non-Bt Refuge Research Articles

Modelling the role of refuges for sustainable management of dual-gene Bt Cotton in West African smallholder farming systems

Modelling is a common tool for evaluating the sustainability of Bt crops. We previously used modelling to show that, under the agro-ecological conditions of West Africa, non-Bt cotton could be used as a refuge to delay the development of resistance to Bt toxins in Helicoverpa armigera. We concluded that Cry1Ac Bt Cotton should be limited to 20% of the total cotton acreage [ Nibouche, S., Martin, P., Vaissayre, M., 2003. A modelling approach of the sustainability of Bt cotton grown by small farmers in West Africa. Res. Pest Manag. Newsl. 13, 55–58]. Here we present results obtained with a cotton cultivar expressing two Bt genes (dual-gene Bt Cotton). Our conclusions are the same as those drawn on the basis of results obtained with one-gene Bt Cotton, i.e. non-Bt cotton refuges are essential for effective prevention of resistance, since wild host plants of the bollworm have not been found to serve as a refuge for this pest during the rainy season. The optimal size of non-Bt cotton refuges depends on bollworm mortality levels on Bt cotton, and on gene flow between rainfed crops and irrigated vegetable crops. Although there is very little data available regarding pest movements and/or migration and crop-pest interactions in the agro-ecological conditions of West Africa, our simulations indicate that Bt cotton should not be grown on more than 20–25% of the total cotton cropping area in order to prevent resistance from developing in the long term.

Simulated Effects of Transgenic Bt Crops on Specialist Parasitoids of Target Pests

Large-scale adoption of transgenic crops that produce Bacillus thuringiensis (Bt) toxins could greatly reduce populations of target pests and their specialist parasitoids. We used a spatially explicit model of host-parasitoid population dynamics to examine effects of Bt crops on parasitoid persistence. We modeled a 9,000-ha region with 900 fields of 10 ha, each planted with either a Bt crop or a non-Bt crop refuge. In the 13,524 simulations we conducted, the pest (i.e., host) was never lost from the region before the parasitoid. The parasitoid was lost from the region in 36% of the simulations. The host was lost after the parasitoid in 12% of all simulations. The probability of regional parasitoid loss increased as the percentage of Bt fields and rotation of field types (Bt and non-Bt) increased and as host reproductive rate and parasitoid attack rate decreased. Regional parasitoid loss also increased as the insecticide spray threshold for target pests in non-Bt fields decreased and as parasitoid susceptibility to insecticide increased. With field types fixed, regional parasitoid loss increased as parasitoid movement increased. However, with field types rotated, parasitoid movement did not influence regional parasitoid loss. Host movement did not influence regional parasitoid loss. Factors not modeled that would reduce risk of regional parasitoid loss include use of alternate hosts by the parasitoid, use of alternate host plants by the target pest, and evolution of resistance to the Bt crop by the pest. The results suggest that risk of regional parasitoid loss can be assessed from the parasitoid life history traits and reduced by increasing the percentage of refuge fields, fixing refuge locations, and increasing the insecticide spray threshold for target pests in refuges.

Effects of Cotton Cultivar on Fitness Costs Associated with Resistance of Pink Bollworm (Lepidoptera: Gelechiidae) to Bt Cotton

Fitness costs associated with insect resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) reduce the fitness on non-Bt refuge plants of resistant individuals relative to susceptible individuals. Because costs may vary among host plants, choosing refuge cultivars that increase the dominance or magnitude of costs could help to delay resistance. Specifically, cultivars with high concentrations of toxic phytochemicals could magnify costs. To test this hypothesis, we compared life history traits of three independent sets of pink bollworm, Pectinophora gossypiella (Saunders), populations on two cotton cultivars that differed in antibiosis against this cotton pest. Each set had an unselected susceptible population, a resistant population derived by selection from the susceptible population, and the F1 progeny of the susceptible and resistant populations. Confirming previous findings with pink bollworm feeding on cotton, costs primarily affected survival and were recessive on both cultivars. The magnitude of the survival cost did not differ between cultivars. Although the experimental results did not reveal differences between cultivars in the magnitude or dominance of costs, modeling results suggest that differences between cultivars in pink bollworm survival could affect resistance evolution. Thus, knowledge of the interaction between host plants and fitness costs associated with resistance to Bt crops could be helpful in guiding the choice of refuge cultivars.

