Theoretically tested remediation in response to insect resistance to Bt corn and Bt cotton: A new paradigm

Various models of density dependence predicted different evolutionary outcomes for Helicoverpa zea, Diabrotica virgifera, and Ostrinia nubilalis using simple and complex resistance evolution models, different dose assumptions and refuge proportions. Increasing available refuge increased durabilities of pyramided Plant-Incorporated-Protectants (PIPs), especially between 1–5%. For some models of density dependence and pests, additional refuge resulted in faster adaptation rates. Significant considerations should be given to a pest's intra-specific competition in simple and complex theoretical models when designing insect resistance management plans. Life-history, refuge, and dose characteristics of a PIP had different effects on the adaptation rate of a generic pest of Bt, and unexpected outcomes occurred. Intrinsic growth rate 'R0' was the strongest evolutionary force, and large R0's reduced time to resistance for a high dose PIP to similar levels as projected for a low dose PIP. This was caused by differential density dependent effects in refuge and Bt fields that elevated generational resistance increases beyond those from selection alone. Interactions between density dependence and R0 were always present and further affected the life-time of the PIPs. Varying 'average dispersal distance' did not affect evolutionary outcomes; however, increasing the proportion of the population engaging in dispersal often increased the durability of high dose PIPs. When resistance genes spread from a hypothetical hotspot, local resistance phenomena developed in the immediate surroundings. Higher growth rates lead resistance to spread faster through the landscape than lower rates. Increasing available refuges slowed adaptation rates to single PIPs and low dose pyramids, although non-linear trends were possible. Integrated Pest Management (IPM) practices at the onset of PIP commercialization slowed pest adaptation rates. For corn rootworm, interspersing non-selective periods with IPM+IRM delayed resistance evolution, yet crop rotation was the best strategy to delay resistance. For bollworm inclusion of isoline corn as an IPM tool did not increase the life-time of the PIP. A local resistance phenomenon for rootworm was maintained immediately surrounding the hotspot; random selection of mitigatory strategies in the landscape slowed adaptation rates while mitigation in the hotspot alone did not. Mitigation extended the life-time of the pyramid minimally for both corn rootworm and bollworm