Imperial County alfalfa growers occasionally suffer losses to summer hay cuttings due to leafhopper infestations. The summer of 2009 included heavy infestations.
Although it is impossible to forecast leafhopper levels for 2010, growers and pest control advisers need to diligently monitor alfalfa fields to prevent losses.
Many species of leafhoppers can be found in alfalfa, but Empoasca sp. is primarily responsible for injury and yield reductions. Three species have been found damaging alfalfa in California: potato leafhopper, Empoasca fabae; southern garden leafhopper, E. solona; and E. mexara.
All three species cause identical injury. The prevalent species in the Imperial Valley include E. solana and E. mexara.
Other leafhoppers associated with alfalfa are distinguished from Empoasca sp. by brown or gray colors. Adult Empoasca sp. leafhoppers are small, one-eighth-inch long, bright green, wedge-shaped insects with piercing and sucking mouthparts. They jump and fly 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 distinguish them from lygus bug nymphs or slow moving aphids.
Empoasca sp. leafhoppers damage alfalfa through the removal of sap. The main concern for hay producers is a type of injury called "hopper burn."
Hopper burn symptoms result from the injection of salivary toxins into the plant during 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 where foliage yellowing 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 re-growth can be severely stunted resulting in yield losses. Hay quality can be affected by severe leafhopper injury by a reduction in protein and vitamin A.
Yellowing and stunting symptoms following a heavy Empoasca sp. leafhopper infestation can carry over into one to two subsequent cuttings, even through the leafhoppers are no longer in the field.
Empoasca sp. leafhoppers attack several other crops. Adult leafhoppers can migrate to alfalfa fields from neighboring crops including sugarbeets.
Monitor alfalfa fields weekly for Empoasca sp. leafhoppers using a standard 15-inch insect sweep net. Also monitor nearby crops including sugarbeets for the leafhoppers and look for leafhopper adult migration when an infested crop is harvested.
When symptoms first appear, sample a minimum of four to six areas over the entire field by taking five sweeps in each area and counting the number of adults and nymphs. Tally the leafhopper counts and divide by the total number of sweeps to calculate the leafhopper number per sweep. Leafhopper infestations usually begin on the field margin. Include the field edges in 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 Empoasca sp. leafhoppers per sweep.
Treat alfalfa scheduled for harvest 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 re-growth 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 tolerate very heavy leafhopper populations without yield loss, but re-growth should be monitored closely.