The weather has finally warmed up enough to allow the alfalfa to grow. When the alfalfa starts to grow it is time to think about alfalfa weevil. Some producers treat alfalfa fields with insecticides each year to control the weevil. Others never spray fields for weevil, they just cut fields early to reduce the weevil populations. The USU Extension publications suggest that we monitor our fields and use given thresholds to determine if a treatment is necessary (http://extension.usu.edu/files/publications/factsheet/alfalfa-weevils89.pdf).
When to Spray
When damage from alfalfa weevils can be anticipated to be minor (as it is in many fields in many years in Utah), chemical treatment to control the weevil is not warranted. Growers are advised therefore to check their fields regularly, and spray only if and when alfalfa weevils become sufficiently numerous to require spraying. Two methods are presented here as alternative ways to determine when to spray.
1) Sweep Net Counts of Larvae
(A sweep net is required. It can be purchased at little cost from several biological supply companies. The county extension office can provide information on how to order a net.) Once the alfalfa reaches 10" or more in height, sweep the plants with a 15 inch diameter net. Take a full sweep from side to side (180x) through the top of the foliage with the net, and then count the number of alfalfa weevils collected in the net. Move to a new location in the field and take another sweep. Repeat this procedure for a total of ten sweeps taken at scattered locations throughout the field. If more than 20 weevil larvae are collected on average per sweep (200 larvae in ten sweeps), then spraying is advised. If 10-19 weevil larvae are collected per sweep, the field should be swept again in 3-5 days. If less than 10 weevil larvae are collected per sweep, the field should be swept again in 7 days. Sweeping will give most reliable results if restricted to calm, clear days (or evenings) with no more than a gentle breeze. Do not sweep when the foliage is damp or wet from rain or dew. Sweep vigorously, but not so vigorously that large quantities of alfalfa are torn off by the net.
For best success, alfalfa fields need to be checked regularly by sweeping before the first cutting. Typically, only older weevil larvae (third and fourth instars) are collected by sweeping. Young larvae are not easily dislodged by the net from their protected feeding sites inside the folded terminal leaves. If the grower sweeps his fields every few days beginning with 10" alfalfa growth, he may collect few weevil larvae at first (if the larvae are still young) even when many larvae are present. Later, as the weevil larvae reach the third instar and move to more exposed feeding sites, the catch in the sweep net will rise dramatically, and in some instances may indicate the need for spraying. Feeding damage will still be minor at this point (the young larvae consume relatively little plant tissue). The grower has 1-2 weeks (depending on the temperature) to spray or cut and kill the weevils before they become highly destructive as fourth instar larvae.
2) Stem Counts of Larvae
Once the alfalfa reaches 10" or more in height, break off 50 stems at ground level and shake them into a bucket to collect and count weevil larvae. Be careful not to knock larvae off the stem before shaking it in the bucket (cup your hand around the terminal to catch larvae should they drop from the plant). The stems should be gathered from scattered locations throughout the field. For example, take ten stems from each of the four corners of the field (30-50 feet in from the field edge) and from the center of the field. Measure the height of the alfalfa at each of these locations. After ten stems have been shaken in the bucket, count the number of weevil larvae collected. Add the five counts of larvae together, and determine the average number of larvae collected per stem.
Control is warranted if (1) more than 2 larvae per stem occur on alfalfa 10- 14" high, (2) more than 2.5 larvae per stem occur on alfalfa 15-18" high, or (3) more than 3 larvae per stem occur on alfalfa more than 18" high. At lower numbers of larvae per stem, sample again in 3-5 days if more than one larva occurs per stem. Sample again in 7 days if less than one larva occurs per stem.
Early Cutting
If alfalfa weevils do not reach destructive levels (20 per sweep or 3 per stem) until sometime during the final two weeks before the expected time of first cutting, the grower should consider early cutting as an alternative to spraying. Use of many insecticides requires an interval of at least two weeks between spraying and cutting to minimize residue contamination of hay. Early cutting will kill young weevil larvae (first to third instars), but fourth instar larvae often survive to pupate (pupae present at cutting will also survive). If more than half of the weevils at early cutting are 1/4-3/8" in length (these are mostly fourth instars), the hay should be removed from the field as quickly as possible and the stubble should be sprayed.
Keep an eye out for the weevil this year.
.jpg)