Entomology Update: Alfalfa Weevil Scouting Notes Back »

This article was written collaboratively by Ada Szczepaniec, former SDSU Extension Entomology Specialist, and  Laura Edwards.


Alfalfa weevils are one of the key pests of alfalfa, and both adult and immature stages feed on this crop. Adults overwinter in protected areas of fields, and while some eggs are laid in the fall, majority of oviposition occurs in the spring. Low temperatures and late planting have delayed insect problems, but it’s not too early to scout for alfalfa weevils.

Adults are present in the fields in spring, early summer and fall but it is the larvae that cause the most serious damage to alfalfa. In high infestations, larval feeding can result in the characteristic greying or whitening of the fields. Larvae start hatching in early spring and will feed inside the folded leaves of new growing tips. Unfolding the leaves with pinhole damage may be necessary to see the young, light to dark green larvae. Feeding damage will start as pinholes in the leaves but soon will increase to give the leaves a ragged and shredded appearance.

Above: Larvae of alfalfa weevil are green with a white stripe running down their sides. They are usually curled up in a C-shape. Damage they cause will start as small pinholes (seen on the clover leaves in the image) and quickly increase resulting in shredded appearance of the leaves.
Photo by: John Rickertson, NDSU

Warm and wet springs will promote growth of pathogens that attack the larvae so weather conditions and soil moisture play a role in terms of severity of alfalfa weevil infestations. Early cutting of fields when possible is recommended to manage alfalfa weevils, and regular scouting is crucial in making sustainable management decisions. Mere presence of these pests in the fields does not warrant pesticide applications. The general threshold (and least precise) is to treat if 30-40% of tips are damaged by the weevils, larvae are present, and early harvest is more than one week away. A bucket method described below is a more precise sampling method and is the preferred technique to sample alfalfa weevils and determine whether pesticide applications are warranted.

Sampling for these insects should start at 190 growing degree days (GDD) accumulation and continue at least once a week. A number of locations in the western and southern parts of South Dakota have reached or surpassed 190 GDD, as seen on the map below in larger, bold font. For up to date information on GDD in your location, visit the South Dakota Climate and Weather website, select Growing days parameter, use base of 48° F, method 80/48, start date January 1, 2013 and end date of your choice. This will give you the current GDD for your location or a location nearby in a graph form or a list if you click on “View data”.


Above: Growing degree day (GDD) accumulation as of April 30th, 2013.
Map by: Ryan Vanderleest and Nathan Edwards, State Climate Office

To scout for alfalfa weevils, start sampling at least 20 paces inside the field, and randomly collect 30 stems while walking in a ‘U’ pattern. Collect the stems into a bucket, clipping them at soli line. Return to your truck and lay the stems on a flat surface, and then beat four-five stems at a time against the side of the bucket to dislodge the larvae and check folded leaves with pinhole damage, as the larvae may be inside. Count the number of larvae and then randomly select 10 of the 30 stems and measure them to the nearest inch to determine the average stem length as well. Use the table below, developed by the Extension faculty at the University of Kentucky, to guide your decision about pesticide applications.

 
190-225 Degree Days (DD)
Number of larvae / 30 stems 27 67 100 130
Plant height 2” 4” 6” 8”
  • If nr of larvae greater than the number in the table for the average height of stems: apply a long residual insecticide.
  • If nr is below the table value but >15: sample again in 2 days.
  • If nr is <15: sample again in 7 days.

 

226-275 Degree Days (DD)
Number of larvae / 30 stems 15 19 20
Plant height 2” 4” ≥6”
  • If nr of larvae greater than the number in the table for the average height of stems: apply a long residual insecticide.
  • If nr is below the table value: scout again in 7 days.
 
276-325 Degree Days (DD)
Number of larvae / 30 stems 37 60 83 105 135
Plant height 4” 6” 8” 10” 12”
  • If nr of larvae greater than the number in the table for the average height of stems: apply a medium residual insecticide.
  • If nr is below the table value: scout again in 7 days.
 
326-375 Degree Days (DD)
Number of larvae / 30 stems 82 105
Plant height 8” ≥10”
  • If nr of larvae greater than the number in the table for the average height of stems: apply a short residual insecticide.
  • If nr of larvae is less than the table value but >20, and alfalfa is <16”: sample again in 2 days.
  • If alfalfa >16” and nr of larvae <20: scout again in 7 days.
376-525 Degree Days (DD)
Number of larvae / 30 stems 52 64 72 80
Plant height 12” 14” 16” ≥18”
  • If nr of larvae greater than the number in the table for the average height of stems: apply a short residual insecticide or harvest if alfalfa is in the 30% bud stage or greater.
  • If nr of larvae is less than the table value ±20: sample again in 2 days.
 
First cutting regrowth (5-7 d post-harvest)
Number of larvae / 30 stems* 20 33 47 60
Number of larvae / 30 stems** 17-20 17-32 23-46 23-59
Plant height 2” 4” 6” ≥8”

* If number of larvae exceeds the table value: spray with a short residual insecticide.
** Sample again in 2 days; f numbers are below these values, no treatments is necessary.

Source: Townsend, L. Alfalfa weevil field sampling program. University of Kentucky, Extension. Entfact-127

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