Imperial County alfalfa growers occasionally suffer losses to summer hay cuttings due to leafhopper infestations. Many species of leafhoppers may be found in alfalfa, but species in the genus Empoasca are primarily responsible for injury and yield reductions.
Three species have been found damaging alfalfa in California: the potato leafhopper, Empoasca fabae, and two southern garden leafhoppers, E. solona, and E. mexara. All three species cause identical injury. The prevalent species in the Imperial Valley are E. solana and E. mexara.
Other leafhoppers associated with alfalfa are distinguished from Empoasca sp. by their brown or grey color. Adult Empoasca sp. leafhoppers are small (1/8 inch long), bright green, wedge-shaped insects that have piercing and sucking mouthparts and jump and fly readily when disturbed. Nymphs are also green, wedge-shaped and run rapidly sideways or backward when disturbed. The unusual rapid movements by the leafhopper and their shape easily distinguished them from lygus bug nymphs or slow moving aphids.
Empoasca sp. leafhoppers damage alfalfa through the removal of sap, but the main concern for hay producers is a type of injury referred to as "hopper burn". Hopper burn symptoms result from the injection of salivary toxins into the plant during leafhopper feeding. An early symptom of hopper burn is a characteristic V-shaped yellow area on the leaf tip. This symptom should not be confused with nutrient deficiencies or diseases, in which yellowing of foliage typically begins at leaf margins. As damage increases, the yellow area spreads over the entire leaf and the field takes on a yellow color.
Alfalfa regrowth can be severely stunted, resulting in yield losses. Hay quality can be affected by severe leafhopper injury due to reduction in both the protein and vitamin A. Yellowing and stunting symptoms following a heavy Empoasca sp. leafhopper infestation may carry over into one or two subsequent cuttings, even through the leafhoppers are no longer in the field.
Empoasca sp. leafhoppers attack several other crops and adult leafhoppers can migrate to alfalfa fields from neighboring crops, such as sugarbeets.
Look for leafhoppers during weekly field monitoring with a standard 15-inch insect sweep net. When symptoms first appear, sample a minimum of four to six areas over the entire field by taking 10 sweeps in each area and counting the number of adults and nymphs. Leafhopper infestations usually begin on the field margin so be sure to include field edges in your samples.
An insecticide treatment should be applied for leafhopper control if the alfalfa crop is two or more weeks away from harvest and if counts reach five leafhoppers per sweep. Treat alfalfa scheduled to be harvested in 10 days to two weeks if counts reach 10 Empoasca sp. leafhoppers per sweep. It is not unusual for leafhopper infestation of treatable magnitude to be confined to the first 50 to 100 feet of the field margin, in which case only the field margin should be treated.
Common sense should be utilized when applying treatment thresholds. Heavy leafhopper infestations on young regrowth immediately after harvest are more damaging than similar infestations later in the growth cycle. Alfalfa under stress from other insects, diseases, or lack of water is more susceptible to injury than stress-free alfalfa. Alfalfa within a week of harvest may be able to tolerate very heavy leafhopper populations without yield loss, but regrowth should be monitored closely.