Keep an eye on your alfalfa fields for signs of alfalfa weevil. Iowa State University extension entomologists keep track of degree-day accumulation and those numbers this spring indicate that the pest started hatching in southern Iowa on March 30-31. In central Iowa counties weevils should be hatching by the middle of April and in northern Iowa weevils should hatch the last full week of April.
"Proper management of this insect requires timely scouting, correct identification, determination of the population level present in your field, and if necessary, cutting the hay or spraying an insecticide," says Marlin Rice, ISU extension entomologist. "Alfalfa weevil larvae can be very destructive to first-cutting alfalfa, so fields should be scouted."
The 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.
How to scout fields for alfalfa weevil
Scouting should begin when approximately 200 degree days have accumulated in fields south of Interstate 80 in Iowa, and 250 degree days in fields north of this highway, says Rice. You can go to ISU's Website at www.ipm.iastate.edu/ipm/icm to keep up with a degree-day map that shows degree day accumulation in the nine crop reporting districts in the state.
You should begin scouting in your district based on the projected hatching dates, he explains. Scouting should start on the south-facing hillsides. Larvae will hatch here first because these areas warm up more quickly than north-facing hillsides.
You can save time when you first scout for alfalfa weevil by using a sweep net. 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," says Rice. "Collect 30 stems and make counts of the larvae in the upper leaves."
"We have information on economic thresholds for alfalfa weevils, management options and techniques for using the stem-collection method for assessing weevil populations," says Rice. You can get this information from our ISU Integrated Crop Management Website or by contacting your county extension office.