Bollworm and looper numbers are unusually high for this time of year in Arkansas, and growers need to be scouting, or in some places, be ready to spray, say entomologists with the University of Arkansas Division of Agriculture.
“The bollworm population has been one of the highest for this time of year that we’ve seen in many, many a year,” said Gus Lorenz, associate department head-Extension entomology for the University of Arkansas Division of Agriculture. “It’s pretty much statewide, but the south seems to be a whole lot worse than the north.”
Because of the early stage of soybean growth, producers need to keep an extra close eye on the crop because it won’t take long for the bollworms to eat enough to hit the 40 percent defoliation threshold.
“We’re telling everyone not to be misled because the beans are small,” he said. “They don’t have as much foliage, and they can’t take as much feeding on them.”
The decision whether to treat has been “field-by-field,” said Scott Akin, extension entomologist for the UA Division of Agriculture. “Some fields have justified treatment, and others simply have not. It is easy to get worried over some defoliation, but soybeans can compensate pretty well in terms of yield, particularly if the damage occurs before bloom.
“Also, it’s important to hold off on that first insecticide spray for as long as possible in soybean so as not to disrupt any potential beneficial insects in the system,” he said.
The bollworms seem to be coming north from Louisiana, since Arkansas’ snowy winter would’ve likely been too hard for overwintering bollworms, Lorenz said, adding that a colleague in Louisiana had noted significant bollworm numbers there two to three weeks ago.
Size is another consideration in determining whether to treat the caterpillars.
“When it comes to feeding rate, the larger the larvae are, the more they eat,” he said. “The amount of leaf tissue a bollworm can consume, during later larval stages is much more than that of earlier stages. If you already have moderate levels of defoliation with medium-sized worms still present, you may want to treat.”
The bollworms “that we’re seeing are on fields where growers planted into vegetation already growing in the fields, then sprayed an herbicide to kill the weeds,” Lorenz said. “That moved the worms off the weed hosts and into the soybeans when the worms are already large.”
The rise of bollworms this season is re-emphasizing the importance of pre-emergence weed control.
“Henbit has been the most common host for the bollworms in this scenario,” Akin said. “In Mississippi, they seem to be migrating from purslane onto Bollgard II.”
Soybean growers are also seeing loopers early, too.
“We’re seeing a different species that we don’t traditionally see a lot of in Arkansas,” he said. “It doesn’t even have a common name like the soybean or cabbage looper, which are more common.”
The difference is that this unusual looper “doesn’t have the resistance to pyrethroids that soybean loopers have,” Lorenz said. “A lot of people are going to be treating them with the high-priced insecticides, when a pyrethroid is going to be more efficient and cheaper to use.”
Lorenz said there doesn’t appear to be any rhyme or reason why the loopers are appearing in one field and not another.
“Don’t think that just because you don’t have loopers in one field, you don’t have it in another field,” he said. “You need to scout every field.”
Soybean growers also should not believe that the loopers will cycle out of their fields. “Ninety to 95 percent of what they eat in their lifetimes will be in those later instars, or stages,” Lorenz said. “That’s when they cause all the damage.”
After visiting his rice plots, Lorenz also warned rice growers statewide to look for an old enemy — water weevils.
“We’re seeing them and growers need to be scouting for them,” he said.