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Alfalfa Weevil Active (05/25/17)

Alfalfa weevil adult and larvae have been reported defoliating alfalfa in Bowman County in southwest ND.

Alfalfa Weevil Active

Alfalfa weevil adult and larvae have been reported defoliating alfalfa in Bowman County in southwest ND. Since fairly large larvae are already being observed, eggs probably successfully overwintered in the southwest last year. Adults are about ¼ inch long and brown-golden with a blunt snout and a dark brown longitudinal stripe in the center of the back. Antennae are elbowed and clubbed. They overwinter in debris and alfalfa stubble. Eggs are laid inside the stems of alfalfa, and are small and cream colored. Mature larvae are about ⅜ inch long with a black head capsule and a green-wrinkled body. A white stripe, running lengthwise, can be observed across the top.

Scouting should begin immediately after egg hatch, and fields should be scouted weekly up through the first cutting. Fields should be scouted in 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. First-instar larvae feeding in rolled leaf tips won’t dislodge easily, so be sure to examine leaf tips for 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 are the factors affecting the economic threshold.   

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.

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Janet J. Knodel

Extension Entomologist

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