The evaluation of resistance risk to Cry2Ab and cross-resistance to other insecticides in Helicoverpa armigera

To delay the resistance of pests to Bt cotton producing Cry1Ac, pyramided cotton has been planted to replace Cry1Ac-cotton. However, the resistance mechanism of insects to Cry2Ab is rarely researched. In this study, a Cry2Ab-resistant Helicoverpa armigera strain (96-2Ab60) was selected in laboratory, which had a resistance ratio of 778.84-fold compared to the 96S susceptible strain. Genetic analysis showed that the resistance of 96-2Ab60 strain was controlled by more than one locus, and inheritance mode was incompletely dominant. The Cry2Ab-resistant H. armigera had high cross-resistance to Cry1Ac (284.28-fold), Cry1Fa (282.50-fold), Cry1Aa (> 71.40-fold), Cry2Aa (30.14-fold) toxins, and low cross-resistance to Cry1Ab (9.94-fold) and Cry1Ca (> 8.05-fold), while it had no cross-resistance to abamectin and spinetoram and negative cross-resistance to Vip3Aa toxin (0.14-fold). The fitness costs of 96-2Ab60 resistant strain were evaluated on toxin-free artificial diet, compared with 96S strain, the life table parameters such as pupa survival rate, pupa weight, oviposition period, hatching rate of egg, r, and lambda in 96-2Ab60 were significantly decreased, and total pre-oviposition and T were significantly increased. There were obvious fitness costs in 96-2Ab60 strain whose fitness value R-f (0.7341) was lower than that of the 96S strain. The larval mortalities of 96-2Ab60 and 96S fed on either DP33B (single-toxin cotton) or Bollgard II (pyramided cotton) were significantly higher than those fed on non-Bt cotton; however, the mortalities of 96-2Ab60 were obviously reduced compared with 96S. These results indicated that although these two Bt cottons could kill part of 96-2Ab60 larvae, 96-2Ab60 already had resistance to them. These results provide useful information to further understand Cry2Ab resistance mechanism and apply pyramided cotton for managing resistance in H. armigera.