Thrips problematic in 2006 Georgia cotton

This past year was a “non-issue” one for cotton insects in Georgia, with very low populations across much of the state, especially late in the season, says Phillip Roberts, University of Georgia Extension entomologist.

“But there still are some important things we observed this past year that we should keep in mind, with hopes of improving our cotton insect management programs,” says Roberts.

Year in and year out, regardless of conditions, growers will see thrips on seedling cotton, he says. “We’re all familiar with the increase we saw in thrips in 2006. One thing we did observe this past year was that we had relatively cool temperatures with slow seedling growth, and that’s when thrips can really hurt you in terms of yield — whenever you have poor vigor and poor growth,” says Roberts.

Another thing growers learned about thrips this past year was that one to two-leaf cotton is much more susceptible to yield injury from the pests than four to five-leaf cotton, he adds.

Five years of data shows a cotton yield increase when growers use a recommended thrips treatment, says Roberts. Thrips, he explains, feed on cotyledons and unfurled leaves in the terminal bud. Damaged cotyledons appear shiny and silvery whereas damaged true leaves are crinkled and misshapen.

“Moderate thrips damage stunts plants and delays maturity,” says Roberts. “Loss of apical dominance or stand loss may occur in severe situations. A preventive systemic insecticide is recommended at planting for control of thrips. A summary of thrips control research trials conducted in Georgia since 2000 indicated that recommended thrips insecticides used at planting increased yield about 90 percent of the time.”

A failure to use a preventive treatment can result in excessive thrips injury and the need for multiple foliar sprays, he says.

Recommended at-planting treatments for thrips control include Temik 15G, Orthene as an in-furrow spray, and the seed treatments Cruiser, Gaucho Grande, and Orthene. Temik granules are applied in the seed furrow at planting and generally provide control of thrips for four-plus weeks after planting, depending on the rate, says Roberts. Orthene in-furrow sprays are seldom used in Georgia but are an option for thrips control, he says. “The seed treatments Cruiser and Gaucho Grande have performed similarly in Georgia and typically provide thrips control for about three weeks after planting. Cruiser is the thrips insecticide component in Syngenta’s Avicta Complete Pak and Gaucho Grande is the thrips insecticide component in Bayer’s Aeris Seed Applied System. The Orthene seed treatment provides thrips control for about seven days,” he says. Roberts emphasizes that slow seedling growth compounds the resulting damage from thrips. “Thrips must feed on seedlings to obtain a toxic dose of the systemic insecticide used at planting. If the plant is not growing, feeding injury is often compounded due to the fact that many thrips have repeatedly fed on the same unfurled leaf in the terminal bud. As seedlings develop, they become more tolerant to thrips injury. “For example, we expect greater yield losses from similar thrips injury symptoms on one to two-leaf cotton versus four-leaf cotton. Cotton seedlings are susceptible to thrips until plants attain five true leaves and are growing rapidly.” In Georgia, Roberts says he is seeing more automatic applications of a thrips insecticide with glyphosate at the four to five-leaf stage, and that’s a bad thing for two reasons.

“One, it is uncommon to observe a yield response to foliar thrips sprays applied at the five-leaf stage. And two, the disruption of natural enemies increases the likelihood of aphid and/or spider mite outbreaks.”

The decision to treat thrips with a foliar insecticide should be based on scouting, he says. “Our threshold is two to three thrips per plant. When scouting for thrips, the presence of immature or wingless thrips suggests that the systemic insecticide is failing and a foliar treatment may be justified. In reality, most decisions to treat thrips are based on plant injury symptoms. When looking at plant injury symptoms, focus on newly expanding leaves.

Recommended foliar insecticides for thrips include Bidrin, dimethoate and Orthene.”

Also in 2006, about 400,000 cotton acres in Georgia were infested with spider mites, says Roberts. “That’s a much higher population than we’ve ever seen in Georgia. We’re just fortunate that these mites never exploded, and that had a lot to do with decisions made by growers and their county agents.”

The insect pest that caused the most questions this past year was the aphid, he says. “We have cotton aphids each and every year, but in 2006, they built to very high numbers and lingered for an extended time. Typically, we wait on a fungus to take out aphids, but that was a very slow process this past year. We cannot show a consistent yield increase from aphid control, but we do know that aphids sometimes impact yield. If stress is already high, aphids could put the plant into cutout.”

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