Transgenic Rootworm Corn: Assessing Potential Agronomic, Economic, and Environmental Benefits

The potential benefits of transgenic rootworm corn (Cry3Bb1 protein) are expected to include: increased root protection; increased intangible benefits to farmers (safety of not being exposed to insecticides, ease of use and handling, time and labor savings, better pest control); increased economic benefits to farmers ($231 million from yield gains [$25-$75/acre relative to no insecticide control, $4-$12/acre relative to control with a soil insecticide] and $58 million in reduced insecticide risks and time savings); reduced incidence of corn stalk rot; and increased yield protection (9 to 28% relative to no insecticide use, 1.5 to 4.5% relative to control with a soil insecticide). If transgenic rootworm corn is planted on 10 million acres, the annual impact will be a reduction of 5,344,462 lb a.i. (75.2%) of insecticide use; increased resource conservation (3.07 to 5.23 million gal of diesel fuel equivalents conserved that would have been consumed in the manufacture and delivery of insecticides); increased water conservation (5,657,734 gal of water not used in insecticide application); conservation of aviation fuel (68,845 gal of aviation fuel not used); reduced farm waste (1,187,035 fewer insecticide containers used); increased planting efficiency; and improved safety to wildlife and other nontarget organisms. Introduction Corn (Zea mays) was planted on almost 80 million acres in the United States during 2002, and it accounts for more than 90% of the total value and production of U.S. feed grains (7). Corn is attacked by a variety of insect pests that can significantly reduce grain yield. Two of the most important pests in the Corn Belt are the western corn rootworm (Diabrotica virgifera virgifera) (Fig. 1), and the northern corn rootworm (D. barberi) (19). Additionally, the southern corn rootworm (D. undecempunctata howardi) and the Mexican corn rootworm (D. virgifera zeae) can be economically damaging in the Gulf Coast states and Texas, respectively (37). Corn rootworms cost farmers nearly $1 billion annually in crop losses and control costs (1). Current corn rootworm control strategies require the use of insecticides or rotation of corn with another crop. Both of these methods are used widely in the Corn Belt, but each has its limitations and has occasionally failed to prevent yield loss. Adult western corn rootworms have developed resistance to foliar-applied insecticides in some areas of the western Corn Belt (22). Likewise, in some areas of the Corn Belt, both the northern corn rootworm and western corn rootworm have adapted to crop rotation, which has long been considered an effective cultural control tactic. Eggs of the northern corn rootworm undergo extended diapause, i.e., they do not hatch the year after they are deposited, when the field is a crop other than corn; rather, they hatch two years later when corn has been planted in the field (5,18). The incidents of failure of crop rotation to control this insect have increased in recent years in Iowa (39). Western corn rootworm adults leave corn to lay eggs in soybean fields that may be planted to corn the following year (20). This problem has escalated the use of insecticides so that 33% and 39% of Illinois and Indiana farmers, respectively, apply insecticides to first-year corn fields for corn rootworm control (Wilson, Rice and Tollefson, unpublished data). Although management of corn rootworms will continue to include the use of insecticides Plant Health Progress 1 March 2004 and crop rotation, planting of transgenic (i.e., genetically engineered) corn that resists rootworm damage and protects grain yields will be an attractive alternative for many growers. Monsanto Company has genetically modified the cry3Bb1 gene derived from Bacillus thuringiensis kumamotoensis to express a Cry3Bb1 protein in corn (9). This Cry3Bb1 protein, a variant of the wild-type protein, is selectively toxic to some species of Coleoptera. The modified cry3Bb1 gene is expressed in the MON 863 corn line and differs from the wild-type cry3Bb1 gene by the addition of an alanine residue at position 2 of the protein and seven amino acid changes (12). The intended technical effect of the genetic modification is to protect corn roots from corn rootworm feeding. On February 25, 2003, the Environmental Protection Agency (EPA) approved transgenic MON 863 corn with the Cry3Bb1 protein for commercial use (10), and Monsanto will market the biotechnology as YieldGard Rootworm corn. Gray (13) stated that “transgenic insecticidal cultivars offer great potential to serve as the most exciting and