Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Reports of heavy alfalfa weevil feeding are coming in from southern Michigan, and cutting is seven to 14 days away. The easiest scouting method uses tip injury. Survey across the field and not just on one side or on an edge. Check tips of 100 stems for feeding. Treat if 40 percent of stems show damage and the field won’t be cut for at least seven days.
Another scouting method is the “stem and bucket” action threshold from Ohio State, which uses both weevil number and crop height. Collect 30 stems at random (snap off at base) into a bucket from a field. Shake the stems into the bucket to knock off big larvae. Don’t worry about the little larvae. Count the total number of larvae in the bucket. Then randomly select ten of the 30 stems and measure the average stem height. (I marked a ruler on the handle of my sweep net.) The treatment threshold, in number of larvae per 30 stems, varies with average stem height in inches, as follows: 12 inch - 30 to 50 larvae; 16 inch - 40 to 75 larvae; 20 inch - 45 to 85 larvae; and 24 inch - 50 to 90 larvae.
If a field is over threshold, cutting is the preferred control method because it preserves natural enemies and pollinators, and saves the cost of application. Also, most insecticides cannot be used within seven to 21 days of cutting, depending on the product. Most fields this year are too early to cut, so they may need to sprayed. When a decision is made to spray, remember to note the pre-harvest intervals (PHI) of insecticides labeled for alfalfa. The PHIs range from zero to 28 days, depending on the product and rate per acre. Also remember, that there are many beneficial insects in alfalfa killed by sprays with longer residuals. Most labels include honeybee warning statements. Bee populations are taking a beating this season. Many of you may have heard of a new, unidentified disease, colony collapse disorder, which has devastated bee hives this spring. Since honey bees pollinate many of our food crops, it is extremely important to preserve the bees that are left. It is a good policy to notify beekeepers in the neighborhood when you do spray, because they may be able to move hives or restrict bee movement for a few hours. Fields in bloom should never be sprayed because all of the insecticides registered for alfalfa are toxic to bees. (These fields should be cut)
After cutting, remember to check for weevil larvae on the regrowth, which can delay or prevent green-up. The threshold after cutting is six to eight larvae per square foot of regrowth.
Dr. DiFonzo's work is funded in part by MSU's AgBioResearch.