There’s an unwanted guest in our alfalfa fields this spring, and the count of these guests is pretty high.
The larvae of the alfalfa weevil, Hypera postica has moved in to munch on the succulent leaves of our developing alfalfa plants. They are hard to see on the plant, but if you see leaves with a “shot hole” type of appearance, then it’s very likely to be the feeding damage of alfalfa weevils.
Field scouting in Ravalli and Missoula counties have documented a high number of weevil larvae actively feeding on plants this June. Because it is hard to see the larvae when walking through a field, we use an insect sweep net to knock the larvae off the plants and into the collection netting, where they can be counted.
As weevil populations increase we can see chew holes and skeletonized leaves on the alfalfa plants, and in bad years with a high population the field can take on a “frosted” appearance, where the tops of the plants start to turn yellow from all of the feeding damage.
Alfalfa weevils overwinter as adults in the residues and stubble in alfalfa fields and the grass boundaries surrounding the fields before traveling into the field as the spring weather reaches a consistent temperature above 50 degrees. Adult alfalfa weevils don’t do much feeding themselves during the spring, but instead mate and lay eggs on the stems of the developing alfalfa plants.
As the alfalfa weevil eggs hatch, the very small larvae will feed inside of the developing buds. The alfalfa weevil will develop through four stages of larval development, called instars.
The first two instars are small, hard to see, and don’t contribute significantly to the overall feeding damage on alfalfa plants. But the third and fourth instar stages of the larvae are bigger, feed openly on the plant leaf surfaces, and can cause more economic loss in both the volume and quality of the first cutting of hay. In years with extremely high populations of weevil activity, there can be a 30-40% loss in volume of the first cutting of alfalfa.
As the summer temperatures get hotter the developed larvae will spin a cocoon and stay in a kind of summer hibernation through the heat of the summer months in the soil, or attached to plant stems. They can emerge in the fall, or stay in hibernation until the next spring.
There are two strategies producers commonly employ when combating alfalfa weevil infestations that are high enough to warrant taking action. The economic threshold to instigate the use of control actions is when two or more larvae are found per stem on plants, or when more than 20 larvae are collected per sweep of a collection net.
Farmers with a high infestation of weevils can cut alfalfa fields early, to remove the foliage and stop further loss and damage from the weevil larvae. Alfalfa weevil larvae like the cooler, moister and shady conditions found under the canopy of the plants. When farmers cut the crop it exposes the larvae to heat and direct sunlight, and will kill a significant number of them.
But farmers who cut the field early as a control mechanism have to rely on the weather to cooperate, giving farmers a good window of sunny and dry days to get the crop down, baled, and stored off the field. We don’t always get those dry windows in the early summer.
It is also important when using this strategy to get the windrows of hay baled and up out of the fields in about seven days. If left longer the larvae can crawl under the windrows of hay and feed on the regenerating alfalfa plants beneath it, carrying the impact into the second cutting.
Use of insecticides to control weevil populations is another option. There are several different insecticides with different active ingredients labeled for use to control alfalfa weevil. It will be important to consider all of the label directions when choosing the pesticide tool to use. Some have an interval between application and harvest, and others do not.
A very important consideration in using insecticides as a weevil management tool is the developmental stage of the alfalfa field we are considering for control options. We should not use insecticide treatments in an alfalfa stand that is near or in bloom to protect pollinator populations that will be visiting the flowers.
The Ravalli County Extension office can help with field monitoring protocols and the establishment of an economic threshold for action on the damage being caused by alfalfa weevils, and the office has advice on the selection of any tools for management.
Alfalfa weevil populations shift in presence and impact on the fields from year to year based on temperatures in the early spring and the window of development for the larvae. This year is shaping up to be a year with a big population in some fields. Get out in the field and do some scouting, take a measure of where your visitors are at.
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