Update on Pink Bollworm Resistance to Bt Cotton in the Southwest

Monitoring of Arizona pink bollworm (PBW), Pectinophora gossypiella, susceptibility to Bt toxin Cry1Ac has been conducted annually since 1997. PBW were collected from cotton fields located throughout the Southwest in 2002, cultured in the laboratory, and tested for susceptibility to Cry1Ac using diet-incorporation bioassays. A total of 13 collections from Arizona were successfully reared and bioassayed. Six California collections and one collection each from New Mexico and Texas were also tested. Laboratory selection of pink bollworm collected in Arizona in 1997 and exposed to Cry1Ac in diet produced a strain capable of survival on Bollgard cotton. Subsequent studies showed that 10 ug Cry1Ac/ml of insect diet was a reliable diagnostic concentration for detection of pink bollworm that were homozygous for resistance to Cry1Ac. On this basis, resistant PBW were detected in 2002 in only 2 out of 13 Arizona strains. The overall frequency of resistant PBW in 2002 strains tested from Arizona was 0.17% and ranged from 0.0 to 1.7%. One of six California collections evaluated had a single resistant survivor. No resistant pink bollworms were detected in the single New Mexico and Texas collections evaluated. Resistant PBW were significantly more abundant in Arizona in 2001 and 2002 than they were in 1998, 1999 or 2000. However, the frequency of resistant survivors in bioassays was low in 2001 and 2002, and markedly lower than in 1997. Field evaluations of efficacy of Bt cotton were conducted by the Arizona Cotton Research and Protection Council in adjacent pairs of Bt and non-Bt fields at 43 Arizona locations. Pink bollworms were found in an average of 23.3% of non-Bt bolls statewide. Bolls from Bt cotton fields yielded an average of 0.144% (range 0 to 1.300%) infested bolls. Of these, all but three of the pink bollworm recovered from plantings of Bt cotton were from bolls that tested negative for Cry1Ac.