Research Highlights

Does Rain Wash Off Recent Insecticide Applications?

By Laura Temple

Pop-up rain showers or thunderstorms that don’t show up on the weather radar commonly occur during Southeast summers. Throughout that time of year, insects can reach treatment thresholds in soybean fields.

“South Carolina farmers typically spend between 5 and 20 percent of their total annual input costs on insecticides,” says Michael Plumblee, Extension corn and soybean specialist and Assistant Professor of Agronomy at Clemson University. “If it rains immediately after an insecticide application, do they need to re-treat?”

Plumblee only found vague or non-existent statements about rainfastness on common soybean insecticide labels. In 2021, the South Carolina Soybean Board agreed to invest soy checkoff funds to answer this question for farmers.

“I teamed up with Clemson entomologist Jeremy Greene,” he says. “We designed a trial to simulate rainfall after applying a full rate of bifenthrin, a common soybean pyrethroid insecticide found in products like Brigade^®.” 

They found that fitting a high-clearance sprayer with the largest nozzles available and spraying water mimicked unexpected showers well. One tank of water was approximately equal to a 0.3-inch rainfall on each plot, similar to a typical pop-up event. 

“Usually, the duration and intensity of these showers creates more of an issue than the volume of precipitation,” Plumblee says. “The large nozzles created drops similar to these rains.”

The insect complex, consisting of the species composition and population levels, was monitored frequently. Pests included multiple species of stink bug, kudzu bug, threecornered alfalfa hopper, soybean looper, velvetbean caterpillar, grasshoppers and more. Overall, insect populations were low in 2021. Stink bugs — including green, southern green, brown and redbanded species — finally reached the economic threshold late in the season. The economic threshold is when pest levels get high enough that the cost of damage from the pests will soon be more than the cost of treatment.

After the researchers treated the replicated plots with bifenthrin insecticide, they simulated rainfall at five different intervals after application: immediately to 30 minutes, 1 to 1.5 hours, 3 to 4 hours, 6 to 7 hours, and 24 hours. The team also simulated rainfall on untreated soybeans at the same time intervals to see how a pop-up rain would affect insect populations.

Consistent Insect Control

Plumblee and Greene’s team counted the pest complex 1 day, 7 days, and 12 days after the insecticide treatment. When analyzing the data with a focus on stink bugs, all treated plots displayed an adequate level of pest control. Though the immediate to 30-minute interval showed a spike in stink bug population a week after treatment, the count dropped several days later to be statistically similar to the rest of the treated plots.

“Based on one year of research, control remained similar regardless of when the insecticide was washed off,” Plumblee says. “The general consensus was that we saw no yield effect from the insecticide wash-off. However, we had a good growing season with late pest pressure that wasn’t likely to impact yield.”

He leveraged the soy checkoff investment in 2021 to secure other funding to continue this trial in 2022. He would like to have multiple years of data before making firm recommendations to farmers. 

He notes that the treated plots had significantly less pressure than the untreated plots.

“So far, it appears that the simulated rain didn’t impact insect populations or insecticide efficacy,” he continues. “If these findings hold true as we continue the trial, pesticide applicators will know they have more flexibility when applying insecticides. But it also means that scouting should continue regardless of pop-up rains, because we don’t have evidence that they knock back pest populations in soybeans.”

Plumblee and Greene have conducted a similar trial in cotton that incorporates additional insecticides with different residual periods, some of which are also used on soybeans. If they find other insecticides have a higher sensitivity to washing off in cotton, they plan to test those products in soybeans.

“We expect to be able to refine recommendations as we learn more,” he says. “Farmers need to cover a lot of ground, and when insects reach economic thresholds for treatment, timing matters. This research will help them understand insecticide efficacy if it rains shortly after application and determine if resprays are needed or not. They will be able to make decisions both to protect yield and profitability.” 

Published: Aug 29, 2022