Assessing the role of non‐cotton refuges in delaying Helicoverpa armigera resistance to Bt cotton in West Africa

Abstract

Non-cotton host plants without Bacillus thuringiensis (Bt) toxins can provide refuges that delay resistance to Bt cotton in polyphagous insect pests. It has proven difficult, however, to determine the effective contribution of such refuges and their role in delaying resistance evolution. Here, we used biogeochemical markers to quantify movement of Helicoverpa armigera moths from non-cotton hosts to cotton fields in three agricultural landscapes of the West African cotton belt (Cameroon) where Bt cotton was absent. We show that the contribution of non-cotton hosts as a source of moths was spatially and temporally variable, but at least equivalent to a 7.5% sprayed refuge of non-Bt cotton. Simulation models incorporating H. armigera biological parameters, however, indicate that planting non-Bt cotton refuges may be needed to significantly delay resistance to cotton producing the toxins Cry1Ac and Cry2Ab. Specifically, when the concentration of one toxin (here Cry1Ac) declined seasonally, resistance to Bt cotton often occurred rapidly in simulations where refuges of non-Bt cotton were rare and resistance to Cry2Ab was non-recessive, because resistance was essentially driven by one toxin (here Cry2Ab). The use of biogeochemical markers to quantify insect movement can provide a valuable tool to evaluate the role of non-cotton refuges in delaying the evolution of H. armigera resistance to Bt cotton.

Potential shortfall of pyramided transgenic cotton for insect resistance management

“…Accordingly, in pests with low susceptibility to Cry1A and Cry2A toxins, crossresistance between Cry1A and Cry2A will generally hasten evolution of resistance. In accord with results from other simulation models (14,15,20,28), the modeling results presented here (Figs. 5 and 6) indicate that resistance to a two-toxin pyramid evolves faster when each toxin of a pyramid does not kill all individuals resistant to the other toxin, as seen with H. zea (Fig.…”

“…Although most of the optimal conditions for pyramids apply to this model system, they may not apply for some other pestBt crop combinations, particularly when pests have inherently low susceptibility to one or more of the toxins in the pyramid (19,20,28,46). Here we found several deviations from optimal conditions for H. zea and Bt cotton producing Cry1Ac and Cry2Ab.…”

“…As shown here and in other studies using simulation models, nonrecessive resistance can accelerate the evolution of resistance to two-toxin cotton (14,15,20). Fitness costs, which occur when fitness on non-Bt host plants is lower for Btresistant than susceptible insects, can slow evolution of resistance, particularly to two-toxin plants (19,32).…”

“…Redundant killing, which occurs when each toxin produced by a two-toxin Bt cultivar kills all insects resistant to the other toxin, is essential for optimal success of the pyramid strategy (14,15,20,28). …”

“…Survival of susceptible H. zea larvae on two-toxin cotton can reach 5% during the growing season (5,(16)(17)(18), which could reduce the efficacy of the pyramid strategy. Furthermore, as Bt cotton plants age, toxin concentrations decline, which could increase survival of pests that have inherently low susceptibility to Bt toxins (19,20). Redundant killing could also be undermined if selection for resistance to one of the toxins causes cross-resistance to the other toxin (5,14,19,21).…”