Insecticide restrictions important as soybeans mature

An early soybean harvest is fast approaching amid the worst drought in decades, making it that much more important for growers to pay attention to restrictions on use of insecticides as plants near maturity.

Even though soybean growers still are seeing spider mites and aphid emergence, a Purdue Extension entomologist warned against applying insecticides past the R5, or beginning seed, growth stage.

"Whether it’s due to spider mite damage or in combination with the drought and heat, maybe soybean fields are stressed this season and will likely see an early harvest,” Christian Krupke said. “Keep in mind that although active spider mite populations, or even bean leaf beetle leaf or pod feeding, might still be found, all insecticides have a pre-harvest interval that is stated on the label.”

Those intervals range from 18 to 45 days before harvest, depending on the insecticide.

Soybean plants begin to senesce or “shut down” once they reach the R5 growth stage, so insecticide applications are not enhancing yields at that point, Krupke said.

"We don’t have any mite-specific data to go on, but for soybean aphids we have not seen yield benefits when treating at or beyond R5, regardless of pest pressure,” he said. "There is nothing to be gained by wiping out insects feeding on what is essentially dying foliage - it’s not contributing anything to yield at this stage, anyway.”