Scout for Potato Leafhopper Populations in Your Alfalfa Fields
This is the time of year that alfalfa growers should turn their attention to potato leafhopper. I have yet to see this insect pest this year, but it is bound to arrive soon,  perhaps with rains that could arrive in Pennsylvania in the next few days. Potato leafhoppers (Figure 1) do not live in Pennsylvania all year, but migrate here each spring on storm fronts, typically arriving in June. Growers would be wise to keep an eye out for increasing populations of this pest, as once feeding is evident, economic damage has already been done. Potato leafhoppers are perennially the most damaging pest of alfalfa in Pennsylvania, but they can also cause economic damage in other legume forages and some vegetable crops. Their feeding reduces yield and quality (especially lower protein content) and can decrease stand longevity.
Potato leafhoppers can be frustrating due to the sporadic nature of their infestations—even in bad leafhopper years, some fields escape damage. Once potato leafhoppers colonize alfalfa fields, adults deposit eggs into stems and leaf veins. In warm weather, these eggs will develop into adults in about three weeks, so populations can increase quickly. Potato leafhoppers have straw-like mouthparts and extract plant juices. Heavy feeding disrupts nutrient flow within plants, causing yellow triangles to form at the leaflet tips ("hopper burn"), but this evidence of damage does not develop until 7-10 days after feeding begins. As feeding continues, damage worsens (Figure 2), and the chlorotic areas spread toward the base of the leaflet. Once hopper burn is evident, economic loss has occurred.
If damaging populations develop, early harvest or insecticides are often the only choices. Early harvest can stop the damage, but regrowth should also be scouted to determine if the next cutting also develops damaging populations. To target leafhoppers most effectively, populations should be sampled and treatment applied only when economic thresholds are exceeded. In my experience, regular scouting with a sweep net and use of economic thresholds can limit the need for insecticides to once a summer. Scouting details and economic thresholds are available in our potato leafhopper fact sheet.
If you want to think about improving potato leafhopper control in the coming years, consider planting potato leafhopper-resistant varieties of alfalfa. These varieties limit leafhopper populations because they are covered with fine hairs (the hairs are actually "glandular trichomes"), which decrease leafhopper feeding. Another option is to mix other forages in with alfalfa. Alfalfa/orchardgrass stands (or other combinations) are much better at tolerating leafhopper damage than pure stands of alfalfa. Lastly, adopting IPM can go a long way to controlling leafhoppers. Spiders and other natural enemies can kill potato leafhoppers, and using integrated pest management (IPM) to spray insecticides only when economic populations develop will help maintain these allies in pest control. In fact, we have growers in Pennsylvania who do not treat their alfalfa fields with insecticides and claim they do not see potato leafhopper damage; natural enemies would be the best explanation for this lack of damage.
