Jack L. Baldwin, Merrill, Thomas A.
LSU AgCenter entomologist Dr. Jack Baldwin says soybean insect pests, especially the leaf-feeding ones, generally have been light this year, but he cautions August always is a critical month for major insect outbreaks.
"Later maturing soybeans are always at risk to defoliators (insects that feed on leaves) during this period, because foliage retention is necessary for proper seed development," Baldwin said.
He stresses that the defoliation level from insect damage in soybeans should not exceed 20 percent to 25 percent after the bloom stage.
The LSU AgCenter expert says the soybean looper and the velvetbean caterpillar are the two most important foliage-feeding caterpillars.
"Soybean looper infestations are increasing in some soybean production areas," Baldwin said, adding, "They can be an economic problem in most areas, but infestations tend to be more explosive where soybeans are grown in the vicinity of cotton."
Velvetbean caterpillars also have been seen during the earlier part of the month, Baldwin said, adding that heavy infestations are more likely to occur in late August or early September.
"These two caterpillars can be present in high numbers at the same time in some areas," the LSU AgCenter entomologist warned. "Although they damage soybeans in a similar manner and are equally damaging, the insecticide recommendations are not always the same."
Baldwin said certain insecticides labeled for use with soybeans will control both caterpillars, but some will not.
"That is why it is important to determine which foliage-feeding caterpillar is present, or if both should be target pests in a particular field," he said. "Furthermore, their economic thresholds or time to treat are different."
If a sweep net is used to sample the field, the threshold for the soybean looper is an average of one-and-a-half caterpillars per sweep, while that for the velvetbean caterpillar is three caterpillars per sweep, according to Baldwin.
The LSU AgCenter entomologist also cautioned there are several foliage-feeding beetles that are pests of soybeans – the most important of which is the bean leaf beetle.
Bean leaf beetles eat small, round holes between the leaf veins, as compared to caterpillars, which cause more jagged, irregular holes, he explained.
"Bean leaf beetle populations already have been heavy in some fields, and infestations often increase in mid-August," Baldwin said, adding, "Bean leaf beetles also have the potential to feed on pods."
Banded cucumber beetles, spotted cucumber beetles and the grape colaspis are other small, foliage-feeding beetles that can be found in soybean fields, according to Baldwin, who said although they seldom are serious problems by themselves, they contribute to the overall level of defoliation when major pests are present.
In addition, the LSU AgCenter entomologist warned stink bugs are always a threat to soybeans, and the threat usually increases late in the growing season.
"Stink bugs often will migrate to late-maturing soybeans as other crops begin to mature and become less attractive," he said, explaining, "Stink bugs feed on the pods and damage the developing seeds. Depending on the infestation level, this can cause a yield loss, a quality reduction or a delay in maturity."
Baldwin said Piezodorus guildinii, a red shouldered stink bug, has been a serious problem in the southern part of the state. "This is a rather new stink bug pest to Louisiana soybeans, and it is more difficult to control than the more-established green and brown stink bug species," he said.