Potato Leaf Hopper (PLH) in 2006
By Mike Stanyard – Field Crops & IPM
The spring storms finally started to arrive in WNY on May 11. This is much to the relief of many WNY producers who are ahead of planting schedule and have thousands of acres of corn and soybeans sitting in dry soil. The coming of the first storms reminds me that these storms bring us more than just rain.
There are many insect pests that hitchhike their way to NY on storm fronts. Some of these include black cutworm (BCW), common armyworm (CAW), and potato leafhopper (PLH). Since June is dairy month, let’s talk more about the potato leafhopper which could be building populations in second cut alfalfa at this time.
Biology
Adult leafhoppers are quite small (1/8”), slender, and pale green. Though small, they are strong jumpers and fast fliers when disturbed. Unlike the alfalfa weevil, the PLH does not over winter in New York and therefore, severity of infestations cannot be based on past years’ populations. Every year is a different story! Adults migrate each year from the Gulf States (Louisiana and Mississippi) up and into the Northeast.
Leafhoppers normally arrive anywhere between mid-May and mid-June. As of May 12th, I have not found any while sweeping for alfalfa weevil. However, I’m sure the first wave of early insects has come in this week on many of the storms that have passed through the area. Upon arrival, females immediately begin laying eggs within the leaf veins and petioles. Females lay approximately 3 eggs per day and live from 6 to 8 weeks. Eggs hatch within 7 to 10 days, and the young nymphs emerge. PLH nymphs look identical to the adults except they lack wings and are a yellowish-green. The nymphs feed on the underside of the leaves using their straw-like mouthparts to suck out plant juices.
Damage
These first PLH are not a concern for maturing first-cut alfalfa fields. Weather permitting; most of these fields will be cut the last week of May and the first week of June. The tender new regrowth is the most vulnerable stage along with new spring seedings. When PLH inserts its mouthparts to feed, it injects a toxin with its saliva. This toxin clogs the vascular system and causes the leaves to turn yellow from the tip back. This yellowing is commonly called “hopper burn”. If feeding continues, stunting, reduction in hay quality, and yield loss can result.
Management
Alfalfa fields should be monitored for PLH prior to evidence of their feeding damage. Once plant symptomology is apparent, unrecoverable losses have already occurred. Monitoring for PLH is done through the use of an insect sweep net. Using a pendulum motion, sweep the net through the tops of the alfalfa plants in a side to side motion while walking. Count the number of sweeps as the net passes in front of you. Cornell recommends taking five sets of sweeps (10 sweeps per set) per field and calculating a PLH (adults & nymphs) per sweep for each set. Remember; when taking random sweep samples from a field make sure it is an accurate representation of the entire field!

Action thresholds for PLH management are based on plant height and leafhoppers per sweep.
Stem Height Leafhoppers/sweep
Under 3 in. 0.2
3 to 7 in. 0.5
8 to 10 in. 1.0
11 to 14 in. 2.0

If PLH’s exceed 2.0/sweep and alfalfa is within 1 week of cutting, no action is needed. If alfalfa will not be ready to harvest, use one of the insecticides listed on page 75 of the 2006 Cornell Guide for Integrated Field Crop Management. However, be careful of the pre-harvest intervals of each product (varies from 7 to 14 days) required after application!
Leaf Hopper Tolerant Varieties
More and more producers are planting leafhopper resistant alfalfa varieties. They have come along way with tolerance expression since its first release in 1996. These resistant varieties have glandular hairs that make it less susceptible to PLH feeding. Economic damage can still occur from PLH feeding if populations get high enough, especially in the seeding year.
Consideration should be given to the need for insecticide treatment of PLH resistant alfalfa when PLH are abundant during the first year of establishment. Where treatment of first year PLH resistant alfalfa is considered, many university researchers recommend that the action thresholds used on resistant alfalfa should be increased about twofold.
For more information or training in field scouting, contact Mike.
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