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Bt cotton expressing Cry1Ac insecticidal protein was introduced in India in 2002. Within a span of 12 years, Bt-cotton constituted more than 90% of cropping area covering 11.6 mHa. This large scale cultivation favours resistance evolution in target pests. One of the major factors that influences rate of resistance is frequency of resistance alleles. Using F 2 screen methodology and a discriminating dose, we estimated the level of resistance to Cry1Ac toxin in field populations of Helicoverpa armigera (Hübner). We report an increase in Cry1Ac resistance allele frequency in field populations of H . armigera collected in Telangana and Andhra Pradesh states of India. Collective allele frequency was 0.085 (95% CI) for 2013 and 0.035 (95% CI) for 2014, which was 65 times and 29 times higher than the initial frequencies estimated during 2004. To prevent further increase in Bt resistance frequency in H . armigera , it is necessary to integrate other resistance management tactics that preserves pest susceptibility to Bt cotton. [ABSTRACT FROM AUTHOR]

Fall armyworm, Spodoptera frugiperda (J.E. Smith), is a major pest of many crops and a cross-crop target of transgenic maize, cotton, and soybean containing Bacillus thuringiensis (Bt) genes. Some of the current Bt maize products for controlling lepidopteran species contain the Bt event MON 89034. The objective of this study was to determine the frequency of resistance alleles in field populations of S. frugiperda collected from Louisiana and Florida, U.S. to Cry1A.105, one of the two Bt genes in MON 89034. A total of 150 F 2 two-parent families of S. frugiperda were established using single-pair mating of field-collected individuals in 2011, which included 79 families from two locations in Louisiana and 71 families from one location in Florida. F 2 screen was conducted to detect resistance alleles in these families to Cry1A.105 protein in maize plants. Four out of the 79 Louisiana and 14 out of the 71 Florida families were identified to possess resistance alleles to the Cry1A.105 maize plants. Thus, the corresponding frequency of resistance alleles to Cry1A.105 maize was estimated to be 0.0158 with a 95% credibility interval (CI) of 0.0052–0.0323 for the Louisiana populations and 0.0559 with a 95% CI of 0.0319–0.0868 for the Florida populations. The resistant families survived on whole Cry1A.105 maize plants and demonstrated a significant level (>116-fold) of resistance to the Cry1A.105 protein in a diet-incorporated bioassay. These findings suggest that resistance allele frequency in S. frugiperda to single-gene Cry1A.105 maize in the U.S. southeast region apparently is not rare, most likely due to the selection of Cry1F resistance and its cross-resistance to Cry1A.105. [ABSTRACT FROM AUTHOR]

High-dose/refuge is the most common resistant management strategy used to delay or prevent pest adaptation to transgenic plants expressing Bacillus thuringiensis Berliner toxins. The success of this strategy depends, among other factors, on a low initial resistance allele frequency. In this study, we used genotypic and phenotypic methods to estimate the frequency of resistance to Vip3Aa20 toxin in Spodoptera frugiperda (J. E. Smith) in Brazil. In genotypic monitoring (F 2 screen), 11 populations were screened from 2013 to 2014 in diet-overlay or excised-leaf bioassays. In phenotypic monitoring, 50 populations were screened from 2013 to 2015, by exposition of neonates at a diagnostic concentration of 3600 ng/cm 2 of Vip3Aa20 in diet-overlay bioassay. The resistance allele frequency to Vip3Aa20 overlay on diet or expressed on corn leaves was similar 0.0012 (95% CI, 0–0.0033) and 0.0011 (95% CI, 0–0.0031), with an overall frequency of 0.0009 (95% CI, 0–0.0021). The resistance allele frequency does not differ among populations from different Brazilian states. In the phenotypic monitoring, only three populations MS19, MT24 and PR51 from Chapadão do Sul (MS), Lucas do Rio Verde (MT) and Castro (PR) showed larval survival ranging from 0.87 to 0.97%, differing from a susceptible reference population. In summary, the frequency of resistance to Vip3Aa20 toxin in Brazilian populations of S. frugiperda was low. Therefore, if other factors such as recessive inheritance of resistance and high level of refuge compliance are met, the risk of resistance to Vip3Aa20 in S. frugiperda populations in Brazil can be minimized. [ABSTRACT FROM AUTHOR]