The potato leafhopper, Empoasca fabae, is perhaps the most abundant insect pest in New York State alfalfa fields.
It is potentially the most damaging of the alfalfa pests because of the debilitating effect of its feeding on newly established alfalfa stands and stunting of regrowth alfalfa.
In many respects the potato leafhopper is quite different from the other major insect pests of alfalfa in New York. It does not overwinter in New York or the Northeast. Instead, it migrates into NYS each year from the South. The numbers and time of arrival vary each year, depending on the spring climate, weather patterns, and spring crop production in the South. It can arrive from mid-May to late June.
The type of feeding is also different. Weevils are basically chewing insects, consuming leaf tissue. The leafhopper is a sucking insect, removing plant sap from the vascular system of the plant. In the process of removing sap, leafhopper insects leave in the plant a salivary secretion that causes hopper burn or tip burn, resulting in injury to the plant. The characteristic yellowing or reddening of the alfalfa leaflets during the summer months are a result of the salivary toxin injected during feeding. By adversely affecting the vascular system, leafhopper feeding reduces photosynthesis, decreases productivity, stunts the plant, and sometimes kills young seedlings. A third difference is that the leafhopper does not occur in distinct generations or peaks. The adults are very long-lived and the generations continue to overlap and increase through mid-August.
The potato leafhopper is a small, wedge-shaped green insect about 1/8 inch long. It has long hind legs that allow it to hop like a grasshopper, and it has very powerful wings that allow it to fly quickly. Adults and nymphs walk backward and sideways as well as they walk forward. The leafhopper feeds on the underside of the alfalfa leaflet and stems, sucking sap from the veins. Adult females also implant eggs in the veins with the aid of a sharp ovipositor. Females lay about three eggs a day over a six- to eight-week period. Eggs, which are not visible to the naked eye, take about nine days to hatch, depending on the prevailing temperature. Nymphs, which are very pale green and hard to see on the plant, are miniature versions of the adult but have no wings. They go through five stages, or molts, in about two weeks before they become adults. The entire life cycle takes a little longer than three weeks.
We do not yet have a reliable method to forecast damage by the leafhopper because damage relates to the density of the leafhopper and the age and state of the stand at the time of infestation. We do know that very early detection of the leafhopper-before damage appears-is essential for good management. Very young plants and plants in very early stages of regrowth are the most sensitive to leafhopper damage. Damage is intensified by moisture stress during drought. In general, if the alfalfa plant is more than 14 inches tall before the leafhopper begins feeding on it, no reduction in yield will result. If leafhoppers infest the alfalfa regrowth when it is 2 to 4 inches tall, however, densities as low as one per sweep can cause economic damage under the right circumstances. Potato leafhopper damage is intensified by drought. Under severe drought conditions, where dry weather conditions are expected to continue, leafhopper action thresholds can be halved, especially for new seedings.
Potato leafhoppers are detected by sweeping the field using a standard 15-inch-diameter insect sweep net. While walking forward, swing the net into the tops of alfalfa stems using a pendulum motion. Count the number of sweeps taken each time the net passes in front of you (see Fig. 4.10.2). Five sets of sweeps (ten sweeps per set) collected from different areas of the field are generally used for making management decisions. Count PLH found in each individual set of ten sweeps.
The management decision is made by comparing the number of leafhoppers (adults and nymphs) per sweep with the height of the alfalfa using the following chart as a guide:
|Average stem length||Leafhopper/sweep|
|less than 3 in. (new seedlings)||0.2|
|3 to 7 in.||0.5|
|8 to 10 in.||1.0|
|11 to 14 in.||2.0|
|15 in. or above||If leafhoppers exceed 2.0 per sweep and if regrowth is within 1 week of harvest, no action needed. If not, use a short-residue insecticide.|
The two lowest treatment levels are specifically for use in new seedings, which warrant protection at lower leafhopper densities.
Potato Leafhopper Resistant Alfalfa: Recent advances in the development of PLH resistant alfalfa have made the planting of resistant alfalfa a viable alternative to insecticides for the management of leafhoppers. Planting the newest generation of PLH resistant alfalfa hybrids is strongly suggested for the management of PLH in both clear alfalfa seedings and in stands mixed with grass species. Please refer to the alfalfa variety table to evaluate the different available PLH resistant alfalfa varieties.