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Alfalfa Weevil Scouting (05/20/21)

North Dakota gained about 100 Accumulated Degree Days (ADD) units from last week. This week we have a low of 265 ADD in the northeast area to a high of >400 ADD in the northwest to south central areas (see map).

North Dakota gained about 100 Accumulated Degree Days (ADD) units from last week. This week we have a low of 265 ADD in the northeast area to a high of >400 ADD in the northwest to south central areas (see map). Start to scout alfalfa fields for adult alfalfa weevils after 300 ADD, and for larvae from 371 through 595 ADD. The heaviest feeding occurs as mature larvae appear (from 439 through 595 ADD), usually mid-June through mid-July. Most of North Dakota is in the adult weevils to small, early larvae stages, causing damage which will appear as light feeding injury or pin-holes in leaf clusters.

To assess the insect DD model, go to the NDSU’s NDAWN website and Applications – Insect DD. Then, click on the Map tab and select 48 F for your base temperature and Degree Days (DD) for your map type. Then, click Get Map.

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How to Scout for alfalfa weevils:  To determine if alfalfa weevil is present or absent in a field, a sweep net is a good tool to use for detection. But, for determining if your alfalfa needs to be treated with an insecticide or not, use the stem-bucket method. Fields should be scouted weekly up through the first cutting. Scout 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.

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

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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