify which type of pest is ruining their crop before spraying.

Soybeans come with their own villainous Rogue’s Gallery when it comes to caterpillars: the cabbage/soybean looper, the green cloverworm, the velvetbean caterpillar and the corn earworm.

“They may seem to be lookalikes at first glance but a producer should be able to easily distinguish between them with a closer inspection,” said Tom Royer, Oklahoma State University Cooperative Extension entomologist and integrated pest management coordinator.

Cabbage/soybean loopers are light green in color. As they mature, they develop white stripes, two on top and two on each side. They have three pairs of prolegs, two pairs on the abdomen and one pair at the anal end.

“Larvae move in a characteristic ‘looping’ motion,” Royer said. “They reach about 1.2 inches in length at maturity.”

Green cloverworm larvae can be confused with cabbage/soybean loopers because they move in the same “looping” manner. However, they are pale green in color with two longitudinal stripes. They have four pairs of abdominal prolegs, three pair on the abdomen and one pair at the anal end. At maturity, they measure about 1.15 inches in length.

Velvetbean caterpillars vary in color from green to brown to black. As they mature, they develop white stripes, one on top and two on each side. They have five pairs of prolegs, four pairs on the abdomen and one pair at the anal end.

“Larvae thrash about violently when disturbed,” Royer said. “They reach about 1.9 inches in length at maturity.”

Corn earworms – also known as soybean podworms – are generally pod feeders. They have five pairs of prolegs, four pairs on the abdomen and one pair at the anal end. Body color varies with the host plant, ranging from shades of pink, yellow, green, brown and black.

“Larvae typically have darker or lighter stripes running lengthwise on the body and can be identified by the presence of short, sharp micro-spines between the hairs on the body,” Royer said. “Larvae reach about 1.2 inches in length at maturity.”

Following positive identification, soybean producers then need to assess the level of defoliation in their crop.

“In soybeans, randomly select six leaflets – two from the lower, two from the middle and two from the top of the canopy – in five locations about the field, then estimate the percent of defoliation by averaging the defoliation level from 30 leaflets,” said Brian Pugh, OSU Cooperative Extension area agronomy specialist for the state’s Northeast District.

OSU recommended treatment thresholds vary.

“At the three-leaf stage to beginning of bloom, the treatment threshold is 35 percent average defoliation,” Pugh said. “From bloom to pod fill, the threshold is 15 percent to 20 percent average defoliation. From full pod fill to maturity, the treatment threshold is 35 percent to 40 percent average defoliation or 5 percent to 10 percent of the pods exhibiting damage.”

Selection of insecticides can depend on the predominant species of caterpillar present in the field.

“It is also important to be aware that soybean loopers are often resistant to pyrethroid insecticides so there are more limited choices for control,” Royer said.

Control guidelines and information on registered insecticides are available online at http://osufacts.okstate.edu by consulting OSU Extension Current Report CR-7167, “Management of Insect and Mite Pests in Soybean.”

Soybean growers needing additional assistance should contact their OSU Cooperative Extension county office, typically listed under “County Government” in local telephone directories.

Oklahoma produces more than 10 million bushels of soybeans annually, according to USDA National Agricultural Statistics Service data.