Some alfalfa producers have noticed what appears to be a yellowing, or bleached appearance in their second crop alfalfa, along with less than acceptable re-growth after the first cutting? Sometimes a lack of adequate soil moisture or a nutrient deficiency causes a similar appearing problem, but if growing conditions have been favorable, the lack of growth may have been caused by potato leafhoppers. Potato leafhoppers can be an important pest on crops such as alfalfa, dry beans, and potatoes. Injury results from direct feeding on leaves by adults and the tiny nymphs. What makes this pest especially harmful is that early symptoms are very subtle and can easily be missed. Another concern is that once the leafhoppers arrive, they can continue to feed and reproduce.

How to identify the Potato Leafhopper:

Adults are about 1/8-inch long and are elongated with short, bristle-like antennae. They are lime green with translucent, greenish wings, which are held at a roof-like angle over the body. Eggs are white in color, elongated and are about 1/24-inch long. Eggs are inserted in the plant stems and petioles. Nymphs are the immature stage and resemble the adults, but are wingless. When first hatched the nymphs are approximately 1/32-inch long. Nymphs are pale green and 1/8-inch long when fully grown. The potato leafhopper passes through five nymphal stages before becoming an adult.

Potato leafhoppers over-winter in the Gulf States and migrate northward each spring on air currents. These wind drafts can deposit adult leafhoppers on to crops virtually overnight. After arrival, adults locate forage crops for food and a place to lay eggs. Adult potato leafhoppers readily fly or run sideways when disturbed, Females deposit one to six eggs daily on plant stems, leaf petioles, and leaf veins. The nymphs hatch in about 10 days. Like adults, they suck sap from leaf veins and run sideways when disturbed. Nymphs molt five times over a period of one to two weeks before maturing into winged adults. There may be several overlapping leafhopper generations (usually two) per year in Minnesota. By late June, leafhopper populations may increase sufficiently to damage hay crops. The feeding of both immature and adult leafhoppers is damaging to forage crops. Potato leafhoppers inject saliva into plants as they feed, resulting in abnormal cell growth and interference with transport of fluids in the leaves. A visible result of this damage is the characteristic "hopperburn," which is a wedge-shaped yellowish pattern on the tips of the leaves. Prior to this a slight yellowing or curling of leaves occurs, which is often overlooked. Damage reduces forage yield and quality in several ways. Damaged plants may be stunted, with heavily infested fields experiencing as much as a 50% yield reduction. Damage also results in substantially lower protein levels. General stand vigor is decreased, leading to slow regrowth following cutting and potentially increased winterkill.

It is important that producers check alfalfa fields for leafhoppers shortly after the first cutting and into July. A heavy-duty sweep net should be used to properly sample for potato leafhoppers. Take 20 sweeps in five representative areas of the field and determine the average height of the crop at each location. Economic control thresholds for alfalfa vary with crop height: 3 “ height – 0.2 adults or nymphs per net sweep; 4-6” height – 0.5 adults or nymphs per net sweep; 7-12” height – 1.0 adults or nymphs per net sweep; and over 12” height - 2.0 adults or nymphs per net sweep. If the infested crop is almost ready to cut, remove it first, check the re-growth and spray if needed. The potato leafhopper can remain a threat throughout the season, so checks should continue until the final hay cutting is made.

Cutting of forages is an effective method for reducing leafhopper damage. Adult leafhoppers will leave the fields when it is cut. The wingless nymphs will remain behind, but without foliage to feed on they quickly die. If a field is at or above threshold and is mature, cutting is the preferred control measure. There are alfalfa varieties on the market that are tolerant to leafhopper feeding or deter leafhopper feeding.

An insecticide treatment is justified when leafhoppers exceed thresholds and the crop is not yet mature enough for harvest.

Insecticides approved for use to control leafhoppers in alfalfa include: permethrin*, Baythroid*, carbaryl, dimethoate EC (Cygon, De-Fend), Furadan*, Lannate*, Lorsban, Malathion 57 EC, Mustang Max*, Proaxis* and Warrior*.

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