Treatment Thresholds, Proper Timing Keys to Plant Bug Management

As of May 15, USDA reports that 37 % of the U.S. cotton crop has already been planted. That means young cotton will be maturing quickly. And plant bugs will be waiting.

Dr. Scott Stewart with the University of Tennessee says that growers need to be aware that they may be dealing with multiple species of plant bugs, including tarnished plant bugs and clouded plant bugs. But regardless of the species, the treatment thresholds remain the same.

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“We’ve done this research collectively for a number of years in the Mid-South,” says Stewart in a video post on the UTCrops blog. “I want people to have confidence in our thresholds, and the first thing they need to know is what they are. Plant bugs are annoying, but they are manageable. And don’t forget to account for stink bugs and other pests.”

At Squaring

During the first two weeks of squaring, the threshold is a little more aggressive, triggering treatments if eight or more tarnished plant bugs are found per 100 sweeps with a sweep net. The reasoning is simple: because square population is lower at that time, doesn’t take very many plant bugs to knock square retention back.

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“Any time prior to bloom, if you start getting into that 80-85% square retention range, I would consider reducing, but not disregarding, the thresholds and cutting them back maybe 30-40-50% based on what your square retention is,” notes Stewart. “If you have very poor square retention, you probably need to get on a scheduled spraying to help build that square population back up.”

By the third week of squaring, the threshold relaxes to about 15 plant bugs per 100 sweeps. Since square populations are bigger at this time, it takes more plant bugs to knock a higher percentage of squares off.

At Bloom

At first bloom – especially the second week of bloom – Stewart recommends a shift from sweep nets to drop cloths, which work better capturing immatures, which is the biggest part of the plant bug population at that time. On a drop cloth, treatment would trigger at three or more plant bugs. With a sweep net, the threshold is 15 or more per 100 sweeps.

“Once we get into bloom, I start giving growers some cheats when you’re dealing with a complex of plant bugs and stink bugs,” adds Stewart. “For clouded plant bugs, I tell people to treat them equivalent to 1.5 tarnished plant bugs because, on a per bug basis, they do a little bit more damage. They’re a little bit bigger and may also be more likely to feed on bolls. I also tell people to consider stink bugs as three tarnished plant bugs.”

What Treatments Work Best?

The results of insecticide trials conducted in 2020 using two applications over three rating dates based on treatment thresholds were similar to tests from prior years and in other Mid-South states. Those studies showed that the top three insecticides for reducing plant bug populations were Diamond, Orthene and Transform.

“However,” points out Stewart, “the tests used multiple treatments of the insecticides, and that’s not necessary what I would recommend. I’d recommending mixing or alternating insecticides to match your pest population. Growers need to do a good job of managing plant bugs, or they’re potentially leaving a bale of cotton on the ground.”

What’s Coming?

Stewart says that some growers and consultants will have Bayer’s new ThryvOn Bt technology for some large scale testing this year in anticipation of a full-scale launch in 2022. To date, the technology has shown excellent efficacy in reducing thrips injury.

“For tarnished plant bug management, what we’ve seen is the technology delays and reduces the buildup of plant bug populations,” he explains. “And because of that, we don’t have to make as many insecticide applications – maybe one for thrips and one for plant bugs and stink bugs. I think that’s typical of what we’re going to see when this technology is introduced, but we don’t really know for sure yet. One thing we do know is we’ll still have to manage and treat for tarnished plant bugs. It’s just going be less.

“The good news is our current thresholds work very well with this technology,” he points out. “If you follow those thresholds, you’ll likely realize those insecticide savings.”

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