Now is the time to start scouting for the alfalfa weevil in southern Iowa. Proper management of this alfalfa pest requires timely scouting, correctly identifying the insect, determining population levels, and if necessary, cultural or chemical control options.
Why should fields be scouted 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 their way 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 on the back page of this newsletter at the growing degree days chart. Find the city in the left column that is nearest to you, then look down the alfalfa weevil column on the far right for the degree days at that location. Newly-hatched larvae can usually be found when 200 or 250 degree days have accumulated in your area.
Didn't scouting once start at 300 degree days? Yes. However, research conducted by Todd DeGooyer, Department of Entomology at Iowa State University, found that alfalfa weevils were laying some of their eggs in the fall, and that these were starting to develop before winter. The result was 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 quicker than north hillsides, and larvae will hatch here first.
How do I scout for alfalfa weevil larvae? After hatching from eggs laid in the stems, the larvae will crawl up the stem and begin feeding inside the newly-developing leaves. Collect a 30-stem sample, and look for the larvae in these leaves. When you collect stems, take care not to break them too hard, or the larvae may be knocked off. An extremely effective way to collect all of the larvae is to grab the tip of the plant in one hand, then break the base of the stem with the other hand, or cut it with a knife. Count large larvae by beating the stems inside a five gallon bucket. Small larvae hidden in the leaflets are not so easily dislodged. Pull the newly-developing leaves apart to accurately count these smaller larvae.
What do alfalfa weevil larvae look like? They typically have a very dark head, almost black, and are pale green with a white stripe down the back. They are about 116 inch long when they hatch and may be light yellow in color. After feeding for several days, they turn green. They are 516 inch long when fully grown.
Are there any other insects that 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.
When should weevils be controlled? Traditionally, entomologists have stated that if two or more larvae are found per stem and 40 percent of the stems show any leaf feeding, then the best option is to cut the hay, if possible, 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. Entomologists Robert Peterson, Steve Danielson, and Leon Higley at the University of Nebraska have developed new economic thresholds based upon their recent research. The value of their research is that these new economic thresholds actually consider economics, that is, the value of the crop, the cost of treatment, and the damage caused by the alfalfa weevil larvae. These 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.
The Nebraska thresholds are easy to use. First, determine the control costs in dollars per acre, then estimate the forage value in dollars per ton. Where these two values intersect on the chart, you find the number of alfalfa weevil larvae per stem needed to justify chemical control. For example, if the control cost is $10 per acre and the forage value is $75 dollars per ton, then an average of 4.8 larvae per stem would have to be found before the control would be economically justified.
What if the weevil count is below the economic threshold? Resample in 3 to 5 days. Chemical management may be needed then, or possibly a decision can be made to cut the crop early because of an increasing weevil population.
What chemicals are labeled for alfalfa weevils? There are several. See the chart, but read and follow all label directions before using any insecticide.