Scout now for alfalfa weevil

Alan Leslie
Agriculture Extension Agent, Charles County

After a relatively mild winter, we are getting reports of high numbers of alfalfa weevils causing damage to alfalfa fields in Southern MD. Eggs that were laid by alfalfa weevil adults last fall are all hatching now, and early spring is when the larvae can cause the most damage to alfalfa stands, reducing the yield and quality of the first cutting and potentially setting the entire stand back for the rest of the year. Now is the time to be scouting for larvae and to consider chemical treatment to prevent economic yield loss. Alfalfa weevil larvae look very similar to caterpillars, with a dark head and a green body with a white stripe running down the middle. Early signs of injury from alfalfa weevil larvae appear as pinholes in leaves, but extensive feeding will skeletonize leaves, leaving plants with a distinct gray color. Sweep nets do not do a very good job of catching small larvae, so the best way to scout your fields is to cut stems and beat them on the inside of a bucket to knock the larvae loose. To effectively sample a field, you should collect at least 30 stems from across the field, and beat them vigorously inside the bucket. Make sure to keep track of the exact number of stems you sample, so you can calculate the average number of larvae per stem. Thresholds for control depend on the average height of the stand at the time you sample, with the threshold increasing in taller plants (Table 1).

Table 1.Threshold numbers of larvae per plant for different average stand heights that would warrant chemical control

*At these stand heights, prompt or early harvest can also control the larval infestation

We have also gotten reports of less than adequate control of alfalfa weevil populations using synthetic pyrethroids (group 3A; beta-cyfluthrin, zeta-cypermethrin, permethrin, lambda-cyhalothrin, cyfluthrin). Because synthetic pyrethroids have historically been such a cheap and reliable chemical class, they have been widely used on many pests, and there are many examples of insect species becoming resistant to this class of insecticide. Currently we do not have good data on the extent to which local populations of alfalfa weevil are becoming resistant to pyrethroids, but it is always a good idea to rotate modes of action of insecticides sprayed over your fields to help prevent resistant populations from developing. Other chemical classes that give effective control of alfalfa weevil include carbamates (group 1A; methomyl, carbaryl) organophosphates (group 1B; phosmet, malathion, chlorpyrifos), and oxadiazines (group 22A; indoxacarb). If you have any remaining chlorpyrifos in your inventory, this may be a good opportunity to use it up, since the state of Maryland will likely introduce legislation to ban its use this year or in the near future. Remember to always read and follow the label whenever making any insecticide applications; the label is the law!