Glandular haired (GH) alfalfa varieties with resistance to potato leafhopper (PLH) have been commercially available since 1997. Although the presence of glandular hairs on the leaves and stems is thought to be responsible for PLH resistance, the mechanism by which this occurs is poorly understood. In fact, evidence exists with GH alfalfa for the all three main types of crop resistance to insects to be operating: antibiosis, nonpreference, and tolerance.