I have gotten a few phone calls from growers who say they are seeing some spotty damage in their alfalfa fields caused by alfalfa weevil. Due to our cool weather temperatures through this point in May, I would not expect much alfalfa weevil activity yet, but growers should be scouting fields to detect any possible early damage. Alfalfa weevil overwinters in our area as an adult beetle that becomes active when temperatures get above 48 degrees F. Adults lay eggs on the stems of alfalfa plants and eggs hatch as growing degree day heat units begin to accumulate. The young larvae pass through 4 developmental stages called instars before they quit feeding and pupate. The larvae feed on alfalfa leaves and the feeding damage from the first two instars is most evident on the growing tips of the alfalfa plants. The third and fourth instars are more voracious feeders and they can cause significant defoliation of alfalfa plants and economic yield loss when they are present in large numbers.
It is only necessary to apply an insecticide as a rescue treatment if the population of the weevil larvae is heavy enough to cause economic yield loss. Although growers may find some larvae and feeding damage, in my experience the need for an insecticide application is rarely justified for two reasons. The first is that the alfalfa weevil is often brought under control biologically. There are 3 parasitic wasps and a fungal pathogen that often knock populations down. Secondly, the impact on yield due to weevil feeding declines as the alfalfa plant height increases. Treatment generally focuses on alfalfa plants 12 inches or less in height. Once a plant reaches 16 inches in height, an early harvest is the preferred control option.
Scouting is done by collecting a series of 10 alfalfa stems from 3 to 4 random locations within the alfalfa field. Each stem should be pinched off or cut off at the stem base and placed top down into a bucket. When ten stems have been collected, the stems should be vigorously shaken and beat against the side of the bucket. This action will dislodge 3rd and 4th instar larvae stages. It may take a visual inspection of stem tips to detect the 1st and 2nd instar stages. An insecticide rescue treatment is recommended if more than 1 larvae per stem are found on alfalfa 9 inches or less in height and when more than 2 larvae per stem are found on alfalfa 12 inches in height.
Contact the Wayne County Extension office at 330-264-8722 for more information about alfalfa weevil.