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Crop and Soil Environmental News, June 2000
Curt Laub, Research Associate and Rod Youngman, Extension Specialist,
Department of Crop and Soil Environmental Sciences, Virginia Tech
Potato leafhopper populations are weeks ahead of their normal levels, and are approaching threshold levels in some fields, especially in the Piedmont. Alfalfa fields in Augusta, Campbell, Montgomery, Orange, and Rockbridge Counties were sampled for potato leafhopper between June 1 and 5. Alfalfa stem length in these fields ranged from 6 to 18 inches. Leafhopper populations ranged from 0.3 per sweep in Montgomery County to 1.0 per sweep in Orange County. The action threshold changes depending on stem length, so taller alfalfa can withstand higher numbers of potato leafhoppers.
Almost all the potato leafhoppers counted were adults, which tells us that high numbers of eggs are being deposited. These eggs will be hatching in the next few weeks and we expect very high numbers of nymphs to start feeding. Nymphs are already present in Orange County. Damage to alfalfa may be severe, especially to fields that are just now being harvested. Fortunately, second-cutting alfalfa regrowth has been excellent. Growers are advised to monitor all alfalfa fields for potato leafhopper, especially those fields that have recently been cut. Procedures for monitoring potato leafhopper and recommended insecticides for controlling this insect are described on pages 126-128 of the 2000 Virginia Pest Management Guide for Field Crops-Extension Publication no. 456-016 (or go to http://www.ext.vt.edu/pubs/pmg/fc4a.pdf to download a PDF file of the Field Crops Insects section from the Pest Management Guide-then scroll down to page 126).
With plenty of soil moisture allowing rapid growth, alfalfa may soon reach the point at which spraying to control potato leafhopper is considered uneconomical. Any alfalfa that is taller than 14 inches and has leafhopper levels exceeding the thereshold should be harvested early rather than sprayed.
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