Beyond Bollgard: Insect-resistant Cotton Varieties

Billy Leonard, Bagwell, Ralph D., Micinski, Stephen  |  10/26/2004 1:55:58 AM

B. Rogers Leonard, Stephen Micinski and Ralph Bagwell

The first caterpillar-resistant transgenic cotton varieties (Bollgard) were approved by the U.S. Environmental Protection Agency (EPA) in 1996. The Bollgard technology has successfully reduced the frequency of sprays for caterpillar pests by about half. The excessive costs of controlling pests with foliar insecticides and the risk of economical yield losses allowed Bollgard to become widely accepted by producers. In 2002, more than 75 percent of Louisiana’s total cotton acreage was planted to Bollgard varieties.

Bollgard varieties produce the Cry1Ac protein from Bacillus thur-ingiensis. This protein is toxic to the larval stages of many lepidopteran (moth) pests of cotton, but the primary targets in cotton include the tobacco budworm and the pink bollworm. Bollgard demonstrates only limited activity against other pests such as armyworms, soybean looper and bollworm. If these pests are present at high densities or populations persist for an extended period, supplemental insecticide applications are justified to prevent yield loss.

Recent advances in genetic engineering technologies within the agrochemical industries have produced a second generation of caterpillar-resistant cotton germplasm. These cotton lines contain two separate B. thuringiensis proteins and have improved the target spectrum of caterpillar pests. Monsanto’s new genetically engineered cotton (Bollgard II) was derived by incorporating the Cry2Ab protein from B. thuringiensis into commercially available Bollgard cotton varieties. Levels of Cry1Ac protein expression in Bollgard II are similar to the levels of Cry1Ac expressed in Bollgard; there-fore, tobacco budworm control is also similar.

Dow AgroSciences has employed a similar strategy and developed another multiple protein product (WideStrike) with efficacy against a wide range of caterpillar pests. The Wide-Strike cotton lines express Cry1Ac and Cry1F proteins from B. thuringiensis strains.

Assessing Performance

LSU AgCenter entomologists have had the opportunity to evaluate the performance of the new transgenic cottons against the common insect pests in Louisiana. Experimental cotton lines containing the Bollgard II technologies have been tested in field and laboratory trials since 1998. Evaluation of WideStrike technology began in 1999. The objectives of these studies have been to document the efficacy of these products against a spectrum of caterpillar pests and to determine their optimum role in an overall cotton insect pest management system.

Field trials of Bollgard II and WideStrike demonstrated satisfactory control of mixed Heliothine (tobacco budworm and bollworm) populations and late-season foliage feeding insects including soybean looper and beet armyworm. A summary of 11 tests indicated Bollgard II had the lowest average number of Heliothine-infested fruiting forms and associated damage compared to that in conventional cotton and Bollgard cotton (Table 1). Bollgard II also demonstrated satisfactory control of foliage-feeding insects compared to conventional non-transgenic and Bollgard cottons (Figure 1). In a similar series of trials, WideStrike demonstrated successful control of Heliothines and foliage-feeding insect pests (Table 2, Figure 2).

In addition to Bollgard and WideStrike, Syngenta Corporation is developing a vegetative insecticidal protein (VipCot) technology. Although fewer trials have evaluated the performance of the VipCot trait, the preliminary data indicate a target efficacy spectrum similar to that of the other technologies. As transgenic technology continues to evolve in the future, LSU AgCenter scientists will evaluate additional insecticidal plant proteins.

Insect Management Strategies

Bollgard II and WideStrike will further reduce the need for foliar sprays of insecticides against caterpillar pests of cotton; however, reduction in spraying inadvertently gives rise to more secondary cotton pests that can lead to significant yield losses. Several secondary pests of cotton, including a complex of tarnished plant bugs and stink bugs, will increase their pest status as the need for insecticides is further reduced in Bollgard II and WideStrike cotton fields. Recently, LSU AgCenter scientists have intensified efforts to develop cost-effective solutions for managing the tarnished plant bug and stink bug complex. Using Bollgard II or WideStrike as the primary strategy against caterpillar pests, research projects focus on emerging pest problems. Sampling procedures, establishing action thresholds for treatment initiation, efficacy trials for foliar insecticides and studying the interactions of multiple pest problems are examples of cotton IPM research that will improve the successful implementation of novel transgenic products.

Conclusions and Recommendations

The multiple protein products, Bollgard II and WideStrike, have demonstrated satisfactory control of caterpillar pests including tobacco budworm, bollworm, soybean looper and beet armyworm. These technologies will further reduce and, in some instances, eliminate a requirement of supplemental control of caterpillar pests with foliar applications of insecticides in cotton. Neither of these two products is active against noncaterpillar pests, which will likely increase the occur-rence of tarnished plant bug and stink bug problems.

B. Rogers Leonard, Professor, Macon Ridge Research Station, Winnsboro, La.; Stephen Micinski, Associate Professor, Red River Research Station, Bossier City, La.; Ralph Bagwell, Associate Professor, Scott Research and Extension Center, Winnsboro, La.

(This article appeared in the fall 2003 issue of Louisiana Agriculture.)

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