Adults and nymphs are present in fields now and should be monitored for population increase on 2nd crop. Potato leafhopper populations can build quickly, particularly as temperatures increase.
Alfalfa: Continue Potato Leafhopper Scouting
Eileen Cullen, Extension Entomologist, UW Entomology Department
Thursday, July 13, 2006
Many UW Extension county agents, growers, consultants, and coop agronomists began scouting in late May for arrival of potato leafhopper adult migrants, and monitoring of new seedings. Adults and nymphs are present in fields now and should be monitored for population increase on 2nd crop. Potato leafhopper populations can build quickly, particularly as temperatures increase.
Potato leafhopper inserts its beaklike mouthpart into the food-conducting tissue within leaves to extract plant sugars, minerals and other compounds. Potato leafhoppers inject a toxin into the plant which inhibits water and nutrient transport. Symptoms include plant stunting, and yellowing of the leaves in a V-shaped pattern starting at the leaf tip (“hopper burn”). Both the nymph and adult stage feed.
Potato leafhoppers: small (1/8 inch), green, wedge-shaped. Adults and nymphs look similar except that adults have wings and nymphs have wing pads. Leafhopper nymphs move sideways when disturbed.
Because leafhopper population densities vary from year to year (depending on migration patterns of overwintered adults from southern U.S.), and from field to field, the only way to accurately determine damage potential is by monitoring fields on a weekly schedule, beginning with second crop.
To obtain an accurate population estimate, sample for leafhoppers when alfalfa is dry, and avoid field edges. Cold, wet or windy conditions may temporarily knock adults and nymphs from plants, resulting in an inaccurate sweep count. Use a standard 15-inch diameter insect sweep net. A total of 100 sweeps should be taken throughout the field, in an approximate “M-shape” through the field to obtain 20 consecutive sweeps in each of 5 randomly selected areas.
Economic thresholds are based on average number of potato leafhoppers/sweep. Keep a running total of the number of leafhoppers caught and divide by 100. Nymphs are not generally recovered in the bottom of the sweep net with adults, but found along the collar of the net (along the wire hoop rim of the net). Nymphs move quickly, typically sideways, when disturbed.
The larger the alfalfa the more PLH it takes to cause economic damage. This occurs both because the older plant is more tolerant of damage and because the PLH will be on the plant less time before it is harvested.
|
Alfalfa Height (inches) |
Potato leafhoppers per sweep |
|
Do not spray if you are within 7 days of your normal cutting schedule* |
|
|
3 inches |
0.2 |
|
4 to 6 inches (avg. height of 6-inches) |
0.5 |
|
8 to 11 inches |
1.0 |
|
12 inches or greater |
2.0 |
* Following second crop cutting, continue to assess regrowth for leafhoppers.
Potato leafhopper has 3-4 generations per year in the Midwest, so this is an insect to continue monitoring for the remainder of this season. Natural predators and parasites of the potato leafhopper do not typically provide consistent economic suppression of leafhopper. However, cutting alfalfa usually kills a high percentage of the nymphs and forces adults out of the field in search of food. Consult UW-Extension bulletin A3646 Pest Management in Wisconsin Field Crops for insecticide label information (potato leafhopper rates, pre-harvest interval, restricted entry intervals, and precautions to avoid honey bee kill).
Reference
Pest Management in Wisconsin Field Crops 2006. UW-Extension Publication A3646
http://cecommerce.uwex.edu/