The refuge concept in insect resistance management : its history and future application in South Africa
Genetically modified (GM) crops developed for insect control express cry genes from the bacterium Bacillus thuringiensis (Bt). These genes produce target specific insecticidal proteins that protect the plant against insect pest attacks throughout the growing season. The largest threat to the continued success of these insecticidal GM crops is the potential development of target pest resistance. Models and theories suggested several possible strategies to delay the development of resistance. Of these strategies the high-dose/refuge strategy was selected as the optimal insect resistance management (IRM) option and is currently implemented throughout the world. The high-dose/refuge strategy comprises planting Bt maize plants that produce high doses of the toxin and non-Bt plants (refugia) in close proximity to one another. The theory behind this strategy is that the high dose of toxin kills nearly all the individuals of the target pest while the refuge area sustains susceptible pest individuals that survive on the crop and mate with survivors on the Bt crop. Recent reports of resistance development to Bt crops has raised questions about the refuge concept. In the cases where resistance has developed it was largely ascribed to non- compliance to the prescribed refuge requirements or non-functionality of the refuge approach. The Bt crops used throughout the world were developed in North America against the insect pests that occur there. Since there are differences in the lifecycles and behaviour of insect species targeted by Bt crops the refuge areas also needs to be specified for different target pests. For example, the currently used IRM strategies do not differentiate between polyphagous and monophagous pests and also do not take into account differences that exist in biology and behaviour of different pest species. These IRM strategies have also been developed mainly with large scale commercial farming systems in mind and do not take into account farming systems in developing countries. Current IRM strategies need to be revised and adapted for use by small-scale African farmers.