Scout for alfalfa weevil larvae

Alfalfa weevil larvae are now being found in southeastern Iowa. Proper management of this alfalfa pest requires timely scouting, correct identification of the insect, determining population levels, and if necessary, cultural or chemical control.

Why should I scout fields for alfalfa weevils? Large populations of alfalfa weevils can be very destructive to the crop. They remove leaf tissue, beginning with the new leaves in the top of the plant, then work down the stem to other leaves. This reduces forage quality and quantity.

When should scouting begin? Begin scouting at 200 degree days in fields south of Interstate 80 and at 250 degree days in fields north of Interstate 80. Look at the growing degree day chart at the right. Newly hatched larvae can usually be found when 200 or 250 degree days have accumulated in your area.

Didn't we start scouting at 300 degree days several years ago? Yes. However, research conducted by Todd DeGooyer in the Department of Entomology, Iowa State University, found that alfalfa weevils laid some of their eggs in the fall and that these were starting to develop before winter. He observed that eggs hatched earlier than we anticipated, especially in southern Iowa. Therefore, the degree day scouting date was changed to more accurately reflect the average hatching time.

Where should I start scouting? Begin on the south-facing hillsides. These areas warm up more quickly than north hillsides, so larvae hatch there first.

How do I scout for alfalfa weevil larvae? Collect a sample of 30 stems. Look for the larvae in the upper leaves. After hatching from eggs laid in the stems, the larvae will crawl up to the newly developing leaves. When you collect stems, take care not to break them too hard, or you may knock the larvae off. Grab the tip of the plant in one hand, and break the base of the stem with the other hand or cut the stem with a knife to collect all of the larvae effectively. Beat the stems inside a five gallon bucket to count the large larvae. Small larvae hidden in the plant tip are not easily dislodged, so you will have to pull newly developing leaves apart to accurately count smaller insects.

What do alfalfa weevil larvae look like? They have a very dark head, almost black, and are pale green with a white stripe down the back. They are about 1/16 inch long when they hatch and may be light yellow in color. After feeding for several days, they turn green. They are 5/16 inch long when full grown.

Do any other insects look like alfalfa weevil larvae? Yes. Larvae of the clover leaf weevil look very similar, but they are larger, have a light brown head, and often have the white stripe edged with pink. Clover leaf weevil larvae usually hide during the day around the base of the plant and feed mostly in the lower leaves at night. They rarely cause economic yield losses and should not be counted as part of the alfalfa weevil sample.

When should alfalfa weevils be controlled? If two or more larvae are found per stem and 40 percent of the stems show any leaf feeding, the best option is to cut the hay if you can do so within the next five days. This method of cultural control avoids the use of insecticides. If the crop is not mature enough to cut, chemical control may be an option, depending on the economic thresholds.

Are the economic thresholds for chemical control different? Yes. University of Nebraska entomologists have developed new economic thresholds which consider crop value, cost of treatment, and the damage caused by the alfalfa weevil larvae. The new economic thresholds are for early bud stage alfalfa, and third and fourth stage larvae, the two largest stages during which 90 percent of the damage is done.

To use the economic threshold chart at right (not available here), first determine the control costs in dollars per acre, then estimate the forage value in dollars per ton. The average number of alfalfa weevil larvae per stem needed to justify chemical control is located where these two values intersect on the chart. For example, if the control cost is $10 per acre and the forage value is $75 dollars per ton, an average of 3.4 larvae per stem justifies chemical management.

What if the weevil count is below the economic threshold? Resample in 3 to 5 days. Chemical management may be needed then, or the crop may have reached a stage where it can be cut.

What chemicals are labeled for alfalfa weevils? The chart lists several. Read and follow all label directions before you use any insecticide.

Updated 04/27/1995 - 1:00pm