Evolution of Helicoverpa armigera resistance to Bacillus thuringiensis (Bt) cotton producing Cry1Ac is progressing in northern China, and replacement of Cry1Ac cotton by pyramided Bt cotton has been considered to counter such resistance. Here, we investigated four of the eight conditions underlying success of the refuge strategy for delaying resistance to Cry1Ac+Cry2Ab cotton, a pyramid that has been used extensively against H. armigera outside China. Laboratory bioassays of a Cry2Ab-selected strain (An2Ab) and a related unselected strain (An) reveal that resistance to Cry2Ab (130-fold) was nearly dominant, autosomally inherited, and controlled by more than one locus. Strong cross-resistance occurred between Cry2Ab and Cry2Aa (81-fold). Weaker cross-resistance (18- to 22-fold) between Cry2Ab and Cry1A toxins was also present and significantly increased survival of An2Ab relative to An on cotton cultivars producing the fusion protein Cry1Ac/Cry1Ab or Cry1Ac. Survival on Cry1Ac+Cry2Ab cotton was also significantly higher in An2Ab than in An, showing that redundant killing on this pyramid was incomplete. Survival on non-Bt cotton did not differ significantly between An2Ab and An, indicating an absence of fitness costs affecting this trait. These results indicate that a switch to three-toxin pyramided cotton could be valuable for increasing durability of Bt cotton in China.
Transgenic crops have revolutionized insect pest control, but their effectiveness has been reduced by evolution of resistance in pests. We analyzed global monitoring data reported during the first two decades of transgenic crops, with each case representing the responses of one pest species in one country to one insecticidal protein from Bacillus thuringiensis (Bt). The cases of pest resistance to Bt crystalline (Cry) proteins produced by transgenic crops increased from 3 in 2005 to 16 in 2016. By contrast, in 17 other cases there was no decrease in pest susceptibility to Bt crops, including the recently introduced transgenic corn that produces a Bt vegetative insecticidal protein (Vip). Recessive inheritance of pest resistance has favored sustained susceptibility, but even when inheritance is not recessive, abundant refuges of non-Bt host plants have substantially delayed resistance. These insights may inform resistance management strategies to increase the durability of current and future transgenic crops.
In pests with inherently low susceptibility to Bacillus thuringiensis (Bt) toxins, seasonal declines in the concentration of Bt toxins in transgenic crops could accelerate evolution of resistance by increasing the dominance of resistance. Here, we evaluated Helicoverpa zea survival on young and old cotton plants that produced the Bt toxins Cry1Ac and Cry1F or did not produce Bt toxins.
Although transgenic cotton producing insecticidal proteins from Bacillus thuringiensis (Bt) is a cornerstone for pink bollworm control in some countries, integrated pest management remains important for bolstering sustainability of Bt cotton and is critical for controlling pink bollworm where Bt cotton is not available or where this pest has evolved resistance to Bt cotton. Here, we used data on moth captures in gossyplure-baited pheromone traps and boll infestations for 163 Bt and 152 non-Bt cotton fields from Arizona to evaluate accuracy of chemical control decisions relying on moth trapping data and capacity of Bt cotton to suppress survival of offspring produced by moths. Assuming an economic injury level of 12% boll infestation, the accuracy of decisions based on moth captures corresponding to economic thresholds of 6%, 8%, and 10% boll infestation increased from 44.7% to 67.1%. The association between moth captures and boll infestation was positive and significant for non-Bt cotton fields but was not significant for Bt cotton fields. Although chemical control decisions based on trapping data were only moderately accurate, pheromone traps could still be valuable for determining when moth populations are high enough to trigger boll sampling to more rigorously evaluate the need for insecticide sprays.