As with fine wines, time has done great things for leafhopper-resistant alfalfa varieties.
“The industry has come a long way increasing the level of resistance,” says Marc Sulc, an Ohio State University forage agronomist.
A few years ago, actual resistance levels were 35% at best, says Sulc. Now some varieties have at least 75% resistance.
“By planting resistant varieties, a farmer has the potential to save on insecticide applications and increase his yields when leafhopper populations get ahead of him,” he states.
“Our research clearly shows yield benefits to using leafhopper-resistant varieties in the presence of leafhoppers,” adds Dan Undersander, University of Wisconsin extension forage specialist.
Seed companies introduced the first generation of leafhopper-resistant varieties in 1997. Those glandular-haired varieties fell far short of growers' expectations. Their resistance levels were low, plus their yields dragged in the absence of the pests. But today's newest varieties are much better on both fronts, notes Undersander.
Potato leafhoppers remove plant nutrients and fluids by piercing and sucking. They inject a toxin into the plant during feeding, which causes the leaves to turn yellow and restricts the flow of nutrients. Continued feeding stunts plants, reducing yield. The reduction can be as high as 20-25% for the entire growing season and 30-50% for a single cutting, depending on leafhopper numbers.
In 2001 Ohio State trials, glandular-haired varieties cut first-year yield loss by 82% compared to smooth, susceptible varieties, Sulc reports. Up to 1 ton/acre more dry matter was harvested in the seeding year with resistant varieties. In those trials, resistant varieties lost yield only in the first cutting. In later cuttings, yields were the same as those of sprayed susceptible varieties.
“This means that a farmer may only have to spray once in a seeding year, saving on insecticide costs,” says Sulc.
New seedings are especially sensitive to leafhoppers. So Undersander recommends spraying first-year alfalfa when leafhopper numbers exceed the economic threshold for susceptible varieties, even when a resistant variety is planted.
“This very important management tool hasn't been emphasized enough,” he says. “Infestations during the seeding year will reduce yields in subsequent years, even if potato leafhoppers aren't present after the seeding year.”
He also encourages growers to plant leafhopper-resistant varieties if they're using a cover crop because it usually isn't possible to spray through that crop.
Another recommendation: Double the economic threshold for spraying if the resistant varieties you're using have greater than 50% leafhopper resistance.
“In many regions and in most years, this will eliminate the need for spraying,” says Undersander. “And, in areas where infestations are very severe, it will reduce the number of sprayings needed.”
While there's no way to predict leafhopper infestations, farmers should grow resistant varieties if they don't carefully scout for leafhoppers, says Sulc.
“The best way to control leafhoppers is to use insecticides, resistant varieties or a combination of both,” he states.
Leafhopper Economic Thresholds
|Source: Marlin Rice, Iowa State University; and Steve Lefko, Monsanto, 1999|