We have had a mostly cool, and sometimes wet, March. These temperatures have set the weevil hatch back a few days from last year, but projected degree days indicate the hatch is just around the corner in southern Iowa. Proper management of this pest requires timely scouting, correct identification, determination of population levels, and if necessary, cutting the hay or spraying an insecticide. Alfalfa weevil larvae can be very destructive to first-cutting alfalfa, so fields should be scouted. Larvae remove leaf tissue, beginning with the new leaves at the top of the plant, then work down the stem to other leaves. This feeding reduces forage quality and quantity.
Scouting should begin at approximately 200 degree days in fields south of I-80, and 250 degree days in fields north of this highway. The map indicates the accumulated degree days across the nine crop reporting districts. Begin scouting in southern Iowa based on the projected hatching dates. Scouting should start on south-facing hillsides. Larvae will hatch here first because these areas warm up more quickly than north-facing hillsides.
Save some time by using a sweep net when first scouting a field for alfalfa weevil larvae. A sweep net can quickly and easily determine whether larvae have hatched in the field. If no larvae are found in the net, then move on to the next field. However, if larvae are found in the net, then collect 30 stems and make counts of the larvae in the upper leaves. Next week we will present information on economic thresholds for alfalfa weevils and management options.
Projected degree days (base 48°F) through April 12 and predicted hatch date.
This article originally appeared on page 65 of the IC-496 (5) -- April 3, 2006 issue.