Cultivation Of Bt Cotton Research Articles

Frequency of Bt resistance alleles in Helicoverpa armigera in 2007–2009 in the Henan cotton growing region of China

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is a key insect pest of cotton in the Henan cotton growing region of China. In this region, cotton is grown on small acreages in rich agricultural landscapes, contrary to cropping systems in the United States or Australia. Under such cropping regimes, naturally occurring refuges (with non-Bt plants) may be sufficient to prevent H. armigera resistance development to Bt toxins. In order to gain a timely understanding of the evolution of resistance of H. armigera to Bt toxin after continuous cultivation of Bt cotton for c. 10 years, we assessed the frequency of alleles conferring resistance to Cry1Ac toxin in field populations of H. armigera sampled from Xinxiang County in Henan province during 2007–2009. Screening F 1 and F 2 generations from isofemale lines, derived from female moths trapped in the field, were used with a discriminating dose of Cry1Ac diet to estimate the frequency of resistance alleles. Totals of 625, 516 and 488 isofemale lines were screened for the F 1 generation in 2007, 2008 and 2009, respectively. Resistance gene frequency in Xinxiang fluctuated between 0.0000 and 0.0005, and it did not increase significantly from 2007 to 2009. Based on the relative average development rates (RADR) of H. armigera larvae in F 1 tests, no substantial increase in Cry1Ac tolerance was found in the Xinxiang region over the 3-yr period.

Effects of repeated cultivation of transgenic Bt cotton on functional bacterial populations in rhizosphere soil

The impact of multiple-year (0-5 years) cultivation of transgenic Bacillus thuringiensis (Bt) cotton on the functional bacterial populations in rhizosphere soil was investigated. The transgenic Bt + CpTI cotton line SGK321 and a non-Bt cotton line Shiyuan321 were planted in four fields in which Bt cotton had been continuously cultivated for 0, 1, 3, and 5 years. Rhizosphere soil samples were collected at the seedling, squaring, flower and boll, and boll-opening stages of cotton. Numbers of bacteria involved in nitrogen-fixing, organic phosphate-dissolving, inorganic phosphate-dissolving, and potassium-dissolving were measured with cultivation-dependent approaches. The data presented here showed no consistent statistically significant differences in the numbers of different groups of functional bacteria between rhizosphere soil of Bt and non-Bt cotton in the same field, and no obvious trends in the numbers of the various group of functional bacteria with the increasing duration of Bt cotton cultivation. These studies suggest that multiple-year cultivation of transgenic Bt cotton may not affect the functional bacterial populations in rhizosphere soil.

Assessment of biological and biochemical indicators in soil under transgenic Bt and non-Bt cotton crop in a sub-tropical environment

There is concern that transgenic Bt-crops carry genes that could have undesirable effects on natural and agro-ecosystem functions. We investigated the effect of Bt-cotton (expressing the Cry 1Ac protein) on several microbial and biochemical indicators in a sandy loam soil. Bt-cotton (MRC-6301Bt) and its non-transgenic near-isoline (MRC-6301) were grown in a net-house on a sandy clay loam soil. Soil and root samples were collected 60, 90, and 120 days after sowing. Soil from a control (no-crop) treatment was also included. Samples were analysed for microbial biomass C, N and P (MBC, MBN, MBP), total organic carbon (TOC), and several soil enzyme activities. The microbial quotient (MQ) was calculated as the ratio of MBC-to-TOC. The average of the three sampling events revealed a significant increase in MBC, MBN, MBP and MQ in the soil under Bt-cotton over the non-Bt isoline. The TOC was similar in Bt and non-Bt systems. Potential N mineralization, nitrification, nitrate reductase, and acid and alkaline phosphatase activities were all higher in the soil under Bt-cotton. Root dry weights were not different (P > 0.05), but root volume of Bt-cotton was higher on 90 and 120 days than that of non-Bt cotton. The time of sampling strongly affected the above parameters, with most being highest on 90 days after sowing. We concluded from the data that there were some positive or no negative effects of Bt-cotton on the studied indicators, and therefore cultivation of Bt-cotton appears to be no risk to soil ecosystem functions.

A Modified F1 Screening Method for Detecting Resistance Gene Alleles to Bt Cotton in Helicoverpa armigera (Lepidoptera: Noctuidae)

Large-scale cultivation of Bt cotton places high selection pressure on target insects and, consequently, may prompt resistance evolution in pest populations. To better monitor Bt-resistance allele frequency in field populations, a modification of the F1 screen was developed to screen F1 progeny derived from single pair mating between field-collected males and laboratory resistant females (designated as F1 screen). This method was used to survey a field population of Heliocoverpa armigera (Hübner) for resistance alleles at the same loci in the resistant strain. After treatment of the F1 progeny derived from more than 260 single pair mating lines with Bt cotton, there was no clear separation of resistant genotypes from susceptible genotypes based on survival rates which demonstrated a linear distribution within a certain range. We further analyzed larval growth data of the F1 progeny and found a correlation between larval body weight and survival rate. The maximum correlation was obtained when F1 larval body weight reached ≥0.7 mg. To avoid underestimation, body weight ≥0.6 mg was used to initially separate potential positive lines (resistant genotype). After that, a corrected survival rate of >21.3% (the minimal theoretical rate of >25% for F0 males to carry sr and rr genotypes) was used as the criterion to examine F2 progeny derived from single sib-mating of F1 adults and to verify whether the potential positive lines carried resistant alleles.

