High Plains IPM Guide - Alfalfa: Alfalfa Weevil
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Alfalfa: Alfalfa Weevil
Sue Blodgett and Frank B. Peairs revised Mike Brewer, Frank Peairs, Jay Donahue, original
Contents |
Introduction
Alfalfa weevil is the most destructive insect of alfalfa hay in the intermountain western region of the United States. Both larvae and adults feed on the alfalfa; the larval stage is the damaging stage, lowering yield and quality.
Identification (and life cycle/seasonal history)
Adult weevils are snout beetles approximately 3¦16 inch long; they are light brown with a dark brown stripe from the head to about three-quarters down the back, narrowing as it progresses down the back. Older weevils may have a less distinct stripe. In colder regions where adults do not become active until spring, over wintering is thought to occur outside alfalfa fields. In warmer regions where adults are active during the winter, overwintering is thought to occur within alfalfa fields. When temperatures warm to about 48°F (9°C) in the spring, the weevils become active. The females lay eggs a few days after emerging from over wintering sites. The females chew holes in alfalfa stems, laying from 5 to 20 eggs in each hole. The eggs are tiny, about 1¦50 inch long, oval yellow in color when first laid and turning dark brown before hatching. Egg clusters can be found by first checking for small punctures and splitting open the lower 1/3 of stems. Egg laying begins in April (warmer areas) or May (cooler areas, Montana) and extends through June or later. Each female will lay between 400 and 1,000 eggs. New larvae hatch and emerge from stems after seven to 14 days, depending on temperature. Weevil larvae are about 1¦20 inch long when they first hatch. They range in color from cream, to pale green, and are curved with shiny black heads. A white stripe running down the middle of the back may be visible and becomes more distinctive as the larva matures. At this stage a 10X hand lens is necessary to identify the weevil larvae. The coloration and shape is characteristic throughout the four larval stages, referred to as "instars." Fully-grown larvae are up to 3¦8 inch long and are wider in the midsection than at either end of the body. First and second instars feed in the tightly folded leaves of stem buds. When half to full grown, the larvae tend to move onto open leaves near the terminals. Larval development is completed in about three to four weeks, with the peak damaging larval populations often coinciding with the first cutting of the crop. Fully-grown larvae move into the plant crowns and soil debris to pupate. The larvae spin loosely woven, net-like cocoons, in which they pupate. Adults emerge from the cocoons in seven to 14 days depending on temperature. They feed on the alfalfa for a short time before entering a summer diapause or aestivation, reducing their activity for the rest of the summer. Adults may be found in protected sites in and around the field. There is one generation per year in the intermountain region of the United States. A second generation of alfalfa weevil may develop in warmer regions, but does not occur at economic levels.
Plant Response and Damage
Larvae feeding in the folded leaves can heavily damage stem terminals, but initial damage is not always clearly visible. The closed, overlapping foliage of the stem terminals should be unfolded to detect feeding damage. Heavily infested stands have a grayish or frostlike appearance due to the dried defoliated leaves. At high weevil densities, foliage can be stripped; leaving only skeletonized and ragged leaf fragments and stems. Yield losses of 30 to 40 percent of the standing hay crop are possible under extreme population levels. Damage also may reduce hay quality due to loss of leaf tissue, leaving only the lower quality stems. Damage to regrowth buds may occur when the plants break dormancy and after first cutting. Larval feeding on the regrowth after first cutting may be concentrated in strips coinciding with windrow locations, especially if the first cutting was taken early due to heavy weevil infestation and larvae survived under the windrows. Damage to regrowth may retard plant growth and result in yield reduction and encourage weed establishment.
Monitoring
Timing of sampling
Estimation of the weevil instars present in the field can be calculated using degree-days. Alfalfa weevil development increases at a nearly constant rate as the temperature rises above 48°F (9°C.). The amount of warm weather required for weevil larvae to complete development is measured in units of degree-days. For the alfalfa weevil, degree-days are accumulated after 1 March for each 24-hour period in which temperatures exceed 48°F (10°C). Using this technique sampling should begin when 148 degree days have accumulated. Weather stations in MT and WY are included at http://ippc2.orst.edu/wea/ that is linked to a degree day calculator. This timing coincides with the peak occurrence of second instars. In years of unusually warm or cold springs, weevil development may occur earlier or later than normal. The alfalfa weevil population relative to the estimated date of first harvest may influence management choices. Third and fourth larval instars cause most of the economic damage, so initiating sampling at the peak occurrence of second instars should provide adequate sampling prior to economic weevil populations.
