July and August are important months to scout for potato leafhoppers in alfalfa and alfalfa mixed hay fields. The potato leafhopper (PLH) is a major economic pest of alfalfa. Both the PLH adults and nymphs have piercing-sucking mouthparts that allow them to feed on the sap within the plant. While feeding, the PLH inject a saliva that contains an enzyme that reduces nutrient movement in the plant. Heavy feeding by PLH causes plant stunting, resulting in yield loss. Additionally, PLH feeding causes a decrease in the crude protein content of the plant, a quality loss.
Heavy PLH feeding can result in reduced growth in the following cutting. First year stands are especially susceptible to PLH damage and heavy feeding can result in a shortened stand life. The classic symptom of PLH feeding damage is a wedge-shaped yellowing of the leaf tips, commonly called “hopper burn”. We see this mainly on the younger leaves at the top of the plant. If you observe hopper burn symptoms yield and quality losses have already occurred, so early scouting is encouraged.
The only correct way to scout for PLH involves the use of a sweep net to get an accurate assessment of PLH numbers. One sample consists of 10 side to side or pendulum sweeps of the net while walking forward through the field. After each 10-sweep sample, stop and count the number of both adult and nymph PLH in the net. Take several height measurements of the alfalfa plants in each sample area. Record or note your results. Repeat this process 3 to 5 times for each 25 acres of alfalfa.
The average number of PLH adults and nymphs per 10-sweep sample plus the average height of the alfalfa determines the economic treatment or action level. Under normal growing conditions, if the average number of PLH is equal to or greater than the average height of alfalfa consider a rescue treatment. The exception to this treatment level is if the alfalfa is a glandular haired or PLH resistant variety. For PLH resistant alfalfa varieties, the treatment threshold is three times higher.
Often the rescue action involves spraying an insecticide on the field, especially when alfalfa is less than fifteen inches tall. As alfalfa approaches a normal harvest height or maturity, another rescue action is to take an early harvest cut. Harvesting alfalfa will temporarily eliminate the presence of PLH nymphs and will cause the dispersal of adult PLH out of the alfalfa stand. With this option, it is important to scout and monitor the new regrowth for PLH numbers.
More information about scouting for PLH in alfalfa is available on the Wayne County Extension web site at: go.osu.edu/WaynePLHscout Included on this page is a link to an OSU Extension fact sheet about PLH in alfalfa including how to scout for PLH and management options and a link to a short video showing how to use a sweep net to sample for PLH.
Rory Lewandowski is an OSU Extension Agriculture & Natural Resources Educator and may be reached at 330-264-8722.
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