The tiny, winged insect is carried north each year in the thunderstorms fed by the moist Gulf of Mexico winds. Swept up from the fields of Alabama, Mississippi and other Southern states, thousands of the little yellow-green pests are sent swirling across the Midwest and dropped off to begin feeding on almost anything with green leaves.
To get the nutrients it needs, the potato leafhopper punctures an alfalfa leaf at midrib and injects a salivary solution that breaks down the cell walls and plugs up the leaf veins, eventually causing a distinctive, yellow, V-shaped pattern on the leaf known as "hopper burn."
It's a quiet pest, hard to spot, and the damage shows up well after the initial infestation. In time, the entire leaf yellows and curls, stunting the plant.
"By the time most farmers see the damage, it's too late," Shade says. "Even then they may not know what's happening. Many of them believe their alfalfa just naturally turned yellow in midsummer."
Those stunted, yellow plants can cost a grower about half a ton per acre of yield from the next cutting. The affected alfalfa also loses protein, making it less valuable to high-end users such as dairy cows. Leafhopper damage makes it harder for the perennial plant to survive the winter and can reduce the overall life of the stand. Despite all that, Purdue pest management specialists say it's still tough to get growers to make the connection between the tiny pest and a big problem.
Purdue pest management specialist Larry Bledsoe recalls a story about a Purdue entomologist who several years ago tried to convince an alfalfa grower in northern Indiana that his yellow, stunted field could be rescued by spraying the leafhopper nymphs before they could wreak havoc.
"The entomologist persuaded the grower to let him spray a small part of a field on a hill overlooking the grower's home as a test plot. Without saying a word, the entomologist sprayed the grower's initials in a corner of the field," Bledsoe says. "When the rest of the field turned yellow from leafhopper damage, the initials stood out bright green and a foot taller."
But timing the spray can be tricky, and it demands a lot from producers, Bledsoe says. They must start about mid-May, sampling the fields weekly with sweep nets to get an estimate of leafhopper populations. Then the growers have to weigh insect pressure against intended harvest dates and the cost of treatment. "Most producers, with the exception of dairy producers who require high-quality alfalfa, either spray indiscriminately or, more likely, just take the loss," he says.
Some growers spray fields after they notice the distinctive, yellow pattern on the leaves. "It's too late then. The damage is done, and killing the insect won't bring the alfalfa back," Bledsoe says. "It's a classic case of what we call 'revenge treatment.'"
McCaslin, president of Wisconsin-based Forage Genetics Inc., says the leafhoppers had had it their way for way too long. "There really wasn't an effective, foolproof method of handling the potato leafhopper. It was a huge gap in the package we were designing to help farmers get the best possible yields, persistence and forage quality."
In an attempt to fill that gap, Shade and former Purdue agronomist Tommy Thompson began experimenting with 60 different species of alfalfa for insect resistance to the alfalfa weevil, pea aphid and, especially, the potato leafhopper. In 1973, they found a glandular-haired alfalfa that didn't seem to be affected by any of the chief alfalfa foes.
As Shade later learned, most insects that landed on the plant's distinctive long hairs either didn't recognize the plant as a food source and flew away, or were trapped by a sticky substance at the heads of the hairs. While he couldn't rule out the possibility the plant had a moderate toxicity, Shade noted that the trapping mechanism stopped most of the small leafhopper nymphs.
This bio-physical insect resistance turned out to be key. But crossing the wild, long-haired species with conventional alfalfa wasn't easy. A genetic mismatch between the wild and the domesticated species meant that, out of every 1,000 plants crossed, only six carried the resistance characteristics.
Shade and colleague Laurie Kitch eventually crossed two species of the wild, glandular-haired alfalfa with common alfalfa to produce a hybrid that came within 70 percent of the yield of the best conventional alfalfa varieties. Deciding in 1984 it was time for commercial companies to take over, they released the germplasm to the public and sent seeds to any breeder who wished to experiment with its potential.
Several, including McCaslin, took them up on it. This past summer, more than 10 years later, Bledsoe and Purdue forage agronomist Keith Johnson began testing several commercial varieties of glandular-haired alfalfa that have their roots in the Purdue plants.
Bledsoe says initial results in Indiana have been encouraging. "In a plot of susceptible alfalfa, you might find counts of 50 potato leafhopper nymphs. Then, in a plot of glandular-haired just five feet away, the count would be 10 or 11." Johnson says it's one of the most exciting things to happen to alfalfa in years.
McCaslin says it's thanks to Purdue. "To me it's the perfect example of public-private research partnership. Although it took 10 years of plant breeding research by private seed companies to bring the new products to market, it could not have been accomplished without the pioneering work at Purdue."
Next spring, McCaslin says, there will be half a dozen leafhopper-resistant varieties available, all of which can be traced back in part to Purdue. He predicts that in five to 10 years almost all new alfalfa varieties will have leafhopper resistance, and that will make for greener fields, better profits, fewer insecticide sprays -- and some very unhappy leafhoppers.
Sources: Dick Shade, (765) 494-4612; e-mail, firstname.lastname@example.org
Larry Bledsoe, (765) 494-8324; e-mail, email@example.com
Keith Johnson, (765) 494-4800; e-mail, firstname.lastname@example.org
Mark McCaslin, Forage Genetics, (608) 786-2121
Writer: Chris Sigurdson, (765) 494-8415; e-mail, email@example.com
Purdue News Service: (765) 494-2096; e-mail, firstname.lastname@example.org
NOTE TO JOURNALISTS: Two color photos are available. One, called Shade/Leafhopper1, shows the "hairy" alfalfa developed at Purdue to resist the potato leafhopper or download here; the other, called Shade/Leafhopper2, shows the difference between healthy alfalfa and plants that have been attacked by leafhoppers or download here.
The glandular hairs on the leaves and stem of this Purdue University-developed alfalfa variety help keep the potato leafhopper pest at bay. (Photo by Dick Shade)
Color photo, electronic transmission, and Web and ftp download available. Photo ID: Shade/Leafhopper1
The stunted, yellow plants at the bottom of the picture showing damage from potato
leafhoppers can cost a grower about half a ton per acre of yield from the next cutting.
At top are healthy alfalfa plants that are resistant to the potato leafhopper. (Photo by Dick Shade)
Color photo, electronic transmission, and Web and ftp download available. Photo ID: Shade/Leafhopper2
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