Minnesota’s ample snow is good for alfalfa, but it can also favor
insect pests. Overwintering alfalfa is most vulnerable in March and April when freezing temperatures follow a thaw. However, “the significant blanket of snow over much of the state provides protection until alfalfa breaks dormancy,” explains Dr. Craig Sheaffer, University of Minnesota Extension forage agronomist. Plants have been insulated from extremely low temperatures during the winter, so he expects little risk of injury. The snow insulation also kept soil temperatures near freezing, above the levels that kill alfalfa plants.
However, there are still some risks of alfalfa injury due to ice sheeting and heaving. Since soil temperatures have been at or near freezing, the risk for ice sheeting is low. If it occurs, it’s more likely to be in low spots.
As April approaches, the freeze-thaw cycle can jack up alfalfa roots – called heaving – in moist soils. If it raises plants more than an inch, plant survival and productivity could decrease.
If heaving affects large field areas, Sheaffer suggests rotating to corn. Small areas can be interseeded with ryegrass and red clover, but not alfalfa due to allelopathy potential.
Even though winter injury risk is low, assess alfalfa in early May for any stand issues.
Now is a good time for frost seeding with red clover. Freeze-thaw cycles will bury seeds scattered on the surface, then germinate when temperatures warm. For spring seeding of alfalfa, plant as soon as possible to boost forage yields.
The snow also protects insect pests. “To manage significant alfalfa pests, use an integrated approach that includes insecticides, natural enemies, cultural control, and host plant resistance,” encourages Dr. Anthony Hanson, University of Minnesota Extension IPM educator.
Natural enemies can control some pests, but unneeded insecticides may kill the beneficial, leading to pest population flares. Overuse can also increase insecticide resistance and input costs.
Rotate insecticide groups at least annually and every three years, if possible, to reduce the risk of developing insecticide resistance. Scout to identify risks and treat them when economic thresholds are reached. “Insurance” applications do more harm than good. Finally, use full-labeled rates of insecticides. Low labeled rates can increase insecticide resistance risks.
The alfalfa weevil and potato leafhopper should have a regular scouting program as part of a pest management plan.
The alfalfa weevil was introduced to the U.S. in 1904. In the 1980s, a parasitic wasp from its native range was introduced to suppress it. While it worked for a while, the wasp was very sensitive to insecticides.
The larvae feed on alfalfa leaves, giving them a skeletonized look. Treatment thresholds are based on number of larvae – in addition to other factors – so scouting is essential.
If alfalfa is close to harvest, mowing and raking damages larvae and exposes them to the elements. Harvesting the crop as baleage or haylage will also quickly remove the crop and larvae protection.
With pyrethroid resistance likely –not yet confirmed – in Minnesota, using these Group 3 insecticides are not recommended unless failure issues haven’t occurred.
Consequently, insecticide options are limited. Group 1A organophosphates – malathion or phosmet – will achieve about 50% control. However, control increases to about 90% when they are combined. The Group 22 insecticide, indoxacarb, is an effective option, but it was hard to find last year.
Historically, alfalfa weevil damage occurred at about the first cutting and possibly into the second. In western Minnesota, Hanson saw fields where damage extended into later June and early July, much later than expected. A new western strain that is one to two weeks later than the predominant eastern strain may have arrived in Minnesota.
Potato leafhopper migrates from the south each year. It tends to build up and becomes a late-season pest, although issues don’t occur every year.
Resistant alfalfa varieties with no yield drag are available. Including grasses with alfalfa will also help keep leafhopper numbers down. Insecticide treatment thresholds are based on alfalfa height, number of leafhoppers, and treatment cost, so scouting is important.
For more information from the University of Minnesota Extension, visit extension.umn.edu/crop-production.
Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program.
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Extension Communications at extdigest@umn.edu. The University of Minnesota Extension is an equal-opportunity educator and employer.
