Thrips at about 1/16-inch long are among the smallest of cotton insects, but cause big problems to Texas High Plains farmers.
In 2007, 1.7 million acres in the Texas High Plains were treated for thrips at a cost of $14.4 million. Still, growers lost 57,651 bales to these tiny pests. For the United States the respective figures were 4.6 million acres, $17.2 million, and 145,040 bales.
Thrips pose a major threat during the seedling stage because they damage plant tissues that will subsequently produce leaves and fruit. Heavy infestations cause stand and yield reductions.
Control measures include seed treatment, systemic insecticides applied to the soil before planting, and foliar application of insecticides.
"Seed treatments are not always effective, especially if an infestation extends beyond the intended effective control period,” said Mark Arnold, research associate, Texas AgriLife Research and Extension Center at Lubbock. “Systemic insecticides can be expensive on a light thrips season and often even high-input growers choose not to use them. Also, the window for foliar application is very short. If the producer waits until thrips damage is evident, it may be too late to spray.
"Thrips-resistant or tolerant varieties would reduce producers' monetary losses significantly, or at least contribute to a more effective integrated pest management strategy for thrips,” Arnold said. “To that end we have formed a researcher group with the objectives of finding thrips-resistant germplasm, and discovering the plant properties that confer that resistance," Arnold said. "To accomplish these objectives we developed and implemented a very extensive screening program."
The researchers began the screening program with acquisition of small seed lots from the wild and obsolete-race stock cotton collections from the USDA, France, and Russia. They increased the seed lots by growing them under controlled conditions in greenhouses. These additional seed are being used by the researchers for field and greenhouse screening tests.
"We begin the initial phase of the screening process by growing thrips-infested wheat in the greenhouse. When the wheat approaches the desired thrips-infestation level, we plant six individual seed of each accession and a commercial check variety in paper cups in trays. Five replications of these trays are placed adjacent to the thrips-infested wheat. After three days, the adjacent wheat is chemically terminated. This forces the thrips to migrate the few inches to the adjacent emerging cotton plants," Arnold said.
When the plants have developed four true leaves, researchers rate them for damage on a 0-9 scale. "We excise all plant tissue above the cotyledons of individual plants and place it in a mason jar. Subsequently we wash out the thrips, count them, and measure the surface area of the plants’ true leaves," Arnold said.
"We determine the relative reduction in leaf surface area due to thrips by comparing the leaf surface areas of the damaged plants to those of undamaged check plants that have been kept thrips-free."
Plants that have high levels of thrips colonization and severe leaf damage are eliminated. The remaining plants will have significantly less damage and/or significantly fewer thrips. "These promising plants are retested one or more times until we are convinced they do have a significant level of resistance. Those that pass the test are then screened in a no-choice test," Arnold said.
In this no-choice test, researchers enclose the promising accession plants and thrips together in cages with transparent coverings that will allow the plants to develop normally, but will not allow the thrips to escape. "Thus the thrips have no choice but to feed on the enclosed plants," Arnold said.
Researchers obtain undamaged check plants by growing the promising accessions in like cages, but without thrips. They compare leaf surface areas and biomass losses of the thrips-damaged accessions with those of the undamaged check plants to determine the resistance levels of the accessions.
"Since the inception of the screening program in 2005 we have successfully screened 341 germplasm accessions. Several of them have shown a measure of resistance, and one accession has shown strong resistance," Arnold said.
The researchers have crossed some of the resistant accessions with elite breeding stocks to produce thrips-resistant lines that will be used by breeders and molecular geneticists to produce thrips-resistant varieties.
"We have significantly reduced the testing time for accessions by stacking the procedures so we will continually have tests in all stages of completion. Presently we are able to complete screening of about 400 accessions per year." Arnold said.
The researchers are pleased with their current screening technique. "We have refined our methodology to the point that we can obtain very reliable, repeatable data in a comparatively short time. And we are confident we will continue to find thrips-resistant plants," Arnold said.
Associated with Arnold in the research program are project leader and Associate Professor-Cotton Breeding Jane Dever; research assistants Heather Elkins, Monica Sheehan, Jimmy Mabry, Leslie Wells and Natalia Castillo; and collaborating entomologist at Texas AgriLife Research, project leader and Associate Professor Megha Parajulee. The work was initiated under John Gannaway, who recently retired.
Funding for the thrips screening research project is provided by the Texas Department of Agriculture Food and Fibers Research Grant Program. Greenhouse infrastructure necessary for the project is provided by a Texas state appropriation for Cotton Germplasm and Texas AgriLife Research. Subsequent breeding work to develop agronomically and economically viable cotton lines is supported by the Plains Cotton Improvement Program, a regional check-off program administered by Plains Cotton Growers, Inc.
Statistical data were obtained from "Cotton Insect Losses, 2007," Michael R. Williams, 2008 Beltwide Cotton Conferences, Nashville, Tennessee, Jan. 8-11, 2008.