• teaching tools
• scientific methodologies

Resistance evolution is a real risk. Scientists concur that in the absence of a management strategy insect pests are likely to evolve resistance to transgenic insecticidal crops - such as the Bt crops - and cause crop losses.

Resistance evolution in pests is a manageable problem. Different crops and cropping systems offer different options for resistance management, but a practical management strategy can be developed that is specific to the crop, farming system and the environment in question.

Step 1. Identification of species at risk of resistance

All target species and some non-target species may evolve resistance, but some are more likely to do so than others, depending in part on their association with the crop, likely exposure to the transgene product and the genetic structure of their populations. The idea is to determine which species is at the greatest risk of developing resistance and to concentrate efforts around this weak link.

• analysis of species at risk of resistance in Kenya

Step 2. Dose and dominance

These are technical options that determine whether resistance is 'recessive' or 'dominant'. Dose refers to the concentration of transgene product to which the pest is exposed in the plant. If the resistance trait is functionally recessive (a high dose) it will take longer for resistance to evolve and allows more management options than if resistance is dominant.

Step 3. Assessing the degree of risk

Several factors influence the degree of risk, including the frequency of resistance, mating behavior and movement of adults, host plant use, occurrence of natural 'refuges', fitness cost associated with resistance, and the regional farming system. A refuge in this context is defined as a place where selection for resistance does not occur.

Step 4. Practical resistance management

Focusing on the weak link, practical and practicable management strategies can be designed. Most of these strategies require some designed implementation of refuges.

• resistance management strategies for smallholder farmers in Kenya

These resistance management strategies can be developed before field release of the GM crop. At the same time, critical information gaps are identified, which provides time for observation and research to develop an even more effective resistance management plan. It is important to consider here what post release monitoring of resistance frequencies is needed. Groundwork for that monitoring approach should be done before field release.

Refuges delay resistance by providing susceptible insects to mate with the resistant ones from the GM crop

pink bollworm

stemborer in maize

some pests can move between cotton and maize