Bt Cotton Performance and Constraints in Central India

Field data collected in central India over a two-year period showed that Bt cotton hybrids reduced plant protection costs by Rs1,268/ha (Rs41 = US$1), raised the crop yield by 2.01 q/ha, worth Rs6,394/ha, and reduced the time spent by the crop in the field by 15–20 days. Cultivation of Bt cotton involved an additional cost of Rs3,720/ha, but increased the net returns by Rs2,674/ha over conventional hybrids. The higher price of seed, high risks, poor refuge management, incidence of wilt, high rates of discontinuance and poor monitoring were the major constraints reported by growers. The impact would be greater if the technology were embodied in the commonly grown hybrids. Development of transgenic varieties is one of the prime ways of reducing crop costs for the resource-poor conditions of rainfed central India.

Interactions of Bacillus thuringiensis Cry1Ac toxin in genetically engineered cotton with predatory heteropterans

A number of cotton varieties have been genetically transformed with genes from Bacillus thuringiensis (Bt) to continuously produce Bt endotoxins, offering whole plant and season-long protection against many lepidopteran larvae. Constant whole-plant toxin expression creates a significant opportunity for non-target herbivores to acquire and bio-accumulate the toxin for higher trophic levels. In the present study we investigated movement of Cry1Ac toxin from the transgenic cotton plant through specific predator-prey pairings, using omnivorous predators with common cotton pests as prey: (1) the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae), with the predator Podisus maculiventris (Heteroptera: Pentatomidae); (2) the two-spotted spider mite, Tetranychus urticae (Acarina: Tetranychidae), with the predatory big-eyed bug Geocoris punctipes (Heteroptera: Geocoridae) and (3) with the predatory damsel bug Nabis roseipennis (Heteropera: Nabidae); and (4) the thrips Frankliniella occidentalis (Thysanoptera: Thripidae) with the predatory pirate bug Orius insidiosus (Heteroptera: Anthocoridae). We quantified Cry1Ac toxin in the cotton plants, and in the pests and predators, and the effects of continuous feeding on S. exigua larvae fed either Bt or non-Bt cotton on life history traits of P. maculiventris. All three herbivores were able to convey Cry1Ac toxin to their respective predators. Among the herbivores, T. urticae exhibited 16.8 times more toxin in their bodies than that expressed in Bt-cotton plant, followed by S. exigua (1.05 times), and F. occidentalis immatures and adults (0.63 and 0.73 times, respectively). Of the toxin in the respective herbivorous prey, 4, 40, 17 and 14% of that amount was measured in the predators G. punctipes, P. maculiventris, O. insidiosus, and N. roseipennis, respectively. The predator P. maculiventris exhibited similar life history characteristics (developmental time, survival, longevity, and fecundity) regardless of the prey's food source. Thus, Cry1Ac toxin is conveyed through non-target herbivores to natural enemies at different levels depending on the herbivore species, but continuous lifetime contact with the toxin by the predator P. maculiventris through its prey had no effect on the predator's life history. The results found here, supplemented with others already published, suggest that feeding on Cry1Ac contaminated non-target herbivores does not harm predatory heteropterans and, therefore, cultivation of Bt cotton may provide an opportunity for conservation of these predators in cotton ecosystems by reducing insecticide use.

Farm-Level Performance of Genetically Modified Cotton

In this paper, the yield increases resulting from the cultivation of Bt cotton in Maharashtra, India, are analysed. The study relies on commercial farm, rather than trial, data and is among the first of its kind to be based on real farm and market conditions. Findings show that since its commercial release in 2002, Bt cotton has had a significant positive impact on yields and on the economic performance of cotton growers in Maharashtra. This difference remains even after controlling for different soil and insecticide inputs in the production of Bt cotton. There is also significant spatial and temporal variation in this‘benefit’, and much depends upon where production is taking place and on the season.

Regional reversion of insecticide resistance in Helicoverpa armigera (Lepidoptera: Noctuidae) is associated with the use of Bt cotton in northern China

A continuous programme for monitoring resistance of Helicoverpa armigera (Hubner) to commonly used insecticides was undertaken for assessing the impact of Bt cotton planting on the evolution of the pest resistance from 1994 to 2002 in China. The bioassay results showed that the resistance levels per year in field populations of H armigera to lambda-cyhalothrin, phoxim and endosulfan decreased, respectively, to 9-15-fold, 11-14-fold and 6-8-fold in 2001-2002 from 197-262-fold, 52-74-fold and 18-38-fold before the cultivation of Bt cotton in 1997. This significant increase in susceptibility to insecticides is expected to result in a reduction in insecticide application for H armigera control in Bt cotton. It is concluded that Bt cotton is playing an important role in the long-term management of H armigera by increasing the potential for natural and chemical control of the pest.