Although populations of potato leafhopper on alfalfa have been somewhat low in June, a few locations are reporting growing numbers. Second cut has happened or is happening in many locations. But if alfalfa is more than 7 days from a cut, a rule of thumb threshold for plants under normal stress is: when the average number of leafhoppers in a single sample (10 sweeps) equals or is greater than the height of the alfalfa, treatment should be considered if harvest is more than 7 days away. For example, if the alfalfa is 8 inches tall and the average number of leafhoppers per sample is 8 or higher, treatment is warranted. If the average is 7 or lower, the grower should come back within a few days to see if the population is higher or lower.
Vigorous alfalfa can tolerate higher numbers, and stressed alfalfa can tolerate fewer. The threshold should be lowered when the alfalfa is under stress, especially for new seedings made this year. Consider lowering the threshold to half the normal level for new seedlings that are growing slowly because of drought stress. In those situations, there may NOT be a yield response from insecticide treatment in the current growth cycle, especially if drought conditions persist. However, protecting the plants from leafhopper damage now will protect the stand, development of the taproot, and its future yield potential. Potato leafhopper resistant varieties can also tolerate higher numbers. Our data to date indicate the threshold for PLH resistant varieties in established stands is 4 times the normal threshold.
(Source – http://www.farms.com/news/potato-leafhopper-in-alfalfa-109551.aspx)