Degree-day information indicates that alfalfa weevil larvae should have started hatching in southern Iowa on March 30-31. In central Iowa counties, weevils should be hatching by the middle of April, and in southern Iowa, weevils should hatch the last full week of April. Proper management of this insect 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 on the following page indicates the accumulated degree days across the nine crop reporting districts. Begin scouting in a respective district 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 when you first scout for alfalfa weevil larvae by using a sweep net. Information on obtaining a sweep net was given in the March 26 newsletter . A sweep net can quickly and easily determine whether larvae have hatched in the field. If larvae are found in the net, then switch your scouting procedure to the stem-collection method. 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, management options, and techniques for using the stem-collection method for assessing weevil populations.
Marlin E. Rice is a professor of entomology with extension and research responsibilities in field and forage crops. Rich Pope is an extension program specialist in entomology with responsibilities in integrated pest management and pesticide applicator training.
This article originally appeared on pages 91-92 of the IC-498 (4) -- April 2, 2007 issue.