Indiana 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, a Purdue Extension entomologist says.
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 said. 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 said.
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 said. "Five alfalfa stems should be examined in each of five areas of a field for a total of 25 stems for the entire field."
Krupke said each stem should be examined for:
- Evidence of feeding by alfalfa weevil larvae.
- Maturity of the stem, such as pre-bud, bud and flowers.
- Stem length.
- Average weevil larvae size.
"Although large larvae are relatively easy to find, small larvae are difficult to see," he said. "Therefore, developing shoots showing pinhole feeding will have to be unfolded to find the small larvae."
Depending on the severity of infestation, growers might want to treat their fields. A series of infestation thresholds can be used to determine if treatment is necessary and what types of treatments are best. Some infestations require immediate treatment with residual insecticides. Tables of heat units, thresholds and treatment recommendations, as well as more information about alfalfa weevil, are available at http://extension.entm.purdue.edu/fieldcropsipm/insects/alfalfa-weevil.php.
Another option would be to cut alfalfa a few days earlier than originally planned.
The good news for growers is that warm, humid, wet weather encourages disease in alfalfa weevil populations, Krupke said.
"Humidity is detrimental to alfalfa weevil populations because it causes the spread of fungal pathogens," he said. "Warm, wet weather causes natural 'plagues' in weevil populations and is a powerful natural control."