Time to monitor for alfalfa weevil

It is time to scout alfalfa fields for alfalfa weevil in Colorado. 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.

Alfalfa weevil 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. 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.

Field damage can be recognized on heavily infested stands as 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.

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 degrees F (9 degrees 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 March 1 for each 24-hour period in which temperatures exceed 48 degrees F (10 degrees C).

Monitoring techniques: Sweep sampling using a standard sized 38 centimeter diameter net is the most efficient method for estimating larval populations. Sampling should begin when 148 degree days have been accumulated, when the larvae are expected to be primarily second instars and when alfalfa hay has reached at least 10 inches in height.

Ten, 180 degree sweeps are taken while the sampler is walking through the field. Count the number of larvae per sweep and repeat this sampling procedure, taking a minimum of three samples for fields up to 20 acres, four samples for fields up to 30 acres and five samples for larger fields. Survey for alfalfa weevil larvae in a predefined pattern based on field acreage. Weevil infestation may be patchy or uniform depending on terrain, weather and soil. Inspection for weevils in every distinctive section of the field will aid in determining the pattern of the infestation.

Bucket method or stem count method may also be used to determine the number of weevil larvae per stem. Take three six-stem samples in fields one to 19 acres, four samples in fields 20 to 29 acres, and five samples in fields 30 acres and bigger. The tools and supplies needed for this method includes a three or five gallon light-colored bucket, a white cloth, a hand lens, paper and pencil. Use the following steps to survey and estimate alfalfa weevil densities (larvae per stem).

The simple economic threshold for a sweep sample is 20 larvae per sweep. The simple economic threshold for the stem sampling method is 1.5 to 2 larvae per stem. For calculating detailed economic threshold, check the High Plains IPM guide at https://wiki.bugwood.org/HPIPM.

Management of the weevil: Insecticide applications and early harvesting are the most common growing season management strategies.

Cultural control: A non-insecticide control measure for alfalfa weevils is an early first harvest if an economic infestation is not detected until late in the growth of the first cutting. Harvesting alfalfa in an immature stage provides good control of larvae for the first crop. Rapid removal of hay will accelerate larval mortality due to desiccation by direct sunlight. An early first cutting tends to cure more rapidly because lighter windrows dry quickly, and forage quality is enhanced by higher crude protein and lower fiber content. Additional steps should be taken to ensure that surviving larvae do not cause economic damage to the regrowth. If larval survival under the windrows is high and baling is delayed (e.g., due to rainfall), damage to regrowth may be exacerbated. Regrowth should be inspected at a height of one to two inches to determine larval density.

Chemical control: If damage becomes unacceptable as harvest approaches, an early harvest or “rescue” insecticide treatment may be necessary. Use care in applying insecticide when alfalfa is approaching bloom: refer to the Pollinator Protection section for guidelines on minimizing insecticide contact of pollinators. Also, consider the waiting period before harvest for different insecticides. Generally, harvest or insecticide applications should happen before bloom if weevils are a problem. For effective products check the High Plains IPM Guide at https://wiki.bugwood.org/HPIPM.