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
Glufosinate and insecticides
Ignite can be applied over-the-top of Liberty Link cotton from emergence to the early bloom stage. Glufosinate is marketed under the trade name Ignite in Liberty Link cotton and Liberty in other Liberty Link crops. At the time this research was initiated, glufosinate was marketed only as Liberty. Liberty was applied alone or in combination with 12 insecticides (Bidrin, Orthene, Centric, Intruder, Trimax, Capture, Karate Z, Baythroid, Steward, Tracer, Denim and Intrepid) to evaluate broadleaf weed control. Control of hemp sesbania, redroot pigweed, pitted morningglory, prickly sida and sicklepod ranged from 85% to 100%, with Liberty applied alone at the three- to four- or seven- to eight-leaf growth stage. Insecticides co-applied with Liberty did not reduce weed control.
Co-application saves money
With the exceptions noted for Envoke, co-application of Staple, glyphosate or glufosinate and insecticides are possible without negative consequences to control of weeds or insects evaluated in this research. In general, herbicides had minimal impact on insecticide efficacy. The negative effects of insecticide co-application with herbicides were observed primarily on weeds that previous research has shown to be less sensitive to the herbicide anyway. For these weeds, another herbicide would be applied. Control of early-season insects and broadleaf weeds can be accomplished with co-applications. Producers are cautioned to consult the individual pesticide labels for restrictions on tank-mixtures when considering weed and insect management strategies.
Donnie K. Miller, associate professor, and Gene Burris, professor, Northeast Research Station, St. Joseph, La.; Richard W. Costello, former graduate assistant, Department of Plant Pathology and Crop Physiology, Baton Rouge, La.; Robert G. Downer, associate professor, Department of Experimental Statistics, Baton Rouge, La.; James L. Griffin, professor, Department of Agronomy and Environmental Management, Baton Rouge, La.; B. Rogers Leonard, professor, Macon Ridge Research Station, Winnsboro, La.; Joseph H. Pankey, former graduate assistant, Department of Plant Pathology and Crop Physiology, LSU AgCenter, Baton Rouge, La.; and John W. Wilcut, professor, Department of Crop Sciences, North Carolina State University, Raleigh, NC.
(This article was published in the summer 2004 issue of Louisiana Agriculture.)