Benefits of Insect-Resistant Bt Crops

Insect-resistant Bt crops offered positive benefits to growers in two ways: by reducing insecticide costs and increasing yields. Since Bt crops eliminate the need for insecticide applications, major impact of insect-resistant crops has been the reduction in insecticide use targeted for key pest control. Insect-resistant crops express toxic proteins during much of the crop season as a result of which supplemental insecticide applications are not needed for pest control. Unlike herbicide-tolerant crops, insect-resistant crops increased crop yields due to enhanced levels of insect control. Overall, direct grower benefits from insect-resistant Bt corn, cotton, potato and sweet corn were reported to be $112 million in 1999 in the United States (EPA, 2000). By 2001, net income of US growers was reported to increase by $228 million from Bt corn and cotton plantings alone (Gianessi et al., 2002).

Corn

A primary benefit of insect-resistant corn has been the opportunity to control a pest that previously escaped control and reduced yields. Though modest, adoption of Bt corn led to reductions in insecticide use. Since the introduction of Bt corn, acreage sprayed with insecticides for ECB control has been reduced resulting in over one million fewer acres treated for ECB (Carpenter and Gianessi, 2001). Only a minor acreage, about 5%, is treated for ECB control in the United States, which is the reason for the modest reduction in insecticide use due to Bt corn (Phipps and Park, 2002). Additionally, the insecticides used against the ECB are also used to control other insect pests to which Bt trait does not provide resistance and would still be applied regardless of ECB.

Yield gain and economic benefit from Bt-corn fluctuates based on variability in ECB infestation levels. While yield increased and resulting economic benefits were lower in low infestation years, Bt-corn delivered a significant economic benefit when ECB outbreaks occurred (Alstad et al., 1997). As a result, net returns have been higher for Bt corn in spite of seed premium and technology fees (Fernandez-Cornejo and McBride, 2000). On an average, yield advantage from Bt-corn ranged from 4% to 8%, depending on the levels of ECB infestation (Marra et al., 1998).

A significant benefit of Bt-corn is decreased secondary pathogen infestations (e.g. ear rot) due to reduction in entryways left by ECB (Alstad, 1997). Fusarium ear rot is the most common ear rot disease in the corn belt; it has been found in nearly every corn field at harvest. The severity of this disease is usually low, but it can reduce yield and quality. Symptoms of Fusarium ear rot are often highly correlated with ear damage by ECB. The primary importance of this disease is its association with mycotoxins, particularly the fumonisins. Fumonisins are a group of mycotoxins that can be fatal to livestock and are probable human carcinogens (Munkvold and Desjardin, 1997). The importance of fumonisins in human health is still a subject of debate, but there is evidence that they have some impact on cancer incidence (Marasas, 1995). Multi-year studies showed that kernel feeding by insects, extent of ear rot infestation and fumonisin levels in Bt corn were significantly lower than conventional corn (Munkvold et al., 1999). Volunteer corn in the following season has been reduced, as ears dropped due to ECB infestation are less with Bt corn (Alstad, 1997).

Depending on the prevalence of ECB populations, Bt-corn influences the local ECB population (Alstad, 1997; Andow and Hutchison, 1998). It is possible that planting non-Bt-corn near Bt-corn could suppress ECB populations in non-Bt corn and this localised benefit is called the halo effect. Similar effects may be noted with other insect-resistant crops.

