With potato leafhopper densities approaching economic threshholds in some parts of the country, University of Illinois Extension entomologist Mike Gray is encouraging alfalfa growers to begin scouting for the potentially destructive insects.
Gray notes that he received a report from Bond County (southwestern Illinois) last week that leafhoppers had reached densities of 1.0 per sweep in some stands of alfalfa measuring 4-7" tall. "When alfalfa is 6-12" in height, a density of 1.0 leafhopper per sweep has the potential to cause economic losses," he says.
According to Gray, the value of hay per ton, height of the alfalfa and the cost of insecticide application per acre are the key factors to consider when making a decision about treating infestations. "For example, the economic threshold for potato leafhoppers would be 0.5/sweep (using a 15" sweep net) for alfalfa 4-8" in height, with hay valued at $120/ton and an insecticide application cost of $12/acre. If the value of hay is reduced to $60/ton, and the insecticide treatment cost remains the same, the economic threshold increases to 1.0/sweep."
Adult potato leafhoppers are 1/8" long, pale green, wedge-shaped and equipped with needle-like mouthparts that they use to suck fluids from plants and inject their saliva. As a result of their feeding activity, the distribution of nutrients within the plant is disrupted. V-shaped yellowish areas may begin to appear on the tips of leaves and is often referred to as hopper burn.
"This discoloration may be misdiagnosed as a disease or nutrient deficiency, most notably boron," Gray says, adding that a boron deficiency in plants is most often confined to younger leaves.
He also points out that large densities of potato leafhoppers can result in protein and vitamin A reductions in alfalfa and lower the nutritional value for livestock. Three to four generations of potato leafhoppers per year typically occur in Illinois. Above-average temperatures throughout the season, accompanied by dry conditions, may lead to increased feeding and damage.