Interaction of herbicides and early-season insecticides on thrips populations and plant growth characteristics on herbicide-tolerant transgenic cotton

Cotton (Gossypium hirsutum L.) is produced in seventeen states from Virginia to California. In 1999, the United States produced approximately $3.8 billion dollars of cotton. Cotton production in Tennessee in 1999 was valued at $142 million.

Major pests of cotton in Tennessee include boll weevil, bollworm, tobacco budworm, plant bugs, stink bugs, and thrips. Thrips are the first insects that attack young cotton seedlings soon after the plants emerge from the soil. Cotton insects in the United States caused approximately $581.5 million in lost lint yield. Losses due to weeds, however, are not as easy to quantify. The most current yield losses that could be found were from 1994, when estimated yield losses caused by weeds in Tennessee were $1.4 million. In Tennessee, growers spent $16.4 million on herbicides in 1994.

Cotton producers have wanted a post emergence herbicide option for weed control because of the ease of application. Previously, growers had to rely upon directed sprays to protect the plants from herbicide contact. Prior to 1995, cotton was the only major agronomic crop without a registered post emergence herbicide available to producers. However, with the introduction of the BXN ® and Roundup Ready ® transgenic varieties of cotton in 1995 and 1997, respectively, producers now have more options for weed control. The premise behind these two types of cotton is that a gene has been inserted into the plant that inhibits certain enzymatic pathways, which makes the plant tolerant to foliar applications of the herbicides bromoxynil or glyphosate.

Research was conducted at both Jackson and Milan, Tennessee, in 1999 and 2000. Three experiments were conducted each year. The objectives of this research were to evaluate the: (1) efficacy of several early-season insecticides for thrips control under two herbicide programs, (2) effects of insecticide on cotton growth, development, and yield, and (3) possible interactions among the insecticides and Roundup Ultra ®, Buctril ®, or conventional herbicides. All insecticide-treated plants had significantly lower amounts of thrips feeding injury in 1999 and 2000. Both adult and larval thrips were affected by insecticide treatment during this study. Densities of adult and larval thrips on aldicarb-treated plants were consistently lower than on untreated plants. In 1999 and 2000, all insecticide treatments protected young plants from thrips feeding. Insecticide-treated plants also had greater amounts of leaf area in each year. Stand densities were not affected by insecticide treatment during 1999 and 2000 at Jackson or Milan. Phorate-treated plants had consistently fewer blooms and a higher node of first position square in 1999 and 2000, indicating a delay in maturity. Lint yields were not affected by insecticide treatment in two of three tests in 1999, while in 2000, two of three tests had higher lint yields. When herbicides were evaluated for impact against stand densities, thrips feeding damage, leaf area, node of first position square, blooms, thrips numbers, and yield, no significant effects were observed.

Insecticides have been evaluated previously, but trials were conducted using conventional cotton varieties. This research will be useful in updating Tennessee's state cotton insecticide recommendations for herbicide-tolerant cotton varieties.