The alfalfa weevil damage season is underway in parts of the state, according to the state Department of Agriculture, Trade and Consumer Protection.
Alfalfa surveys in the southern counties estimate leaf tip feeding injury is still low — 5-20 percent — throughout sampled fields. However, DATCP says feeding pressure is expected to intensify next week. More larvae are transitioning into the larger and most destructive development stages.
The Alfalfa Weevil model offered by the UW-Madison Vegetable Disease & Insect Forecasting Network estimates where scouting for larvae and damage should be concentrated: https://agweather.cals.wisc.edu/vdifn
Red and orange shaded areas, namely in southwestern and south-central Wisconsin, are where scouting is most critical today. Degree day accumulations in these high-risk areas range from 400-500, and weevil larvae are reaching the third and fourth instars, when feeding damage is heaviest.
DATCP notes a red “very high” risk rating on the map does not necessarily mean severe alfalfa weevil feeding. It indicates larvae are in the development stage, where heavy feeding may occur if the weevil population is high in a field. The only way to determine actual alfalfa weevil feeding pressure is to scout fields.
This Week
During the week of May 25-31, more of the state will enter the orange and red high to very high categories. Scouting will be important. The method for assessing alfalfa weevil damage is to collect 50 stems at random from throughout the field, checking each stem for characteristic weevil leaf feeding, and estimating the percentage of stems showing defoliation.
DATCP recommends management when 40 percent of the alfalfa stems have alfalfa weevil feeding and the field is more than seven days from the scheduled harvest date.
Lastly, sweeping fields with a standard 15-inch net is helpful for determining the growth stages and abundance of weevil larvae, but the economic threshold is based on a percentage of stems showing tip feeding across the field and not the number of larvae collected per sweep.
Learn more: https://cropsandsoils.extension.wisc.edu/articles/managing-alfalfa-weevil-in-wisconsin-alfalfa-fields/