Bt cotton and pesticide use in Argentina: economic and environmental effects

This article analyzes effects of insect-resistant Bt cotton on pesticide use and agricultural productivity in Argentina. Based on farm survey data, it is shown that the technology reduces application rates of toxic chemicals by 50 per cent, while significantly increasing yields. Using a damage control framework, the effectiveness of Bt versus chemical pesticides is estimated, and technological impacts are predicted for different farm types. Gross benefits could be highest for smallholder farmers, who are not currently using the technology. The durability of the advantages is analyzed by using biological models to simulate resistance development in pest populations. Rapid resistance buildup and associated pest outbreaks appear to be unlikely if minimum non-Bt refuge areas are maintained. Thus, promoting a more widespread diffusion of Bt cotton could amplify the efficiency, equity, and environmental gains. Conclusive statements about the technology's sustainability, however, require longer-term monitoring of possible secondary effects and farmers' behavior in maintaining refuges.

Relative Concentration of Cry1A in Maize Leaves and Cotton Bolls with Diverse Chlorophyll Content and Corresponding Larval Development of Fall Armyworm (Lepidoptera: Noctuidae) and Southwestern Corn Borer (Lepidoptera: Crambidae) on Maize Whorl Leaf Profiles

To manage insect resistance to transgenic crops that express insecticidal proteins from Bacillus thuringiensis (Bt) Berliner, the U.S. Environmental Protection Agency recommends a refuge-based insect resistance management strategy where a percentage of non-Bt (refuge) crop is grown in proximity to a Bt-expressing crop. An important requirement for this strategy is that the toxin exists at a high effective dose for control of the target pest(s), so that heterozygous individuals in the population do not reach adulthood. Factors that cause reduced levels of toxin in the plant are a threat to this strategy. We quantified Cry1Ab from different areas of the maize, Zea mays L., leaf. In general, the distal tip of the V7 maize leaf had a higher concentration of Cry1Ab compared with the middle section of the V7 leaf, and the middle section of the developing V9 leaf had the lowest concentration of Cry1Ab. When these sections of maize tissue were fed to fall armyworm, Spodoptera frugiperda (J.E. Smith), and southwestern corn borer, Diatraea grandiosella Dyar, there was not a reduction in development or an increase in mortality with tissue that had higher concentrations of toxin. Another study tested the relative concentration of Cry1Ab between the white-yellow, yellow-green, and green portions of the developing ninth leaf within the maize whorl. There were differences in Cry1Ab concentration among these leaf areas. The green tissue had the highest concentration of toxin followed by the yellow-green and white-yellow tissues. Correlations between concentration of Cry1Ab and 5-d fall armyworm larval weights among the three leaf color profiles were all significant and negative, i.e., decreased concentration of Cry1Ab in the leaf tissue resulted in increased 5-d larval weights. There was 100% mortality to the southwestern corn borer larvae fed Cry1Ab maize leaf tissue. Differences in the amount of Cry1Ab in the developing V9 leaf profiles did not alter the absolute susceptibility of the southwestern corn borer to the toxin. In cotton, Gossypium hirsutum L., the amount of Cry1Ac was significantly lower in boll tips where flowers had remained attached compared with normal boll tips. Boll tips where the flowers remained attached are often the site where corn earworms, Helicoverpa zea (Boddie), penetrate Bt cotton bolls. This study demonstrated that, in two diverse plant species, tissue that has low chlorophyll content does not fully express Cry1A. Photosynthesis regulating factors related to mRNA transcription and translation should be studied for their effect on Cry1A production and insect control.

Contamination of refuges by Bacillus thuringiensis toxin genes from transgenic maize.