Cotton

Insect-resistant Bt cotton has provided a tool to cotton growers to control the most damaging pests. Insect-resistant cotton resulted in highest per acre grower benefits and largest reduction in insecticide use among all the insect-resistant crops. In states such as Alabama, growers used the least amount of insecticides on cotton since the 1940s (Smith, 1997). A 1999 estimate by the EPA (2000) suggested a reduction of 1.6 million pounds of insecticide active ingredient use and 7.5 million acre treatments due to Bt cotton. Based on the USDA pesticide use data, growers in six major cotton-growing states reduced insecticide use by 16% and insecticide applications by 25% in 2000 compared to 1995 (Carpenter and Gianessi, 2002). A similar estimate by Fernandez-Cornejo and McBride (2000) also showed that Bt cotton growers applied 2.5 fewer insecticide applications per acre. Though not as dramatic as reductions in insecticide use, insect-resistant cotton led to reduced yield losses as a result of which yield advantage has been realised in many cotton-growing states (Fernandez-Cornejo and McBride, 2000). The overall effect of reduction in insecticide use and gains in yields has been higher net return to cotton growers, despite the technology fee. Grower benefits have increased from 16 million in 1996 to 44 million in 1999 due to Bt cotton (EPA, 2000).

By targeting specific insects through the naturally occurring protein in the plant, Bt cotton reduces the need for and use of chemical insecticides. By eliminating chemical sprays, the beneficial insects that naturally inhabit agricultural fields are maintained and can even provide a secondary level of pest control. This is the reason why Bt cotton adoption is high in areas where boll weevil eradication programmes are in effect as insurance against unchecked bollworm and budworm populations due to elimination of natural predators with the use of malathion.

Evidence states that insect-resistant crops impact local ecosystems favourably. Beneficial insect-feeding bird populations have been reported to be higher in numbers in Bt cotton fields compared to conventional fields (Edge et al., 2001).

A major worry concerning the success of Bt crops, especially cotton, is the potential vulnerability to eventual adaptation by insect pests to Bt toxin. Large-scale deployment of Bt crops will impose selection pressure for pre-existing Bt-resistant insects to increase their numbers resulting in the loss of viability of this environmentally sound pest control practice. Several resistance management strategies have been proposed to slow the evolution of insect adaptation to Bt genes such as refuges, intense field monitoring of insect-resistant plants for potential escapes and alternate control strategies.

To slow the adaptation of insects to Bt toxin, the EPA has mandated that cotton growers should plant at least 4% of their biotechnology-derived crop with conventional cotton varieties and this refuge cannot be treated with any insecticides. The advantage of planting refuges is that they will harbour susceptible insects and thus retard the evolution of insect resistance against the Bt gene. Gould et al., (1997) predicted that Bt cotton could remain efficacious for 10 years with 4% refuge.

Potato

Due to low adoption rates, insecticide use reductions in potato are not as dramatic as in cotton. Based on 4% market share of Bt potato, insecticide use reduction from Bt potato has been reported to be 89 000 less acre treatments with corresponding grower benefit of $9.30 (EPA, 2000) to $11.50 (Gianessi et al., 2002) per acre. Insect-resistant Bt potato has not yet made a significant impact on overall yield.

An indirect benefit of insect-resistant crops, potato and cotton in particular, is the worker safety the technology affords. Insecticides routinely used for pest control in cotton and potato such as organophosphates, carbamates and synthetic pyrethroids are known to cause adverse health effects in workers. Insect-resistant Bt cotton eliminates the need for the use of the above chemicals as a result of which occupational risk is minimised.

Sweet Corn

A notable impact of Bt sweet corn is the reduction in number of insecticide applications. Based on sweet corn acreage planted with Bt varieties in 1999, EPA (2000) reported that reduction in insecticide applications have been 4.3 per acre or a total of 127 000 acre applications in the United States. Reduction in insecticide applications on a per acre basis has been the highest in Bt sweet corn compared to any biotechnology-derived crop. Benefits from improved pest control and reduced application costs have been $5.40 per acre (EPA, 2000). An added benefit to insect-resistant sweet corn is the reduction in yield loss caused by feeding damage of fall armyworm and corn earworm. Season long protection offered by Bt sweet corn resulted in significantly higher marketable yield than conventional varieties (Stegelin, 2000). Overall, once the market penetration of Bt sweet corn increases, growers are expected to note significant reductions in overall insecticide use and enhanced returns.

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