With many looking to make first cutting alfalfa as soon as fields are fit and with others checking regrowth, we switch focus from one alfalfa pest to another. Potato leafhoppers (PLH) are a common alfalfa pest that arrives late into the first cutting, carried to Ohio fields by southern winds. The life cycle of PLH only takes around three weeks for the pest to reach maturity and there can be multiple, overlapping generations per year that can quickly become economically damaging.
To identify PLH, the adults have a lime-green coloration, wedge-shaped, and are roughly 1/8 of an inch in length. The immature nymph stages look similar but are smaller and lack wings. PLH feeding damage, commonly called hopper burn, is seen as the yellowing of alfalfa leaf tips in a V-shaped orientation.
One needs two pieces of equipment to scout for PLH, a 16-inch diameter sweep net and a yardstick. A sample consists of 10 pendulum swings into the crop canopy with the sweep net (see link to video of sweep net sampling of PLH). After 10 swings count the number of adults and nymphs present, be sure to check around the rim of the sweep net for nymphs. For accurate scouting, take multiple samples across the field (the bigger the field, the more samples one should take). It is also best to scout when conditions are dry and not overly windy to get a more accurate sample of the pest population.
Compare the total number of adults and nymphs caught per 10 swings to the height of the alfalfa to determine if the threshold is met or exceeded that would justify the use of a control method (insecticide treatment or early harvest). The action threshold is on average met when the number of adult and nymph PLH caught is equal to or greater than the height of the alfalfa. For example, if 18 total PLH are caught in an 18-inch stand of alfalfa, it is warranted to apply an insecticide or harvest early, instead of losing forage quality and quantity.
For those with fields that are looking to make first cutting, harvest is the best method of control due to the preharvest interval restrictions on insecticide control. As the season progresses into the second and subsequent cuttings, chemical control will be a more common method of control, be sure to read the label and follow the preharvest interval.
PLH is most damaging to new seedings, fresh regrowth, and stressed less-vigorous stands because feeding can quickly remove a larger percentage of leaf area on a younger or diminished stand. Damage caused by PLH most commonly hits its peak in July but can remain in the field into early September if conditions are right, so it is something to scout for the rest of the season.
Extra Resources:
A video outlining the scouting process: Scouting for Potato Leafhopper in Alfalfa
A video overviewing PLH and the damage caused: Potato Leafhopper Identification and Damage in Alfalfa
The OSU factsheet on PLH: Potato Leafhopper on Alfalfa