Transgenic crops producing insecticidal toxins from Bacillus thuringiensis (Bt) are widely used to control pests, but their benefits will be lost if pests evolve resistance. The mandated high-dose/refuge strategy for delaying pest resistance requires planting refuges of toxin-free crops near Bt crops to promote survival of susceptible pests. We report that pollen-mediated gene flow up to 31 m from Bt maize caused low to moderate Bt toxin levels in kernels of non-Bt maize refuge plants. Immunoassays of non-Bt maize sampled from the field showed that the mean concentration of Bt toxin Cry1Ab in kernels and the percentage of kernels with Cry1Ab decreased with distance from Bt maize. The highest Bt toxin concentration in pooled kernels of non-Bt maize plants was 45% of the mean concentration in kernels from adjacent Bt maize plants. Most previous work on gene flow from transgenic crops has emphasized potential effects of transgene movement on wild relatives of crops, landraces, and organic plantings, whereas implications for pest resistance have been largely ignored. Variable Bt toxin production in seeds of refuge plants undermines the high-dose/refuge strategy and could accelerate pest resistance to Bt crops. Thus, guidelines should be revised to reduce gene flow between Bt crops and refuge plants.

Can other host species of cotton bollworm be non-Bt refuges to prolong the effectiveness of Bt-cotton?

The potential ecological risks ofBacillus thurigiensis (Bt) insecticides and Bt-crops have caused increasing concern since their commercial release in the field, among which pests’ resistance to Bt-crops is the major ecological risk. Refuge tactic, which can produce sensitive populations, has proved to be a key and sound resistance management strategy in USA and Australia; however, no tactics have been performed in China where Bt-cotton is mostly planted with other host crops of cotton bollworm. Genetic variation and gene flow among different host populations of the cotton bollwormHelicoverpa armigera were analyzed using PCR fingerprinting method. The results show that maize and castor-oil plant, as well as cotton can take effect as refuges to prevent resistance of cotton bollworm to Bt-cotton, while peanut and sesame are not as suitable for planting with Bt-cotton as refuges in the field as low gene flow was detected among populations on peanut, sesame and Bt cotton.

An economic analysis of non-Bt corn refuges

Planting Bt transgenic corn in large areas of the environment may lead to European corn borer (ECB) [ Ostrinia nubilalis (Hübner)] resistance to Bt toxins. Scientists recommend planting a non-Bt corn refuge of at least 20% of total corn area in close proximity to Bt corn as part of an Insect Resistance Management (IRM) strategy. A partial budget analysis was used to estimate the costs of implementing alternative refuge configurations in the US Corn Belt. A strip configuration is the least cost method of planting a 20% non-Bt corn refuge. A block-U refuge combination is only slightly more costly.The costs are small regardless of configuration choice and it is likely that other considerations, such as potential GMO segregation requirements, will dictate choice of configuration.

Dispersal of Pink Bollworm (Lepidoptera: Gelechiidae) Males in Transgenic Cotton that Produces a Bacillus thuringiensisToxin

To delay evolution of pest resistance to transgenic cotton that produces Bacillus thuringiensis toxin Cry1Ac (Bt cotton), refuges composed of cotton that does not produce Cry1Ac (non-Bt cotton) are mandated by the United States Environmental Protection Agency. One critical assumption of this strategy is that susceptible adults emerging from non-Bt cotton refuges mate with resistant adults emerging from Bt cotton, which requires movement of adults between Bt cotton and non-Bt cotton. To better understand movements of pink bollworm, the key lepidopteran pest of cotton in Arizona, we analyzed the distribution of wild males and dispersal of sterile males during 1997 on a 259-ha block of a commercial farm containing 76 ha of Bt cotton (69%) and 34 ha of non-Bt cotton (31%). The distribution of 28,397 wild males caught in pheromone traps between 22 May and 20 September suggests that many moved at least 400 m from non-Bt cotton to Bt cotton, yet movement was not sufficient to distribute wild males randomly between Bt cotton and non-Bt cotton. The average number of wild males caught per trap in non-Bt cotton was nearly double that for Bt cotton. Of the estimated 176,000 sterile males released on 8 dates from 18 June through 4 July, we captured 4,447 in pheromone traps between 19 June and 24 July, which represents a recovery rate of 2.5%. The results show that 66–94% of captured sterile males dispersed 400 m or less from the release sites. Along with previously published data, the results reported here suggest that, to promote mating between susceptible and resistant adults, refuges for pink bollworm should be close to Bt cotton.