Ohio has had enough warm, spring days for alfalfa growers to start seeing alfalfa weevil emerging in their fields. So producers should be scouting for the pest now instead of waiting to see obvious damage before doing anything about it.
The early season pest is active in both adult and larval forms in the spring, and heavy infestations can be destructive to the alfalfa crop.
In early spring, alfalfa weevil larvae hatch from eggs deposited in the plant stems and begin feeding within the folding leaves at the growing tips, Christian Krupke, a Purdue Extension entomologist says. A heavy infestation of larvae can consume enough foliage that an entire field might take on a grayish appearance.
"When you can see the damage from the road and the field starts to look gray, you've missed the opportunity to treat the field with an insecticide," he says.
Alfalfa larvae damage includes pinholes in the leaves, but as the larvae grow, they will start to chew larger holes that make plants look shredded. Adult feeding damage looks like small, circular cuts along leaf margins, but is minor and typically not a concern.
"Sampling a field to determine the extent of alfalfa weevil damage and average stage of weevil development is best accomplished by walking the field in an M-shaped pattern," Krupke says. "Five alfalfa stems should be examined in each of five areas of a field for a total of 25 stems for the entire field."
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