Donnie K. Miller, Gene Burris, Richard W. Costello, Robert G. Downer, James L. Griffin, B. Rogers Leonard, Joseph H. Pankey and John W. Wilcut

Before the availability of transgenic technology in cotton, weed management programs consisted of herbicide applications to the soil at planting followed by multiple herbicide applications directed underneath the crop in combination with tillage to control emerged weeds. Occasionally, over-the-top herbicide applications were made.

Today, more than 80 percent of the cotton grown in Louisiana is transgenic, and weed control programs have shifted to more over-the-top applications. Weeds are managed in many cases without use of at-planting herbicides or tillage. Two relatively new herbicides, Staple and Envoke, provide both contact and soil residual control of broadleaf weeds primarily. Glyphosate (numerous formulations) and glufosinate (Ignite) provide broad spectrum control of broadleaf and grass weeds in Roundup Ready and Liberty Link transgenic cotton.

For insect pest control in cotton, farmers often treat early in the season for thrips and aphids and later in the growing season for bollworms and tobacco budworms. Intense scouting of insects is recommended during the growing season, and a number of insecticides can control various insect pests once economic thresholds are reached.

Because the need for insect and weed control occurs simultaneously, it would be economically advantageous to farmers to apply insecticides and herbicides at the same time – as long as neither control program is compromised. A team of scientists from the LSU AgCenter and colleagues at North Carolina State University have conducted research on possible interactions between the herbicides Staple, Envoke, glyphosate and glufosinate and a range of insecticide chemistries used for early and late-season management of insect pests in cotton.

Staple and insecticides

Staple herbicide, which can be applied over-the-top of cotton until 60 days before harvest, was applied alone or with eight different insecticides (Orthene, Bidrin, Regent, Trimax, Karate Z, Vydate, Furadan and Dimethoate). The combinations were evaluated for potential interaction effects on broadleaf weed control. At 28 days after application at the three- to four-leaf stage, Staple applied alone controlled hemp sesbania (98%), entire-leaf morningglory (94%), pitted morningglory (93%), velvetleaf (87%) and prickly sida (71%). Co-application of Staple with insecticides did not reduce weed control compared to the herbicide alone.

These eight insecticides were also applied alone or in combination with Staple to evaluate possible interaction effects on thrips control. Staple applied with the insecticides Vydate and Dimethoate (in two of three studies) and Trimax (in one of three studies) reduced insecticide efficacy against thrips larvae compared to the insecticides applied alone. The effectiveness of the other insecticides applied with Staple was not reduced.

Glyphosate and insecticides

Glyphosate can be applied over-the-top of Roundup Ready cotton until the fifth true leaf stage, after which applications should be directed to the base of cotton plants. Glyphosate, formulated as Roundup Ultra, was applied alone or in combination with seven insecticides (Orthene, Bidrin, Regent, Trimax, Karate Z, Vydate and Phaser) to evaluate the interaction effects on broadleaf weed control. Roundup Ultra applied alone in two experiments controlled hemp sesbania (68% to 96%), redweed (73% to 85%), pitted morningglory (71% to 81%) and prickly sida (62% to 71%). Less weed control was noted for hemp sesbania, which is inherently less sensitive to glyphosate.

Weed control was not affected when glyphosate was applied with any of the insecticides. Possible interaction effects on thrips and aphid control were investigated with the insecticides Orthene, Bidrin, Dimethoate and Trimax applied alone or with Roundup Ultra. The addition of Roundup Ultra did not reduce insect control compared to insecticides applied alone.

Envoke and insecticides

Envoke can be applied over-the-top of cotton from the fifth true leaf stage until 60 days before harvest. Envoke was applied alone or in combination with 11 insecticides (Orthene, Vydate, Karate Z, Intruder, Centric, Phaser, Steward, Denim, Intrepid, Tracer and S-1812) to evaluate control of 10 weed species.

With few exceptions, control of palmer amaranth, smooth pigweed, common lambsquarters, jimsonweed and prickly sida was significantly reduced with all herbicide/insecticide co-applications. Previous weed control studies with Envoke have identified these respective weed species as being less sensitive to the herbicide than other broadleaf weeds. Control of sickelpod, pitted morningglory, ivyleaf morning-glory, entireleaf morningglory and tall morningglory was unaffected by insecticide co-application with Envoke.

Control of adult thrips and thrips larvae was also evaluated with co-applications of Envoke and the insecticides. Except for the control of adult thrips with the Envoke/Steward co-application in one of two experiments, the addition of Envoke did not reduce insecticide efficacy compared to insecticide
applied alone.

Glufosinate and insecticides