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Thrips: The tiny bug that can be a huge problemBy Jennifer M. Latzke For many growers, reduced insect pressure tops the list of benefits to growing cotton in Kansas and Missouri and other parts of the northern Cotton Belt. Unlike cotton patches to the south, the northern part of the Cotton Belt doesn't have the large threat of boll weevils, or pink bollworms, or even cotton fleahooper. And that helps growers out tremendously, especially with the shorter growing season earlier planted cotton faces. But, according to the 2007 Cotton Insect Losses report from Mississippi State University, sponsored by the Cotton Foundation, Kansas and Missouri did suffer some insect losses last year, mainly from thrips. In 2007, Kansas cotton growers had an estimated 850,000 acres infested with thrips, out of a total of 1.01 million acres. The damage from thrips alone cost Kansas producers 1,771 bales of production, or about $552,552, according to the 2007 Cotton Insect Losses report. Missouri faced 270,250 acres infested with thrips in 2007, with yield losses of 6,600 bales, or a little more than $2 million in cotton production. For the northern cotton grower, surrounded by millions of acres of wheat where thrips overwinter, this tiny little insect can be a major pest. A tiny nuisance Thrips are less than two millimeters long and may be brown, yellow or gray. They have rasping-sucking mouthparts, which they use to harm the plant tissue and suck its liquids. They find cotton particularly palatable, and will rasp the tender leaves and terminal buds of cotton seedlings. Because of farming cycles in Kansas and Missouri, thrips tend to migrate from maturing wheat in the spring to new cotton seedlings that are just emerging from the soil. Phillip Sloderbeck, a professor at the Kansas State University Southwest Area Extension Office, Garden City, Kan., said thrips is one of the most consistent pest concerns for Kansas and other northern cotton growers, because of nearby wheat production. Wheat fields are food havens for thrips. Ironically, Sloderbeck explained, producers may be unaware they have a thrips problem in their wheat because the pest doesn't do a lot of noticeable damage to maturing wheat in the late spring, and because the insect is so tiny. "With so many million acres of wheat production, there is a huge reservoir of potential thrips," he said. The potential for a thrips infestation is there. "We're usually planting cotton as wheat is drying down," he said. "We'll see large migrations of thrips out of that wheat into cotton. It varies from year to year, but it's pretty consistent." Thrips will also feed on soybeans, alfalfa and a few other crops, but Sloderbeck explained cotton is especially susceptible because it is a small plant, and it's emerging under harsher weather conditions in the north. "We plant as early as we can and that can stress cotton," he said. "Thrips damage is a combination of stressed cotton seedlings with insect damage." Northern cotton growers can't avoid the issue, either, by planting cotton later, Sloderbeck added. Cotton already has such a short growing season and northern growers have to plant it early as they can to get the longest amount of the growing time they can. Further south, into the Texas cotton patch around Lubbock, they can plant later and run into less thrips trouble. According to the Kansas State University 2008 Cotton Insect Management Guide, light thrips infestations will delay plant growth and maturity. Heavy infestations, though, may kill or damage terminal buds, causing the cotton to branch abnormally, or may kill the plant off entirely. The University of Missouri Extension Service's Cotton Pests Scouting and Management guide explains that brown, wrinkled leaves are the typical damage symptoms from thrips. This damage lowers the plant's capability to conduct photosynthesis. If the infected field has poor stand density, or early-season crop growth, the delay in maturity will harm the final yield. Seed treatments are key Controlling thrips damage in cotton, especially in the north, begins with a seed treatment, Sloderbeck explained. "Some seed treatments can be used, and it's probably a good idea for a lot of producers to look at something before planting," he said. "A seed treatment, or a pre-plant treatment can give a producer time to make added control decisions after the cotton emerges if they need to. If a population ends up being light, then they could get by without added treatment. Other years, they might see that they'll need additional treatment. If they don't put anything down, though, they have to be right there in the field making those decisions." Waiting to treat could be disastrous if the thrips infestation is large. It's difficult to spot a thrips infestation from the highway, so KSU Extension advises beginning scouting as soon as cotton plants begin emerging. UM Extension advises using a cardboard box, lined with white paper, placed on the ground and angled toward a group of cotton plants. Shake the plants over the box, and count the number of thrips that fall onto the white background. Growers should sample five plants at each of 10 random locations per field. In some instances, a farmer may want to pull a few cotton plants from the field and bag them for later study. Thrips damage, according to KSU Extension, makes new cotton leaves appear "ratty." UM Extension gives four classifications of thrips infestations: --None; --Low to new leaves with slight browning along the edges and a few observed thrips from sampling; --Medium to new leaves, with considerable browning along edges and silvering on the underside of the leaves. Thrips will be common in samples. --High to leaf silvering, which is terminal injury. Deformed plants will be common and thrips will be numerous in samples. KSU Extension advises treatment when thrips populations are higher than one thrips per true leaf, up to the six leaf stage. "From emergence to small seedling, the cotton is the most vulnerable," Sloderbeck said. "By the time it gets to the four or five leaf stage, it's normally outgrown the thrips problem." Once cotton plants are about four to six weeks old thrips damage is less of a threat and they can recover some, according to the KSU Cotton Insect Management Guide. After the infestation If a farmer finds he has a thrips problem, UM Extension advises a preventive, in-furrow insecticide application or seed treatment for control. Popular choices for thrips control in Kansas include seed treatments of thiamethoxam, or planting-time applications of acephate, carbofuran and phorate, according to KSU Extension. Cool, wet weather, however, will slow the seedling's uptake of insecticides. And, in years of high infestations the in-furrow insecticide may not be enough. Another option would be to apply a foliar insecticide to rescue the crop. UM Extension recommends a rescue insecticide foliar treatment when thrips counts average one or more per plant. Sloderbeck also cautions producers that even with treatments, they may still see thrips on their cotton fields. "You're going to see some thrips even with treatment because they'll be flying in from other fields," he said. "If they're on the field for a while, they'll die off as they feed, but you could see some even after treatment. Your goal is to try to avoid the population increasing, or seeing nips develop on the plants. That's an indication the treatment's worn off." By using seed treatments or pre-plant insecticide applications, though, growers in the northern Cotton Belt may be able to lessen the impact these tiny insects can have on their young crops. Jennifer M. Latzke can be reached by phone at 620-227-1807, or by e-mail at jlatzke@hpj.com. 5/5/08 Date: 5/2/08 Advertisement
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