While plant bugs, aphids and spider mites don't strike fear into the hearts of cotton producers like their pre-Bt cotton predecessors did, make no mistake, their control is extremely critical to a producer's bottom line, according to University of Arkansas Extension entomologist Gus Lorenz, speaking on insect control from squaring to first bloom at Cotton Incorporated's Crop Management Seminar in Memphis.
Lorenz noted that recent estimates indicate that although the number of insecticide applications for heliothines have decreased in recent years due to Bollgard cotton, boll weevil eradication and other factors, the total number of insecticide applications has remained the same, indicating a shift in economic importance for these pests. Here's a closer look at the Big Three:
Recent studies indicate an increasing tolerance to neonicotinoids by cotton aphids due to the increasing use of neonicotinoids for both seed treatment and foliar applications, Lorenz noted.
“We put in a trial seven days after a grower had applied 1.75 ounces of Centric that showed about 40 percent control. When you look back at the historical efficacy of this product, I think we would all agree that the results have been much better in the past. We put three rates of Intruder out and there wasn't a lot of difference in control.”
Samples from the population were sent to the Southern Insect Management Research Laboratory in Stoneville, Miss., and a subsequent bioassay indicated that mortality was somewhere around 15 percent to 20 percent. “This is a good indication that we are developing a situation with aphids. The increased use of neonicotinoids over the past few years has probably triggered this to a large degree.”
Lorenz says there appear to be differences in control of early- and late-season mites. “One of the interesting things about spider mites is that control seems to vary with products and time of season. (Mississippi State University Extension entomologist) Angus Catchot has made the observation that they seem like two different critters. Our data certainly suggests that we can make the mite situation worse.”
Lorenz noted that in 2004, a study in northeast Arkansas indicated very good mite control with Oberon and Kelthane at eight days after treatment, but bifenthrin did not perform well. In another study, in mid- to late season, one of the better treatments in the test was bifenthrin (Capture), while Kelthane did not give us good control.
“It's important that we conduct these trials during the growing season to get this information out to the growers to let them know what's working and what's not. Several growers in Arkansas this year are spending in excess of $50 to $60 an acre on mite control. It is a budget-buster. Even the cheaper products are $15 an acre. The Kelthane is only $9 an acre, but it's only as good as it is efficacious, so if it's not working late season, it's not a very good product.”
Lorenz says a study indicates that in-furrow treatments of Temik for thrips can significantly impact the spider mite situation later in the season. “The study included a survey that showed that if you put out Temik, you had one chance in 170 that you would have to make a spider mite application. On the other hand, if you use a neonicontinoid seed treatment, there was a one in 19 chance that you would have to make a spider mite treatment. That has some serious implications for growers.”
Plant bugs have become the primary pest of cotton in recent years and estimates indicate that Mississippi and Arkansas average as many as five applications per year. With recent studies indicating increasing tolerance to insecticides, including pyrethroids and acephate, Lorenz stresses that applications of insecticides be made only when warranted.
“From all our tests, we still consider Orthene and Bidrin to be our standards and what we compare all other products to. We strongly suggest to our growers in Arkansas to not only use sampling methods like shake sheet or sweep nets, but to look at square retention or the COTMAN program to help them make decisions on whether they need to treat.
“The economic loss due to plant bugs is somewhere around $80 million to $125 million just in three states,” Lorenz said. “We already know that we have pyrethroid-resistant and acephate-resistant plant bugs. We're recommending using the neonicotinoids like Trimax and Centric early and saving the standards, Orthene and Bidrin. The new chemistries, Diamond and Carbine, have looked very good for us in our plant bug strategy, and it's important for us to keep rotating that chemistry.”
A four-state study in the Mid-South conducted to determine the level of plant bug infestation that impacted yield showed the importance of judicious spraying, according to Lorenz. Triggers were: untreated, eight plant bugs per sweep, 16 plant bugs per sweep and automatic applications.
Plant bugs never triggered a spray on the eight-plant bug or 16-plant bug thresholds and four applications were made on the automatic trigger. In two Arkansas plots and a single Tennessee plot, there were no significant yield differences between the untreated and treated plots. Another plot in Arkansas showed a 45-pound yield increase for four applications of Centric.
“I think a lot of us have a zero tolerance for plant bugs. If we see one in the field, we are going to treat. But that's not going to work for us. All these resistance issues and insecticide-flaring situations indicate we need to change the way we're doing business
“That's what those shake sheets and sweep nets are for. That's why we do square retention and COTMAN, so we can tell when we need to make those applications. That's where we're falling short.
“In cotton production, we currently have more tolerance and resistance issues than at any other time in cotton production. It all comes down to overuse or misuse of pesticides. If we want to get a handle on this, we have to start paying attention to our insect control right away.
“With increasing concerns of insecticide resistance, we must scout carefully and use pesticides judiciously. Loss of any of the insecticides currently used could be devastating to economical insect control and cotton profitability.”