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What's eating alfalfa?

If unchecked, potato leafhopper can reduce alfalfa yield and quality and shorten stand life. Here are some tips to help protect your alfalfa from potato leafhopper.

by Gary R. Nielsen and William 0. Lamp

POTATO leafhopper may be the single most important alfalfa pest in the eastern and north central U.S.

Leafhoppers injure alfalfa by sucking sap from stems and leaves and disrupting vital physiological processes. They are most damaging to the second and third alfalfa hay crops (in a four cut system and third and fourth in a five cut system).

As its name implies, alfalfa is not the only crop plagued by this insect. It feeds and reproduces on more than 200 plants, including common crops like potatoes, peanuts and soybeans.

Adult potato leafhoppers are small (1/8 inch long), greenish insects with wings. They fly readily when disturbed. Immature leafhoppers are called nymphs and look like wingless adults. Both feed on plant sap.

Leafhoppers take about three weeks to develop from egg to adult, and several generations can occur each summer. Leafhoppers must reinfest northern crops each season because they cannot survive the winter north of the Gulf states. They migrate north in the late spring and summer and attack alfalfa from June through August.

Initially, leafhopper Injury Is undetectable. About a week after leafhopper begin to feed. alfalfa appears yellow and stunted. At this point it Is too late to undo the damage done to this hay crop. Such hay will have reduced yield and quality.

Because potato leafhopper also reduces carbohydrate reserves in the root. the yield loss may carry over to the next cutting, even to the next season. Winterkill may be greater for leafhopper-damaged alfalfa.

From 1983 to 1988, we estimate leafhopper-induced losses to alfalfa in Maryland ranged from $13 to $27 per acre. However, individual fields had a much greater range of potential loss, with the highest losses at over $100 per acre.

A key component of Integrated pest management (IPM) programs Is the action threshold. This Is the pest abundance level at which action Is necessary to prevent future crop losses.

To apply the action threshold to potato leafhopper In alfalfa, a trained field scout counts the number of potato leafhoppers In each sweep net sample and measures the height of the alfalfa. The latter Is necessary because alfalfa is most susceptible to leafhopper injury during the early regrowth period and less so as height increases and harvest nears.

From this information, leafhopper damage to alfalfa can be predicted and a recommendation is made to apply insecticide or wait and scout again. For the details of leafhopper scouting for your area, contact your local cooperative extension service. Also see the May 25,1989, Hoard's Dairyman, page 446.

At the moment, the most reliable control for leafhopper outbreaks is insecticide sprays. However, our economic analysis indicates that spraying, without scouting alfalfa for leafhoppers, is more costly than no insecticide application at all.

We compared three spray programs: no spray, one preventive spray per cutting (one application each to the second and third cuttings ) and spray applied only when recommended by scouts. The cost of each strategy included hay yield lost to the leafhopper and insecticide application costs.

By far, the most expensive option was preventive spraying at an average of S31 per acre. No insect control was $8 per acre less than preventive sprays. The IPM approach of applying insecticides only when necessary was the least expensive option and cost $22 per acre, including scouting costs.

Thus, costs were least when alfalfa was sprayed at the scout's recommendation and greatest when Insecticide was applied without this Information.

Before you scramble to the sprayer, there are several more subtle ways of protecting your alfalfa. Hundreds of insect species inhabit alfalfa. The majority are plant feeders yet only a few (potato leafhopper, alfalfa weevil, pea aphid, spotted alfalfa aphid, clover root curculio ) cause economic losses. The rest are kept in check by predators, parasites or diseases.

These natural controls, as well as the ability of alfalfa to withstand attack, are influenced by weed control, harvest methods and timing and fertilization. Integrated crop management programs seek to incorporate as many of these practices as feasible to load the dice in the grower's favor.

Grass helps control...

Certain grasses emit volatile chemicals that repel potato leafhoppers and cause females to lay fewer eggs. We have found that field plow sprayed with herbicides to control grass weeds, like giant foxtail, large crabgrass and fall panicum, have more leafhoppers than plots without herbicide sprays. When grass weeds were present, yield and protein loss due to potato leafhopper were about half the losses for weed-free prob.

Other grass/alfalfa mixtures may make leaf hopper attack less severe. For example, we found that the use of oats as a nurse crop for spring seedlings of alfalfa reduces leafhopper attack ). Currently, scientists at Maryland, Michigan and Missouri are evaluating the effects of forage grass/alfalfa mixtures on potato leafhopper damage to alfalfa.

Cutting early eliminates leafhoppers from alfalfa fields .Research conducted In Virginia revealed that, while adults migrate to the surrounding vegetation. about 95 percent of the nymphs die. However, when fields are cut poorly, due to lodging or field irregularities, long alfalfa stubble is left.

Stubble longer than 2 inches insures nymphal survival, as do leafy stubble and low-growing weeds (like dandelion). If sufficient food remains, these nymphs can infest alfalfa regrowth, accelerating injury of the next crop.

Early cutting Is one alternative to chemical control due to high leafhopper mortality at harvest. However, harvesting in the prebud stage reduces root reserves and weakens the stand.

Fertilization Involved. . .

Alfalfa gem Its nitrogen from the atmosphere. Consequently, potassium Is the most Important fertilizer nutrient. Potassium increases the yield, quality (protein) and longevity of alfalfa. This nutrient alto leads to more rapid greening after harvest and greater root growth.

How much of these growth benefits are due to enhanced growth and how much results from Improved plant tolerance (vigor) to Insect attack? We are conducting a field experiment to answer this question.

In 1987, the last year of typical leafhopper damage, we found less leafhopper injury and higher yields of alfalfa In fertilized prob. This was a surprise because leafhopper numbers were greater In plots fertilized at each successively higher rate of potassium (200. 400 and 600 pounds Kg/acre ). Leafhopper injury, or hopperburn, for insecticide-free plots was as low at the highest rate of potassium as plots sprayed with insecticide. Thus, applying potassium at the highest rate was as effective in reducing leafhopper damage as spraying.

Leafhopper infestations may cause severe and unpredictable losses. Many practices affect leafhopper numbers and their damage. In general, a healthy, vigorous stand Is more tolerant to pests than a weak one.

The potential for using grasses to repel leafs hoppers and protect alfalfa Is promising. Research Is In progress to measure the Impact of potato leafhopper on alfalfa grown as a mixture with a forage grass. Until these Interactions are better understood, we strongly recommend field scouting to assess leafhopper populations and coordinate timely and cost effective chemical control. Until alternative management tools are developed, chemical control. as part of IPM monitoring is the best method of reducing losses to potato leafhopper.

Copyright 1989. Hoard's dairyman