Bryan Jensen, UW Extension and IPM Program
Alfalfa
Surprising, at least to me, potato leafhopper (PLH) populations have been relatively high in alfalfa this summer despite frequent and heavy rains. Typically, PLH populations peak during periods of hot dry weather. Go figure. Your next question might be “How long will they stick around”? It is difficult to predict with any hint of accuracy. Typically, you see PLH numbers “slow down” by mid-August. Some years I have seen populations crash in mid-July. Other years they are still causing economic damage after Labor Day. My best answer to the question may seem a little simplistic but is the best answer I have. Continue to scout until you are absolutely certain populations are tapering off.
Soybeans
I have noticed several PLH nymphs on soybeans while spot-checking for soybean aphids. This is not uncommon and they can, under extreme circumstances, become an economic pest in soybean. While you might find references indicating leaf hairs may deter PLH feeding they may still become established. PLH damage to soybean can look a little like K deficiency which can be described as marginal chlorosis appearing initially on the lower leaves. However, unlike K deficiency symptoms, PLH injury may be noticed on all areas of the plant. Furthermore, puckering/crinkling of leaves is often associated with advanced PLH feeding.
At this time of the growing season, PLH scouting is best accomplished using an insect sweep net. Consider treating if you have a field average of approximately 6 leafhoppers/sweep. Before treating I would consider doing a whole plant inspection and look for nymphs which will indicate an established population.