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Ben Thrash, Extension entomologist at the University of Arkansas System Division of Agriculture, gave a talk on cotton insecticides for plant bugs at the Mississippi Agricultural Consultants Association (MACA) 47th Annual Conference at Mississippi State University.

Cotton insecticides for plant bugs

Ben Thrash, Extension entomologist, says insecticide efficacy on plant bugs varies by year and location, but there are a few stand out products.

Cotton farmers who plant early may avoid insect pest problems, including plant bugs, later in the season.

Ben Thrash, Extension entomologist at the University of Arkansas System Division of Agriculture, talked about insecticide performance on tarnished plant bugs in cotton at the Mississippi Agricultural Consultants Association (MACA) 47th Annual Conference at Mississippi State University in February.

He also said insecticide efficacy varies from year to year and location to location.

"A lot of factors, such as landscape diversity, influence pest populations in cotton. Things like rainfall can promote more weeds in ditches and can affect plant bug populations. Also, planting cotton next to corn can make plant bugs much harder to keep in check," Thrash said.

Planting date and thresholds

"A lot of people preach about the benefits of planting early, but it really can help you avoid some insect problems later in the season," he said. "I know the past couple of years haven't been the best for getting your crop in the ground early, but if you can, that's the best thing to do."

In one example on edge effects, Thrash showed a field that had 15 applications of insecticides for plant bugs, but it was planted next to a cornfield.

"Even though this field got sprayed 15 times, you can still see how much plant bugs reduced yield," he said. "If you can plant away from corn, I would highly recommend it."

Thrash recommends using a black drop cloth when checking for plant bugs.

"We have confidence in our thresholds. Our work has shown about three plant bugs per 5 row-feet in the midseason, and we recommend using a black drop cloth," he said. "Some people like to shake cotton out on the ground to check for plant bugs. However, our thresholds are made for using a black drop cloth, so if you're not scouting the same way we are, then our thresholds are not nearly as accurate.

"Our data show that you find 30% more plant bugs on a black shake sheet versus a white one; I imagine the difference between shaking plant bugs out on the ground and using a drop cloth would be at least that stark."

Insecticide data

Thrash collected plant bug efficacy data across the Mid-South including Tennessee, Mississippi, Louisiana, and Arkansas for the last few years.

"I've been combining our data to see if I can find any trends in how our insecticides are performing over time on tarnished plant bugs (TPB) in cotton. So far, I have 232 efficacy trials entered in from Marianna, Ark.; Glendora and Stoneville, Miss.; and Jackson, Tenn.," he said.

The trials spray an insecticide by ground at 10 gallons per acre to evaluate insecticide efficacy on plant bugs.

"Our average plant bug density at these locations is pretty high," Thrash said. "We're running around 25 or 30 plant bugs in our untreated checks every year on 10 row-feet."

Effective insecticides

Data combined from 2012 to 2019, show some of the most effective insecticides used in cotton for plant bugs.

"We don't have a lot of insecticides that consistently work well," he said. "We have acephate, Bidrin, Transform, and Diamond, which are fairly consistent. However, we're only getting about 70% to 75% control from our best products."

The data targets intense plant bug pressure, so depending on the farm, it will affect the level of control observed.

"How these insecticides have performed individually over time across the Mid-South varies by year," Thrash said. "For some reason, it looks like in 2018 we had a general dip in control across all insecticides, and I don't know if that's weather-related, but it's across all three states. Control can be quite variable from year to year on how well most insecticides perform, and it looked like we were getting a general decrease in bifenthrin efficacy over time."

However, in 2019, bifenthrin efficacy seemed to increase slightly compared to previous years.

"Both Bidrin and acephate have shown variable control from year to year," he said.

"We see this variability across time in imidacloprid, which is not providing a lot of control anyway, according to the data.

"Transform was a little different, and it's been providing pretty stable control over time," Thrash said. "It's the most consistent of any of the products that I've looked at, and I think that's probably because it gets sprayed mostly on cotton, sometimes sorghum. Also, it's a newer product, so that's probably why it's performing more consistently over time as it relates to the other products."

Factors: Year and region

Looking at Arkansas, Mississippi, and Tennessee from 2016 to 2018, data show that the test sites in Mississippi had harder-to-control plant bugs.

"From region to region, there can be quite a bit of variability on how well your insecticide applications are performing," he said. "For Arkansas in 2017, Transform was providing the best control. Looking at Mississippi, Bidrin was providing the best control. In Tennessee, acephate was providing the best control over tested products."

The level of control varies year by year too.

"If you look at 2007 Arkansas data, you can see that acephate was providing a lot higher level of control than Bidrin was," Thrash said. "In 2008, Bidrin was providing a lot better control than acephate. If plant bug susceptibility varies from year to year and you're not seeing the level of control with some products you usually use, it might be time to swap to something else. However, I'm not saying to swap from acephate to something like imidacloprid. Acephate is always going to be better than imidacloprid, but a swap from acephate to Bidrin or something like that might be worth considering."

Thrash also recommends Diamond on plant bugs. With Diamond, the adult plant bugs lay fewer eggs and the eggs they lay are less viable than the ones not exposed to Diamond in the field.

"If we look at Diamond alone, you see initially three days after application there isn't a lot of control," he said. "It's not a knockdown product. However, when you combine it with a knockdown product like acephate, it really provides extended residual control over acephate alone."

Thrash said the best two options for residual control are Transform and Diamond.

"Just remember," he said, "plant bug insecticide efficacy can vary by year, so slow down and pay attention to what kind of control you're getting with some of your insecticide applications."

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