effective tool for corn rootworm control in the pest management arsenal.” In addition to insect control, the planting of transgenic rootworm corn is expected to generate numerous agronomic, environmental, and societal benefits. This article will discuss these expected benefits. Potential Benefits of Transgenic Rootworm Corn Increased root protection. Data from field studies indicate that transgenic rootworm corn will be as good as or better than soil insecticides in protecting corn roots from significant corn rootworm larval injury (38,40). YieldGard Rootworm and YieldGard Plus (a stacked hybrid with genes to control both European corn borer [Ostrinia nubilalis] and corn rootworms) were compared against the soil insecticide terbufos (Counter 20CR) and a nontransgenic hybrid with no insecticide (38). Products were evaluated for their consistency, which is the percentage of times a treatment limited feeding injury to 0.25 node or less (greater injury can result in economic yield losses). Both transgenic corn hybrids were 100% consistent in protecting roots from economic damage to roots, whereas the insecticide was only 63% consistent and the untreated nontransgenic offered no protection from insect damage (Fig. 2). Although transgenic rootworm corn does not offer total protection against larval feeding, university data suggest that the quantity of root injury is minor and should not translate into economic yield loss. An added benefit of transgenic rootworm corn is that root protection does not depend upon planting time, weather influences, calibration of application equipment, or soil conditions for optimum performance (24). However, the narrow spectrum of activity against corn rootworm larvae could also be a limitation when secondary pests (e.g., white grubs, wireworms, seedcorn maggots, grape colaspis) are present in economically damaging densities. In fields with secondary pest problems, corn producers may choose to protect the seed and seedling plants with seed-applied insecticides. Fig. 1. Corn rootworm larva (Marlin E. Rice). Plant Health Progress 1 March 2004 Yield protection and economic increases. Over typical ranges for corn rootworm populations, transgenic rootworm corn (genetic event MON 863) would provide a yield benefit of 9 to 28% relative to no insecticide use and a yield benefit of 1.5 to 4.5% relative to control with a soil insecticide (24). For a reasonable range of prices and yields, the predicted value of the yield benefit attributed to event MON 863 would be $25-$75/acre relative to no insecticide control and $4-$12/acre relative to control with a soil insecticide (24). It is estimated that if transgenic rootworm corn had been planted in 2000 on 100% of U.S. corn acres treated with an insecticide (14.2 million acres) for corn rootworm control, at a cost that was equal to per-acre costs for corn rootworm insecticides, the total economic benefit would have been $460 million (3). Of this benefit, $231 million from yield gains would be gained by farmers, $58 million from reduced risks and time savings associated with reduced insecticide use would be realized by farmers, and $171 million would be realized by the technology developer and seed companies. The relative value of transgenic rootworm corn depends on the price of the transgenic seed, field performance, availability, and the price of close substitutes such as other corn rootwormresistant varieties or insecticides (3). Although assumptions of higher prices and a more realistic estimate of adoption of transgenic rootworm corn (percentage of acres) would decrease the potential economic benefits, the benefits would still be substantial. Intangible benefits for farmers. Farmers likely to adopt transgenic rootworm corn would recognize several intangible benefits that bring additional value to this new technology (3). Surveyed corn growers noted that advantages of a transgenic technology combined with an insecticide seed treatment (for minor seed-feeding or root-feeding pests) would include the safety of not handling an insecticide (30% of farmers), ease of use and handling (21%), all-inone product insect control (21%), time and labor savings (14%), and better pest control (14%). The total value of these perceived benefits among likely adopters of the technology is $16.08/acre (Table 1) (3). Table 1. Values placed on various characteristics by respondents likely to adopt transgenic rootworm corn relative to soil-applied insecticides (3). Fig. 2. Corn rootworm feeding injury to nontransgenic (near isogenic) corn (left) and YieldGard Rootworm corn (right) (Marlin E. Rice). Product Characteristic Value ($/acre) Handling and labor time savings 1.94