A common question asked among alfalfa growers in the area over the past few summers has been, “Why is my alfalfa yellow and stunted?”
With all of the rain last spring, growers were expecting high yields on the second cutting but many, especially those with pure alfalfa stands or newly planted stands, were disappointed. The culprit in most cases was the potato leafhopper.
Forecasted weather conditions are beginning to mimic 2018’s summer growing season, so let’s take another look at this pest and try to manage it in 2019.
Potato leafhoppers are the major pest of second, third and fourth cuttings in established fields. They can cause serious damage before the first cutting in fall- and spring-seeded fields. Their feeding produces a distinctive, yellow triangular area at the tip of the leaf that is called “hopper burn.”
Adults of the potato leafhopper are green, wedge-shaped insects less than 1/8 inch long. Eggs are laid in the stems and larger veins of the host plant. Nymphs resemble adults in general shape but are much smaller and do not have wings. These insects use their piercing-sucking mouthparts to feed on plant sap. In warm weather, the leafhopper life cycle takes about three weeks. There can be 3 to 4 generations each year with damage usually ending in mid to late summer. In severe infestations, leafhoppers can be found in large numbers on equipment after a field has been cut.
Potato leafhopper feeding damages vascular tissue and blocks sap flow in leaves, producing the triangular-shaped yellowing at the tip of the leaves called “hopper burn.” This greatly reduces hay quality. Regrowth following cutting can be delayed, plants can be stunted and plant root reserves can be reduced. Once hopper burn is obvious in the field, the damage to the crop has been done.
The amount of damage that potato leafhoppers cause is related to the size of insect population and height of alfalfa. Damaging numbers must be determined before symptoms appear.
Established fields should be sampled after first cutting, while new fields (fall- or spring-seeded) should be sampled beginning around May 25.
Sample each field once a week, starting about 10 days after the first cutting. Use a 15-inch diameter sweep net to take samples at five randomly-selected locations in the field.
Take 20 sweeps at each location and count all potato leafhopper adults and nymphs collected at each location.
Ten stems from the field should be measured to determine average plant height. Also record the number in pre-bud, bud or flower stage to determine if early harvest is a potential control option.
Cutting at around 35 day intervals usually keeps leafhoppers from reaching damaging levels.
If leafhoppers are present in significant numbers and the field is greater than 7-10 days from harvest, an insecticide application should be made. The group 3 insecticides work best on controlling leafhoppers. Some of the more commonly available group 3 insecticides in this area that are labeled for leafhopper control in alfalfa include Warrior II, Mustang Maxx and Tombstone.
For more information on potato leafhoppers in alfalfa or on alfalfa production in general, contact the Hardin County Extension Service at 270-765-4121.