Alfalfa varieties with greater than 50% potato leafhopper (PLH) re-sistance can withstand three times as many leafhoppers as suscep-tible varieties before insecticide treatment is warranted.

That’s according Ohio State University researchers. In a three-year trial, they compared yields from a highly resistant variety and a susceptible variety that received timely insecticide treatments.

Total yield for the untreated resistant variety was 0.8 ton/acre (0.27 ton/acre/year) lower than for the treated susceptible variety. The yield difference roughly equaled the cost of the insecticide treatment, so economic returns were equal for the two varieties.

In the susceptible variety, economic yield loss was observed in 11 of the 14 cuttings. But PLH numbers were much lower in the resistant variety. Economic yield loss was observed in only four of the 14 cuttings, and only when PLH numbers were three to four times higher than the normal economic threshold for susceptible alfalfa.

“We conclude that the economic threshold for alfalfa varieties with high levels of PLH resistance is about three times higher than the economic threshold for susceptible alfalfa, and PLH populations rarely reach this level,” say the researchers.