Does Dollar Cotton Dictate a Change in Plant Bug Thresholds?

With cotton prices nearly double what they were at this time last year, will Mid-South growers be motivated to lower the plant-bug threshold and pull the trigger on insecticide sprays earlier than normal? Will they keep firing away later into the season? Seems logical that they would, right?

“I can understand you would think that. As far as lowering the thresholds because of the increased value of the cotton – I can see why they think that. You have to remember that we call this an ‘economic threshold,’” says Jonesboro, AR, consultant Bob Griffin. “But one problem with this logic are the results from plant-bug research conducted a few years ago. When pre-blooming cotton was treated automatically for plant bugs on sub-threshold levels on a weekly basis, the yields on these automatic treatments were no higher, and many times actually lower, than the untreated checks and treatments based on economic thresholds.

Advertisement

“My thoughts are that this is due to compensation from the plants where a few squares were removed. The cotton plant is very resilient and has a great propensity to compensate,” he continues. “I have often seen that when a plant sets a very high percentage of its squares through bloom. If any stress is incurred, the plant will shed at a much higher rate than plants that had a little less fruit retention at the same time. The bottom line in my opinion is that just by eliminating all pests early season doesn’t necessarily equal greater yields in the end, and definitely doesn’t always mean higher profits.”

Plant bugs are now the number-one cotton insect pest across the Mid-South. That’s been established. What had not been established, as the plant-bug was becoming such a yield-robbing pest, was when to treat.

Dr. Gus Lorenz, an Arkansas Extension Entomologist and IPM Coordinator in Lonoke, says that until four or five years ago, Mid-South Extension entomologists couldn’t agree on much of anything, other than the fact that plant bugs were becoming a limiting factor in cotton production.

Top Articles
Cotton Companion: Adjusting to New EPA Adjustments

“The entomologists in the Mid-South are a tight group and work very well together. The first time we got together, we couldn’t all agree on anything – like what the threshold was, or the correct technique for sampling,” he explains. “We decided that if we couldn’t agree on anything, how can our consultants and growers know when to do it, and how to do it?

“So we started working on a project for, first, deciding what the best sampling methods were. Once we got that done, we started working on our thresholds.”

The entomologists finally decided that up until bloom, a sweep net is the best way to sample, treating when nine to 12 plant bugs are captured per 100 sweeps. Post-bloom, it was determined that using black shake sheets is the best way to sample, and sprays are triggered when the same number of plant bugs are found per three-to-five row-feet.

“We all worked together and the data is very good. We feel very comfortable with those recommendations as they stand today,” says Lorenz. “If growers and consultants use the thresholds to determine when they need to make their applications, instead of just because their neighbor is, or because they think it’s time, or they’re going over the top with Pix, they’ll control plant bugs more effectively. Base your applications on the thresholds. That’s the best way to maintain your profitability.

“You know, it’s all about the bottom line. What history tells us with cotton insects is that insecticide money is better spent from mid-season on, rather than early in the season,” he continues. “We also realize growers don’t plant cotton to have something to spray, they are growing cotton to make a living.”

And by minimizing early season sprays, flaring of aphids and spider mites is less likely to occur. “That’s why we have the thresholds for plant bugs – to help us use our products when the grower is best served, to prevent economic loss, and avoid applications that aren’t needed,” explains Lorenz.

Firing the First Shot
Once economic thresholds have been met, the question becomes what to treat with, and Lorenz says that no one single insecticide is proving to be completely effective.

“We’ve been looking at all of the currently labeled products and it’s become very apparent in those situations where we have high plant bug infestations, a single product is just not adequate anymore,” he explains. “But what we’re looking at is tank mixes, or pre-mixes, or combined chemistries, or insecticide classes, to help us reduce those plant bug populations below the economic injury level.

“There are premixes out there that have been working, like Endigo, which is a combination of Centric and Karate. Another one is Leverage.”

Endigo is manufactured by Syngenta, and has in it thiamethoxam (Centric), a second generation neonicotinoid, and a pyrethroid (Karate). Leverage is from Bayer CropScience and has in it the imidacloprid product Trimax, a neonicotinoid, and the pyrethroid Baythroid.

Also showing promise is the insect-growth-regulator Diamond, manufactured by MANA. Diamond is a third-generation chitin inhibitor that interferes with development of the insect’s ectoskeleton.

Lorenz says the best results with Diamond have been when applications are made during the third or fourth week of squaring.
“When plant bug populations are starting to increase a little bit, we get it in the system with a couple of shots,” he explains. “We have consistently shown about a 150 to 200 pound yield response. So we know it’s working for us, and it’s done a good job.

“It’s not for everybody, but certainly it is a fit for growers who have problems with plant bugs every year,” Lorenz continues. “For growers in areas where plant bugs aren’t that big of an issue, I don’t know how much of an impact it’s going to have for them. But certainly for us, where we’ve tested it in moderate to heavy infestations, we’re very consistently seeing a yield increase associated with it, and better overall control.”

Cease Fire
As far as the end-of season threshold, Griffin says termination of sprays should be based on the boll being safe from a particular insect, not the value of the cotton.

“There are different times to quit, based on heat-unit accumulation, which relates to the maturity or susceptibility of a boll to damage from a particular insect at economic levels,” he explains. “In Arkansas, we have established that when we have an average of five nodes above first-position, upper-most white flower, the field has reached physiological cutout. This means that those white flowers will produce the last economically effective bolls formed on the plants. Just because the price of cotton has increased, doesn’t mean that it is justifiable to treat for insects to protect bolls that will never be harvested.”

0