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Scout for Alfalfa Weevil Larvae (06/20/19)

Larval feeding is at the peak period based on the degree day (DD) accumulations (see map; base development temperature of 48F) for alfalfa weevils in North Dakota.

Larval feeding is at the peak period based on the degree day (DD) accumulations (see map; base development temperature of 48F) for alfalfa weevils in North Dakota. Peak feeding occurs from 504 - 595 accumulated DD when larvae are mature (3rd to 4th instars).

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Although adult weevils will feed on the foliage and cause some defoliation, larval feeding causes most of the defoliation. Mature larvae are about ⅜ inch long with a black head capsule and a green body with a white stripe.

Fields should be scouted weekly up through the first cutting. Walk a “W” pattern or by selecting random sites in the field, with a minimum of five sampling sites per field.

At each sampling site in the field, select a minimum of 30 stems and cut them off at the base. Invert the cut stems into the 5-gallon pail and vigorously beat the plants in the pail to dislodge the larvae.

Count and record: 1) the number of stems sampled, 2) the total number of larvae counted and 3) the height of the alfalfa at the sampling sites. Repeat this procedure for all sampling sites within the field. When finished, total the number of larvae found and divide by the total number of stems sampled to calculate an average number of larvae per stem for the entire field. Then, calculate average plant height for the field.

Several factors must be considered when making alfalfa weevil management decisions. Plant height, estimated yield, crop market value, management costs and plant injury based on the number of larvae per stem must be considered. Threshold numbers in Table 1 are the average number of larvae per stem sampled in the field using the 30-stem sampling method. These economic thresholds apply prior to the first cutting only.

If you are at economic threshold for alfalfa weevils in alfalfa grown for hay, the most cost-efficient management is to cut the alfalfa early before economic loss occurs. This also preserves our parasitic wasps that attack alfalfa weevils, reducing populations naturally.

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Janet J. Knodel

Extension Entomologist

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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