In general, there are three approaches that plant breeders use to develop resistant cultivars

Antibiosis. Plants produce a wide variety of defensive compounds (allelochemicals) that protect them from herbivores. These compounds may reduce growth, inhibit reproduction, alter physiology, delay maturation, or induce various physical or behavioral abnormalities in herbivores. By purposely selecting for plants with high levels of allelochemicals, or by breeding such plants with less resistant ones, it is often possible to develop new cultivars that resist pest injury yet still retain desirable horticultural characteristics.

Antixenosis. A physical or chemical property of a plant can make it so unpalatable that it is largely protected from herbivore attack. This type of resistance is often known as nonpreference. It may involve the presence of feeding repellents (or the absence of feeding attractants), or it may involve physical traits such as hairs, waxes, or a thick, tough epidermis that do not provide the pest with a desirable feeding substrate. Alfalfa, for example, has been bred with hairy leaves to deter feeding by the spotted alfalfa aphid.
Tolerance. Some plant genotypes are simply able to "tolerate" injurious insects better than others. Tolerant cultivars may be exposed to the same pest populations as susceptible ones, but they do not suffer as much injury. Many varieties of field corn, for example, have fairly narrow, brittle stalks. When attacked by European corn borers, these stalks are further weakened and break easily in a windstorm. This wind damage, known as lodging, makes the corn hard to pick with mechanical harvesters. Plant breeders have largely resolved this problem by breeding for corn with thicker, stronger cornstalks. These tolerant cultivars are still attacked by corn borers, but they "stand up" to the injury and insure a harvestable crop.