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(Journal photo by Lacey Newlin.)

Several years ago Oklahoma State University Extension Entomologist Tom Royer started noticing tank-mixing of pyrethroid insecticide with winter top-dress nitrogen becoming more popular across his state with continuous wheat. The practice puzzled him and he began to question the possible advantages wheat growers could be gaining from this process.

“I started wondering if it was really paying for itself and why they were doing it,” Royer said. “I had a pretty good idea that farmers didn’t want to make two trips across the field and that was why they were mixing the chemicals, but I was wondering what benefit they were getting out of the combo.”

Royer had learned from several suppliers in the state this combination of insecticide and nitrogen is an extremely common practice in Oklahoma. Royer knew it was a fairly inexpensive treatment to add to the top-dress because they were using a common generic pyrethroid insecticide product, but as a scientist, he was skeptical of the practice and the value it could add.

Additionally, as a proponent and practitioner of integrated pest management, Royer felt obligated to find out if this practice was really beneficial to continuous wheat. To discover the answer to his quandary, he planted plots of the Gallagher wheat variety in three different Oklahoma locations: Stillwater, Chickasha and Lahoma, with four different treatments applied to them.

The treatments were: no top-dress nitrogen, top-dress nitrogen only, pyrethroid insecticide application only and a combination of top-dress nitrogen and pyrethroid insecticide. These trials have gone on for the last three years with Royer’s team collecting data from them. Royer said his team took an insect pre-count before they made the applications to each plot and another count two weeks after the applications to measure the insects present.

“I’ve got to tell you this is where I have to eat some crow, because what we’ve found is the combination of top-dress nitrogen plus the pyrethroid insecticide on average through all the trials that we did, provided a benefit of about a 5 bushels per acre increase in yield over the untreated,” Royer said. “Combining them together was also the most economical thing to do because it meant they did not have to go over the field more than once. However, as an entomologist, I still wanted to figure out what is going on with this combination of treatments. Why does that pyrethroid response add yield response to our wheat?”

Still on the quest to find the reason behind the yield benefits, Royer expanded the trials in the three locations last year by adding four additional treatments, making a total of eight trials in each location in Stillwater, Chickasha and Lahoma. The new treatments were seeds treated with an insecticide at planting versus untreated. Royer said he also acquired a suction device so he could start sampling plots just as soon as the plants came out of the ground.

“We could sample for any insects that were in the soil, but I was particularly interested in winter grain mites and brown grain mites, because they like continuous wheat and I thought the treatment might be affecting them,” Royer said.

Additionally, he was interested in any aphids that might be building up. Royer said one aspect of the second phase of the trial that has been enlightening as far as the insect and arthropod impact, is that in Chickasha they found mostly aphids; Lahoma had a lot more mites and in Stillwater there as a 50-50 mix of both.

“So at different locations we’re seeing different sets of insects we’re dealing with,” Royer explained.

He said his team plans to continue to do the second phase of the experiment at the same locations for another two years and hopefully by the end of phase two he will finally find the answer as to why a combination of pyrethroid insecticide and top-dress nitrogen provides yield increase in wheat.

Lacey Newlin can be reached at 580-748-1892 or lnewlin@hpj.com.

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