Cooperative
Extension
Service


University of Illinois
at
Urbana-Champaign


No. 19/July 29, 1998

Potato Leafhopper Densities Are Quite High in Many Fields

Several observers have reported that potato leafhopper densities in alfalfa fields this year have been quite high. After establishing their populations early this spring, potato leafhoppers have continued to increase in numbers, apparently not negatively affected by the substantial rainfall we experienced in June. Many alfalfa fields have densities of leafhoppers well above established thresholds. If rainfall is short for the remainder of the summer leafhoppers will inflict economic losses by reducing both yield and quality of the alfalfa.

In previous issues of this Bulletin, we have reviewed the description and life cycle of this key pest of alfalfa. Growers should continue to monitor their fields for leafhopper injury throughout the summer. Densities of this pest will not diminish significantly if a naturally occurring disease organism (Phytophthora radicans) does not take effect. This disease usually has little effect on leafhoppers when the weather is hot and dry. Consequently, leafhoppers likely will plague alfalfa fields until frost.

As has always been the case, we urge growers not to wait until they see evidence of leafhopper injury (that is, yellowing of injured leaves) to control leafhoppers. Yield and quality of alfalfa are already decreasing when yellowing of the leaves appears. Most thresholds in most states are based upon the numbers of potato leafhoppers per sweep of a 15-inch-diameter sweep net. In recent years, we have published dynamic thresholds for leafhoppers that vary according to cost of control and value of the hay. These thresholds, originally developed at Pennsylvania State University, are offered in Tables 1, 2, and 3. Keep on top of leafhoppers this summer.

Table 1. Dynamic treatment thresholds for potato leafhoppers (PLH) on alfalfa that is 1 to 4 inches tall.

Value of hay
(per ton)
Cost of insecticide application
(per acre)
$8$10$12$14$16$20
average number PLH per sweep
$ 600.40.50.60.70.81.0
$ 800.30.40.50.50.60.75
$1000.250.30.40.40.50.6
$1200.20.250.30.350.40.5
$1400.20.20.250.30.30.4
$1600.150.20.30.30.30.4

Table 2. Dynamic treatment thresholds for potato leafhoppers (PLH) on alfalfa that is 4 to 8 inches tall.

Value of hay
(per ton)
Cost of insecticide application
(per acre)
$8$10$12$14$16$20
average number PLH per sweep
$ 600.70.81.01.01.31.7
$ 800.60.60.750.91.01.3
$1000.40.50.60.70.81.0
$1200.30.40.50.60.70.8
$1400.30.350.40.50.60.7
$1600.20.30.40.40.50.6


Table 3. Dynamic treatment thresholds for potato leafhoppers (PLH) on alfalfa that is 8 to 12 inches tall.

Value of hay
(per ton)
Cost of insecticide application
(per acre)
$8$10$12$14$16$20
average number PLH per sweep
$ 602.02.42.83.03.95.0
$ 801.81.92.22.73.04.0
$1001.21.51.82.12.43.0
$1200.91.21.51.82.12.4
$1400.91.051.21.51.82.0
$1600.80.91.01.21.51.8

Kevin Steffey (ksteffey@uiuc.edu), Extension Entomology, (217)333-6652