Improved Alfalfa Varieties Show Resistance Against Forage Crop Insect

April 2, 2002

COLUMBUS, Ohio -- New developments in alfalfa cultivars have proven to be effective in controlling potato leafhopper, an insect that can cause serious damage and yield loss to the forage crop.

Ohio State University 2001 evaluation trials have shown that the use of glandular-haired alfalfa varieties cut yield loss 82 percent compared to smooth, susceptible varieties in the seeding year. In a single alfalfa cutting, yield loss was reduced nearly 67 percent. Anywhere from a third to one ton more of dry matter per acre was harvested in the seeding year with resistant varieties.

Researchers speculate the minute hairs on the resistant varieties aid in reducing potato leafhopper feeding. The cultivars may also exhibit antixenosis, a plant defense mechanism that modifies the feeding behavior of the insect without affecting plant or insect metabolisms.

"The industry has come a long way in increasing the level of resistance in alfalfa cultivars. Where a few years ago, at best, varieties showed 35 percent resistance, now varieties have over 75 percent or better levels of resistance," said Mark Sulc, an Ohio State Extension forage agronomist. "There's no question farmers should be growing resistant varieties if they are not carefully scouting and managing their alfalfa for leafhopper damage."

The potato leafhopper is a small insect that overwinters in Gulf Coast regions. It is carried north each spring via weather fronts and finds its way into alfalfa fields throughout the Midwest. Feeding stunts the growth of the plant, thereby reducing yields. In severe cases, feeding can kill seedlings. Yield reduction can be as high as 20-25 percent for the entire growing season and 30-50 percent for a single cutting depending on the severity of leafhopper populations. There are usually three to four alfalfa cuttings per year.

"Natural enemies of the potato leafhopper exist, but they develop too late and are generally ineffective in controlling the insect," said Sulc. "The best ways to control the leafhopper are to use insecticides, resistant varieties or a combination of both."

In the evaluation trials, conducted at South Charleston, Ohio, researchers analyzed yield potential, growth rate, insect injury and insect populations of unsprayed commercial resistant and susceptible varieties. They found that the use of resistant varieties increased growth rate and reduced leafhopper feeding and insect numbers compared to the susceptible varieties.

"The unsprayed resistant variety lost yield only in the first cutting of the seeding year trial. After that the yield was the same as the sprayed susceptible variety," said Sulc. "This means that a farmer may only have to spray once in a seeding year, saving on insecticide costs." Sulc said that applying insecticide in the seeding year will also help keep leafhopper populations in check. An effective way of measuring leafhopper populations is to count the number of insects per inch of plant height in 10 sweeps. If the number of insects in 10 sweeps is more than the height of the plant in inches, an insecticide application may be warranted.

The Ohio State research on alfalfa cultivars is ongoing, but Sulc said that there is no question farmers are better off planting resistant varieties if they are not managing leafhopper populations in their fields. "The potato leafhopper is the most serious insect pest of forage production year-in and year-out, and it has the potential to build up to economically dangerous levels," said Sulc. "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."

Sulc estimates the economic loss to Ohio's 600,000 acres of alfalfa would range anywhere from $17 million to $54 million annually if a third to one ton of dry matter was lost by planting susceptible varieties and not spraying insecticides to control leafhopper populations. "With resistant varieties it would be a quarter to a half of that loss, and this is just a conservative estimate," he said.

Alfalfa is Ohio's single largest hay crop, being grown on half of all hay acres in the state. According to 2000 Ohio Agricultural Statistics, alfalfa hay yields were the highest since 1994, topping out at 4 tons per acre. Production is valued at over $253 million annually.

For more information on the 2001 Ohio Forage Performance Trials contact Sulc at (614) 292-9084 or sulc.2@osu.edu, or log on to http://www.ag.ohio-state.edu/~perf. For more information on forage production, log on to http://forages.osu.edu.

Author(s): 
Candace Pollock
Source(s): 
Mark Sulc