If potato leaf hoppers (PLH) are going to be a problem in alfalfa, July and August are generally the months when high numbers can build up and cause damage to alfalfa stands. Young regrowth and stands more than 7 days away from harvest are the stands most likely to need a rescue insecticide treatment if PLH is present in high numbers. The PLH is a later season pest because it does not overwinter in Ohio. It lives year round in the gulf coast region and is carried north each year on weather fronts. Approximately 21 days are required to complete the lifecycle. Potato leaf hoppers are a sucking insect and their feeding activity blocks the nutrient flow in the plant. Characteristic PLH feeding damage causes a wedge-shaped yellowing of alfalfa leaf tips. When PLH population levels are high the alfalfa field will have a yellow appearance. Potato leaf hopper feeding causes stunted plants resulting in yield loss in the current cutting and heavy PLH levels will actually reduce the regrowth and yield of successive cuttings. By the time plant symptoms are noticed, the damage has been done and yield will be negatively impacted. Therefore it is important for alfalfa growers to be out scouting alfalfa fields on a regular, weekly basis.
Scouting involves the use of a sweep net. I have had growers call me and tell me that they used the hat on their head to do some sweeps. If they see any PLH in their hat they ask me if they should spray their field. Treatment thresholds for PLH are based on the use of a standard sweep net with a 16 inch rim. Fields should be sampled at 3 to 5 random locations. A single sample is composed of 10 pendulum sweeps of the net. After each 10-sweep sample stop and count both the number of PLH nymphs (wingless development stages of the adult) and adults. Get an average height measurement of the alfalfa. I like to take the height of the alfalfa when I start my 10 sweep sample and again at the end of the 10 sweep sample. If the number of PLH nymphs and adults are greater than the average height of the alfalfa, then either an early harvest or a rescue insecticide treatment should be considered. For example, if the average height of the alfalfa is 8 inches and I count 8 or more PLH nymphs and adults at the end of the 10 sweep sample, this would be considered the treatment threshold level.
Vigorous alfalfa can tolerate higher numbers, and stressed alfalfa can tolerate fewer. The threshold should be lowered when the alfalfa is under stress, especially for new seedings made this year. Consider lowering the threshold to half the normal level for new seedlings that are growing slowly because of drought stress. In those situations, there may NOT be a yield response from insecticide treatment in the current growth cycle, especially if drought conditions persist. However, protecting the plants from leafhopper damage now will protect the stand, development of the taproot, and its future yield potential.
Potato leafhopper resistant varieties can also tolerate higher numbers. Resistant varieties have glandular hairs. The presence of hairs on the alfalfa plant interferes with normal PLH feeding and slows nymph development. The threshold treatment level for these varieties of alfalfa is 3 times higher compared to non PLH resistant varieties.
Early harvest of alfalfa that is above the treatment threshold can temporarily eliminate nymphs by removing their food source and it will cause the adult PLH to disperse and look for another food source. This is a good option if the harvest date is close and/or the pre-harvest interval of an insecticide rescue treatment would interfere with a planned harvest. However, growers must be aware that regrowth 1 to 2 weeks following harvest will be attractive to PLH adults and so scouting of that next harvest regrowth must begin early on. Be aware also that if a neighbor cuts a field after your field has begun to regrow, that the PLH adults from that field may now move into your new regrowth.
For more information about scouting for PLH and/or insecticide options for a rescue treatment, contact the Wayne County Extension office at 330-264-8722. More information about potato leafhopper scouting and thresholds in alfalfa can be found on-line at http://ohioline.osu.edu/factsheet/ENT-33