Insect-resistant, Transgenic Soybeans: A New IPM Tool

Linda Benedict  |  10/26/2004 1:12:55 AM

Matthew E. Baur, Bently J. Fitzpatrick and David J. Boethel

The integrated pest management (IPM) approach to insect control involves multiple tactics. Host plant resistance is one. Pest-resistant plants can reduce pest population growth, the number of pesticide applications and the environmental impact of pesticides. Though some research has been conducted on breeding insect resistance into soybean plants, most soybean breeding programs focus on increasing yield. Transgenic technology, however, allows for the rapid development of insect-resistant plants that also have good agronomic characteristics.

The IPM approach is based on the use of economic thresholds. The economic threshold is the pest population level at which controls are initiated to prevent the loss in yield caused by pest damage to the crop that would be equal to the cost of controlling that pest using chemical intervention. Pest populations are maintained at a level below the economic threshold by monitoring and the as-needed use of insecticides. By employing economic thresholds, the producer and environmental costs are reduced by eliminating unnecessary insecticide applications. Use of economic thresholds also limits the exposure of the pest population to control measures, thereby reducing the possibility of pesticide resistance.

In recent years, researchers involved in developing IPM strategies have studied preventive treatments. This differs from the strategy involving economic thresholds and as-needed applications of insecticides, but still can be viewed as falling within the IPM concept. Preventive treatments can be especially useful in geographic areas that have historical, annual problems with certain insect pests. The transgenic technology in which proteins with insecticidal properties are engineered into plants might be viewed as a preventive measure. As with other preventive treatments, this technology could be useful in areas of Louisiana that historically experience high populations of target pests.

The two target insects for insect-resistant, transgenic soybeans in Louisiana are the velvetbean caterpillar (Figure 1) and the soybean looper (on page 34). These pests feed on the leaves of the soybean plant and can severely limit yield. Velvetbean caterpillar populations can reach damaging levels rapidly. Many producers in areas where velvetbean caterpillar is a significant problem, primarily in the southern part of the state, apply a preventive treatment of Dimilin when plants are in full bloom. In the northeastern part of the state, soybean looper can be a significant problem, especially where soybeans are grown close to cotton. Few preventive measures are accessible for the control of this pest, and this insect has been difficult to control using many of the historically recommended insecticides, such as pyrethroids and carbamates. Both of these insects can be managed using early-maturing soybean varieties planted early in the season. However, this may not be an option because of weather.

In 1998, LSU AgCenter entomologists conducted the first field trials ever to evaluate the effectiveness of transgenic soybean lines developed by Monsanto Co. that contained the insecticidal protein from Bacillus thuringiensis (Bt). That year, and in subsequent years, we have seen exceptional control in the field (Figure 2) and the laboratory of velvetbean caterpillar and soybean looper using these transgenic soybean lines. In addition, there was no direct impact on insect predators often found in these fields. Trials in recent years have continued to examine the effectiveness against insects but have begun to focus on the agronomic properties of experimental lines.

Bt soybeans could be beneficial to producers, but several issues must be addressed before adoption of this new technology:

How would the introduction of another Bt crop affect resistance management in pests that attack several Bt crops? Several insect pests in Louisiana attack multiple Bt crops. The cotton bollworm (also known as the corn earworm) attacks corn, cotton and soybean, and soybean looper attacks soybean and cotton. Resistance management plans for Bt crops already restrict the acreage of Bt corn that can be planted in cotton-growing regions. Bt soybeans could fall under similar restrictions.

What additional cost will be incurred by the producer for this technology? The increase in cost for genetically engineered seed is referred to as the technology fee. For instance, soybean producers pay a technology fee of $10 to $15 for seed with the Roundup Ready technology. Cotton producers also are charged more for Bt cotton seed than for conventional seed. But, it is less expensive to control insects in soybeans than in cotton, so it is unlikely that a technology fee as large as that charged for Bt cotton would be assessed to soybean producers.

What is the cost-to-benefit ratio? Currently, the technology only offers control of lepidopteran pests (moths) and would have to be integrated with management of other major soybean pests such as the threecornered alfalfa hopper and the stink bug complex. Frequently, these pests occur simultaneously with caterpillar defoliators and can be controlled with the same insecticides. This is especially true for the velvetbean caterpillar but not always the case for the soybean looper because of the looper’s resistance to certain classes of insecticides that will still control the other members of the late-season insect pest complex on soybeans.

How will the presence of a second transgenic technology affect the export of soybeans to the world market? Many European markets limit the importation of products produced from transgenic plants. This limits the ability of Louisiana soybean producers to export their product to these markets, because Louisiana producers make use of the Roundup Ready technology. Roundup Ready soybeans are transgenic and at harvest these seed are not kept separate from conventional seed. Therefore, markets averse to transgenic plant products would already discriminate against soybeans produced in this state. The addition
of insect-resistant transgenic soybean would not significantly alter that picture.

The development of Bt transgenic soybeans offers an additional tool to be used in soybean IPM programs. However, the above questions will have to be answered and further studies conducted to evaluate the experimental lines’ agronomic competitiveness before the product will appeal to producers. This technology will complement existing soybean IPM programs and has the potential for enhancing the sustainability and profitability of soybean production in Louisiana.

Matthew E. Baur, Assistant Professor; Bentley J. Fitzpatrick, Research Associate; and David J. Boethel, Associate Vice Chancellor and Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.

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

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