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Superintendent
David Davis
21262 Genoa Road
Linneus, MO 64653
Phone: 660 895-5121
FAX: 660 895=5122
Email: DavisDK@missouri.edu

Forage Systems Update
Vol 11, No. 3

One of the projects underway at FSRC is an alfalfa study addressing the problem of potato leafhoppers. This project compares a standard variety of alfalfa to a variety that has shown to be resistant to potato leafhoppers. The resistant alfalfa plants have glandular hairs on the leaves that discourage the leafhoppers from feeding on the plants.

A second component of the study will address the use of potassium fertilizer to increase the tolerance of alfalfa to leafhopper damage and the fertilizer's effect on the longevity of the stand. Recent studies have shown an increase in the tolerance of standard alfalfa varieties to leafhopper damage with the addition of potassium fertilizer. And it is known that phosphorous and especially potassium fertilizer enhances crown development of alfalfa plants particularly when an adjacent plant dies. The combination of a variety with glandular hairs on the leaves and the application of potassium fertilizer are expected to reduce pesticide use as well as enhance the life of the stand.

This spring we experienced unusually high infestation of alfalfa weevils. Because the plot area was a new seeding, we did not expect to see any weevils. That was not the case. Upon scouting early in the season, we found weevils in great numbers.

When weevils feed they remove the green tissue from the top leaves, leaving the plants with a white, lacy appearance. Scouting was done by collecting 100 alfalfa stems, shaking them vigorously in a white 5-gallon bucket and counting the number of weevils in the bucket. In late April, following a week of warm temperatures near or above 80 degrees, we found an average of 1.75 weevils per stem and nearly 100% of the plants having feeding damage. This exceeded the threshold level of 1 weevil per stem and 30% of the plants with feeding damage for alfalfa 16 inches tall or less. The plot area was treated with WarriorŪ insecticide at 3 ounces/acre. A week following application of the insecticide an average of 0.34 weevils per stem was found.

Temperatures were cooler in early May and we received over 10 inches of rain in the first two weeks of the month. These conditions are less conducive to weevil growth. Weevil populations dropped to 0.08 weevils per stems in the treated area and 0.6 weevils per stem in the untreated border area of the plots. By this time the alfalfa exceeded 16 inches in height. The plants can tolerate more feeding damage at this stage of growth and the threshold becomes 2 weevils per stem.

Now that the first cutting has been harvested from the plots we will focus on potato leafhoppers. Potato leafhoppers do not over winter in Missouri; they migrate north from the Gulf Coast with spring storms and are generally a threat to the second and third cuttings of alfalfa. The alfalfa plots will be scouted weekly through September to determine the leafhopper infestation of the plots. Potato leafhopper damage appears as a wedge shaped yellowing of the leaf tips leaving it with "hopper burn."

Scouting for leafhoppers involves sweeping the plots with a sweep net and counting the insects collected. Twenty sweeps in each of five locations of a field are needed to adequately scout an alfalfa field. When alfalfa is small, stem length of three inches, as few as 0.2 potato leafhoppers per sweep will cause economic damage and warrant treatment. As the alfalfa grows, it can withstand more leafhoppers. When the stems are six inches in length, 0.5 leafhoppers per sweep is the economic threshold and at eight to ten inch stem length, it can withstand one leafhopper per sweep. Once the alfalfa reaches the bud or bloom stage, early harvest is recommended to control potato leafhoppers.