Carbon County Extension
Alfalfa weevil is one of the most damaging alfalfa pests across Utah. Some producers
rely on “calendar spraying”, or applying insecticides at the same time each year,
to hopefully provide control. Weevil activity and growth rate is primarily affected
by temperature. Warming spring conditions and weevil pressure can change dramatically
from year to year. By spraying on the same date annually, applications may be too
early or too late to be advantageous. Once severe plant damage is noticeable from
the roadside, significant yield losses have already occurred and an early harvest
is probably required. Scouting fields can help determine whether weevil larvae have
reached levels requiring treatment. It can also assist in timing treatments, when
necessary, before economically significant damage occurs.
Sweep net scouting
Sweep netting is the most standard method used to estimate larval numbers. Scouting should start when the alfalfa is approximately 10 inches tall. A 15-inch diameter canvas net is briskly swept through the plant canopy using multiple 180° sweeps from one side of the body to the other. Each sweep is taken consecutively with each step forward so that arcs do not overlap. Swings should be vigorous enough to dislodge the larvae from the alfalfa but not so forceful to damage plants. Special care should be taken to angle the net so that insects are knocked into the bag and not onto the soil. Ten consecutive sweeps are taken from one location and counted (see this video for more instructions). Phone clicker counter type apps are useful to quickly count large numbers of larvae. The number of larvae counted is divided by the number of sweeps (ten in this case) to generate the average number of larvae per sweep. By repeating this multiple times throughout different areas of the field, an overall field average can be simply calculated and compared to established thresholds. When scouting early, very small larvae can be difficult to dislodge from inside the protective leaf whorls where they feed. As the season progresses, larvae grow and become easier to dislodge. Scouting should occur more frequently during this time to assess potential damage.
Once the field average larval number has been determined, it can be compared to the current threshold of 20 larvae per sweep. This number is our best estimate of when the value of pest damage occurring is more than the cost of an insecticide application. Once larval densities reach the threshold, management is warranted. When larval averages are below 20 per sweep, it is predicted that application costs are more than the expected yield loss. If the average is 10 to 19 larvae per sweep, the field should be sampled again in three to five days. If less than 10 larvae per sweep are observed, samples should be collected again in seven days. If the field is approaching or at threshold and the field is scheduled to be cut in less than two weeks, producers should consider harvesting early before significant yield loss occurs.
When control is warranted based on scouting, producers should consider what chemicals have been used in the past. Insecticide classes, such as organophosphates or pyrethroids, are grouped based on chemical structure and how they negatively disrupt the pest physiology and lifecycle. These are components of the “mode of action”, now classified by group number, and can be found in the IRAC website. There has been evidence of weevils becoming resistant to pyrethroids in some select areas of the U.S., but it has not been validated in Utah. Modes of action should be rotated so that different modes of action are being used from year to year to reduce the potential for building up weevil resistance to the few insecticide classes currently available.
Key Point: Weevil pressure varies by year (influenced heavily by air temperature) causing the need for early scouting and treatment with insecticide if weevil larvae exceed 20 per sweep instead of using a routine, calendar-based insecticide application.