1. The concentration of a single plant species/variety in ever larger and more extensive monocultures increases its apparency to pests and the number of pest species which colonize it(Strong et al 1984)
2. Generally, high yielding crop cultivars can provide improved conditions for pest colonization, spread and rapid growth
3. Reductions of natural enemies around crops means that natural enemies of pests must come to the crop from increasingly small and more distant non-crop reservoirs, entering crops too late or in too little numbers to prevent pest outbreaks.