Field margins provide a refuge for pest genes beneficial to resistance management

Fencelines and field margins in broad-acre cropping systems are commonly a refuge for weeds, diseases and invertebrates because they avoid many cropping and pest management regimes applied inside fields. As such, fenceline refuges are often managed separately with the goal to reduce pest reinfestation of fields from the margins. However, the implications of these pest control strategies are poorly understood in terms of their impact on pest genes beneficial to pesticide resistance management. Fenceline management, such as selectively reducing pest populations through insecticides, or non-selectively modifying habitat quality by removing host weeds with herbicides, might increase or decrease resistance evolution rates. Indeed, the potential to perform selective and non-selective control of pests separates management of field margins from structured in-field susceptible refuges (e.g. Bt crops). Here, a simulation approach was used to explore the effect of different fenceline management strategies, cropping characteristics and pest genetics on resistance evolution. The analysis was applied to a major crop pest, the mite Halotydeus destructor, for which fenceline treatments of herbicides and insecticides may be applied. Spraying fencelines with an insecticide decreased reinfestation and the overall abundance of mites, compared with not applying insecticides to fencelines. However, in all scenarios tested, resistance evolution was delayed by leaving fenceline refuges unsprayed with insecticides or herbicides. Just as field margins may provide a reservoir for invertebrates beneficial to pest management (e.g. predators and parasitoids), they may also serve as an important refuge for genes beneficial to resistance management.