PINK BOLLWORM RESISTANCE MONITORING ANALYSES
Bruce Tabashnik, University of Arizona
Pink Bollworm Resistance Monitoring Analyses, Project 07-107
During 2012, DNA screening of pink bollworm collected from the field in 2011 did not detect any evidence of resistance to Cry1Ac. These results support the conclusion that pink bollworm resistance to Cry1Ac remained rare in the field in 2011. The scarcity of pink bollworm in 2011 and 2012 is also consistent with the conclusion that this pest remained susceptible to Bt cotton.
Based on a decade of field and laboratory research, the Pink Bollworm Resistance Management Team at the University of Arizona developed DNA screening to detect pink bollworm resistance to Cry1Ac, one of the toxins in Bt cotton that kills pink bollworm. Bollgard cotton produced only Cry1Ac. Newer Bt cottons produce two Bt toxins. Bollgard II makes Cry1Ac and Cry2Ab. Widestrike, which is much less popular in Arizona, makes Cry1Ac and Cry1F, but Cry1F is not effective against pink bollworm. Although Bollgard was the main type of Bt cotton grown in Arizona from 1996 to 2005, its registration expired in 2009, and Bollgard II was the predominant type of Bt cotton grown in Arizona during 2010-2012.
In our bioassays, live pink bollworm larvae feed individually on diet treated with a known "diagnostic" concentration of a Bt toxin (Cry1Ac or Cry2Ab). If they survive, we identify them as resistant. However, an important limitation of bioassays is that they require collection of enough live pink bollworm from the field to establish strains in the laboratory that can be reared for testing of live larvae in subsequent generations. Our DNA screening for resistance to Cry1Ac can be done with preserved, dead individuals of any life stage. In particular, we can test males collected in the field that were caught in traps baited with pink bollworm female sex pheromone.
Cotton bolls collected during 1997-2008 consistently yielded enough pink bollworm to establish new laboratory strains for bioassays each year. However, as the pink bollworm eradication program has progressed, it has become increasingly difficult to collect enough live pink bollworm from the field to establish laboratory strains for bioassays. So far, we have found no pink bollworm larvae in more than 7,000 bolls of non-Bt cotton collected from 14 sites in Arizona during 2012. Therefore, our DNA screening for resistance has become essential.
During 2012, we completed DNA screening of 203 pink bollworm collected from the field in 2011. From these 203 pink bollworm collected during 2011, DNA suitable for screening was obtained and screened from 152 individuals (86 from Arizona, 8 from Texas, and 58 from Baja California). We detected no evidence of genetic changes associated with resistance to Cry1Ac in these 152 pink bollworm.