Entomologists hesitate to answer
Given the current high hay prices, growers should scout for alfalfa insects diligently. But should they treat at lower-than-recommended economic thresholds?
“The entomologists don't really have an answer,” says Mark Sulc, Ohio State University extension forage specialist. Sulc asked insect specialists from several states whether they'd recommend lowering insect thresholds because of hay's higher value.
“The answer is that there is not going to be a good answer, at least not right away,” replies Kevin Steffey, University of Illinois extension entomologist.
“We were happy to go along living with the current thresholds when commodity prices were low and nobody challenged them,” says Steffey. “But now, suddenly, the commodity prices of field and forage crops have turned around dramatically and we're being asked questions that we're simply not prepared to answer well.
“Simplistically, if the commodity price doubles, the economic threshold is cut in half,” Steffey says. “My colleagues in research would take issue with that statement, but my rhetorical response would be: ‘What is the alternative? We aren't going to tell people to go out and solve quadratic equations for insect numbers vs. yield’ … and, quite honestly, neither they nor we have the data.”
The data that is available was collected 30-40 years ago, when hay wasn't considered a high-value crop, says Steffey. And anyone willing to do such laborious and time-consuming work would have a hard time getting funding for it today. Plus, the relationship between insect damage and price isn't linear, Steffey and Sulc say.
But alfalfa weevil threshold data, while old, may still be viable, says Steffey. Alfalfa has changed little compared to corn and soybeans, which are much higher yielding than they were 10 years ago. Some entomologists have produced sliding scales adjusting alfalfa weevil and potato leafhopper thresholds based on commodity prices. But those, too, are now outdated, he adds.
Scientists also don't want to adjust thresholds down because most of them are already on the conservative side, Sulc says. Thresholds are established to give growers time to react and make insecticide applications. Steffey adds that lowering thresholds “puts us into territory where the relationship between insect injury and yield is not known.”
“The potato leafhopper threshold is so low right now on stubble alfalfa that it is almost a no-brainer,” says Steffey. “Leafhoppers are either there or they're not and they're highly destructive.”
Potato leafhoppers are not generally a chronic problem most years in Oklahoma, says Phil Mulder, Oklahoma State University extension entomologist. “Populations of this insect typically overwinter in Louisiana and other southern regions. With spring wind trajectories, they get dumped into states like Nebraska and Iowa and to the east and north from there,” he adds.
Michael Rethwisch, a University of Nebraska-Lincoln extension educator, has witnessed its devastation. “I have never seen leafhoppers as thick in alfalfa as last year in the eastern third of Nebraska. But if you were in the middle third of the state, people there didn't see them.”
Glandular-haired potato leafhopper varieties, less susceptible to hopper damage, can take thresholds higher than recommended, says Rethwisch. In Ohio, Sulc found that established stands of highly resistant varieties didn't show economically significant and consistent yield loss until hopper populations were three times the normal threshold when 10-12 days of regrowth was present.
Rethwisch is commonly asked if it's better to spray for leafhoppers or cut early. Cutting just forces the adults to find another field; spraying breaks the cycle. Growers have to determine if they want to endure the damage or try to stop it, he says.
He and Oklahoma's Mulder are more concerned about alfalfa weevils. “Alfalfa weevil not only hits us with pretty heavy populations, it hits us incredibly early some years,” Mulder says. “We'd like to see growers go by a scouting method where they shake out 30 stems selected at random across a field, count larvae and use a degree-day-based model that looks at the number of degree days accumulated, the height of alfalfa and the infestation that they found in the stem sample.
“If they don't want to scout, a rule-of-thumb threshold is one larva per stem,” he adds. But Oklahoma State makes things easier for weevil watchers. Those who visit agweather.mesonet.org/models/alfalfaweevil/default.html can get help determining if and when applications are needed.
“In Ohio, natural enemies and diseases keep the weevil populations in check most years,” Sulc says. But he encourages Ohio growers to scout fields, since damaging populations do occasionally develop. “In contrast, damaging leafhopper populations are very common, especially in western Ohio,” he adds.
Rethwisch says growers need to monitor fields closely for weevils and leafhoppers. “Just because it's alfalfa, don't ignore it. Growers need to be on top of it. That didn't happen last year and leafhoppers caught a lot of people unawares,” he says.