By Marlin E. Rice and Rich Pope, Department of Entomology
Cold! That is the best way to describe the first three and a half months of 2008. A cool spring has delayed alfalfa weevil hatch this year, however Iowa fields should have accumulated sufficient temperatures for larvae to hatch starting April 23-25 in southern Iowa. Naturally, weevils with hatch at slightly later dates in the central and northern counties of the state.
The top number is accumulated base-48 degree days between Jan. 1 and April 20, 2008. The bottom number is the projected date of alfalfa weevil hatch for each Iowa crop reporting district.
Effective management of alfalfa weevil depends on timely scouting, correct identification and effective determination of insect populations. From that information, better economic decisions can be made that may entail spraying with an insecticide or cutting the hay crop early. 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.
A young alfalfa weevil larva. (Photo by Marlin E. Rice)
Scouting for alfalfa weevil should begin at approximately 200 degree days in fields south of I-80 (the three southern crop reporting districts), and 250 degree days in fields north of I-80. The map indicates the accumulated degree days across the nine crop reporting districts. Begin scouting in southern Iowa based on the projected hatching dates, but remember that scouting should start on south-facing hillsides. Larvae typically will hatch here first because these areas warm up more quickly than north-facing hillsides.
Using a sweep net can save some time when first scouting a field for alfalfa weevil larvae. With the sweep net, a field can be quickly and easily checked to determine whether larvae have hatched. 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 count the larvae in the upper leaves.
Economic thresholds for alfalfa weevils and related management options will be provided in an article during the next week on Integrated Crop Management News.
Marlin E. Rice is a professor of entomology with extension and research responsibilities. Rich Pope is an extension program specialist working in the Corn and Soybean Initiative.