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Crop and Soil Environmental News, July 2000
Rod Youngman, Extension Specialist, and Curt Laub, Research Associate,
Department of Entomology, Virginia Tech
The abundant rainfall this spring and summer has meant near ideal growing conditions for alfalfa in Virginia. Should these rainfall patterns continue through August, many growers could make up to five cuttings of alfalfa this year. Although most growers are beginning to make their third cutting, a potential threat to this cutting, as well as the next, is the potato leafhopper (Fig. 1). This tiny insect (Fig. 2) damages alfalfa by injecting saliva into the plant during feeding which causes a toxic reaction in the plant known as "hopperburn". Initial symptoms of hopperburn include a "V"-shaped yellowing of the leaflet tips (Fig. 3) which can eventually lead to yellowing of the entire leaflet. By the time yellowing is readily apparent across a stand, research has shown that growers can expect about a 30% loss in yield and forage quality. In rare instances, severe leafhopper feeding can "burn" back an entire cutting.
Fig. 1. Potato leafhopper adult and nymph on alfalfa stem.
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Fig. 2. Potato leafhopper nymph on alfalfa leaflet. |
Recent scouting reports in Virginia indicate that potato leafhopper levels continue to be high enough to warrant concern for alfalfa fields in the southwestern and Piedmont areas of the state. Although we have not received any recent reports of high leafhopper levels in the northwestern region of Virginia, growers should continue to scout for leafhoppers to identify any fields that might be above threshold. We do not anticipate leafhopper levels to begin declining until about the first or second week of August.
Fig. 4. Scouting for potato leafhopper using the sweep net method. |
To avoid undue yield penalty from leafhopper, alfalfa fields should be scouted every eight to ten days through the middle of August using the sweep net method described on page 126 of the 2000 Pest Management Guide for Field Crops (Virginia Cooperative Extension publication 456-016). The sweep net scouting method (Fig. 4) is a quick and effective way of determining whether an alfalfa field is above threshold by relating sweep net counts of leafhoppers to plant stand height, and then referring this information to the decision-making chart on page 128 of the 2000 Pest Management Guide. For example, an alfalfa field with ten inches of growth and an average of two leafhoppers per sweep is considered above threshold and should be sprayed immediately according to the decision-making chart.
For those fields where an insecticide treatment is warranted, readers should refer to page 127 of the 2000 Pest Management Guide or to a July 1999 article in Crop and Soil Environmental News entitled: "Performance of commercial insecticides against potato leafhopper".
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