Cooperative
Extension
Service


University of Illinois
at
Urbana-Champaign


No. 15/July 3, 1997

Very Large Numbers of Potato Leafhoppers Are Not Uncommon

Potato leafhoppers arrived in Illinois early this year, and they have survived and prospered. The numbers of leafhoppers in many alfalfa fields have exceeded economic thresholds, so treatment to prevent loss of yield and nutritional quality has been justified. Unfortunately, many growers continue to wait until the alfalfa turns yellow until they investigate the problem. We cannot emphasize enough the importance of sampling for potato leafhopper adults and nymphs before the symptoms of injury develop. Although leafhoppers can be controlled "after the fact," reductions in yield and quality have already occurred when the symptoms of injury are obvious.

A quick review of the life cycle of potato leafhoppers explains how their numbers can increase so quickly. After the adults arrive in the spring, the females begin laying eggs in the stems and leaf petioles. The eggs usually hatch in 6 to 9 days. Nymphs develop through five instars before they become adults. The nymph is smaller than the adult, which is about 1/8 inch long, and is wingless and yellow to yellow-green. When disturbed, nymphs move rapidly sideways or backwards. During the summer, development from egg to adult requires about 3 weeks. As time progresses, generations begin to overlap, so all stages of potato leafhoppers may be present at the same time.

Both adults and nymphs cause injury to alfalfa, but the nymphs cause the serious injury. Initial feeding injury is characterized by wedge-shaped yellow areas at the leaf tips, commonly called "tip burn" or "hopper burn." If densities of leafhoppers are large, entire leaves and even plants turn yellow, then red or brown as they die. Severely damaged plants are stunted and bushy because the internodes stop growing. The damage is exacerbated by hot, dry weather.

Don't wait for the alfalfa to turn yellow: Begin sweeping now! Scout for leafhoppers by using a 15-inch­diameter sweep net. Take 20 "broom" sweeps per location in 5 to 10 sites within the field if the alfalfa is taller than 4 inches. If the alfalfa is shorter than 4 inches, more sweeps are required for a precise sample estimate. Count the number of leafhoppers caught in the net, and divide by the number of sweeps taken. Static economic thresholds are based upon the combination of number of leafhoppers per sweep and height of alfalfa. An insecticide application may be warranted when the following thresholds are exceeded: 0.2 leafhopper per sweep in alfalfa 0 to 3 inches tall; 0.5 per sweep in alfalfa 3 to 6 inches tall; 1 per sweep in alfalfa 6 to 12 inches tall; and 2 per sweep in alfalfa 12 inches or taller.

Dynamic action thresholds have been developed by entomologists at Pennsylvania State University. These thresholds are presented in tables 1, 2, and 3; they vary depending upon the height of the plants, the value of the hay, and the cost of control. These thresholds are based upon limited research and are intended to be used only as an aid to predict when estimated losses may exceed the cost of a treatment. However, they may offer more decision-making power than the static thresholds referred to previously.

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
avg. no. 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
avg. no. 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
avg. no. 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

Keep up with these creatures. Their effects on alfalfa can be devastating if infestations are not controlled in time. Severe damage to one crop of hay may have long-term consequences, too, because the plants do not always recover completely.
Kevin Steffey, Extension Entomology, (217)333-6652