It is difficult to even think about insects when many producers have not even had a chance to work the fields, let alone plant. However, two items of note to think about over the next week or two. First, we are actually approaching the required number of growing degree days needed for peak activity of alfalfa weevil in Ohio's southern counties.
The main damaging stage is the larvae, which feed and injure the plant through defoliation. As a reminder, alfalfa weevil scouting is accomplished by collecting a series of three 10-stem samples randomly selected from various locations in a field. Place the stem tip down in a bucket. After 10 stems have been collected, the stems should be vigorously shaken in the bucket and the number of larvae in the bucket counted. The shaking will dislodge the late 3rd and 4th instar larvae which cause most of the foliar injury. Close inspection of the stem tips may be needed to detect the early 1st and 2nd instar larvae. The height of the alfalfa should also be recorded at this time.
Economic threshold is based on the number of larvae per stem, the size of the larvae and the height of the alfalfa. The detection of one or more large larvae per stem on alfalfa that is 12 inches or less in height indicates a need for rescue treatment. Where alfalfa is between 12 and 16 inches in height, the action threshold should be increased to 2 to 4 larvae per stem depending on the vigor of alfalfa growth. When alfalfa is 16 inches in height and there are more than 4 larvae per stem, early harvest is recommended.
See the OSU alfalfa weevil Fact Sheet for more information on the insect. For insecticides that are labeled for alfalfa weevil, see Bulletin 545-2010. As the temperature warms, alfalfa fields in the more northern part of the state will need to be scouted.
Secondly, a silver lining to the cold weather may be a decrease in bean leaf beetle (BLB) populations early in the season. BLBs overwinter as adults, and typically do not do well in extreme cold winters. Like many other states, we are predicting relatively low BLB populations; however this may change depending on planting conditions and timing. BLB is becoming one of our more serious pests on soybean so early planted fields will still be susceptible and will need scouting.