Reports of alfalfa weevil feeding activity continue to come in from almost all regions of Illinois. The most severe damage and highest numbers of alfalfa weevil larvae have occurred in southwestern, western, and central counties. During the week of May 3, Duane Frederking (Field Sales Agronomist with Pioneer), Bob Harris (Moultrie/Douglas Extension Unit Leader), and David Allen (Planters Effingham Equity) all indicated that the weevils were causing economic damage in fields as far north as Douglas, Moultrie, and Menard counties. Alfalfa weevil larvae also have been observed recently in northwestern counties. David Feltes, Extension Educator/IPM at the Quad Cities Extension Center, indicated that second instars were relatively easy to find, but the injury to the alfalfa thus far has been minimal. Figure 3 shows that heat-unit accumulations above the base temperature of 48°F were at or near 300 in almost all northern counties by May 9. By May 23, projected heat-unit accumulations above 48°F will be well beyond the required amount for significant alfalfa weevil activity (Figure 4).|
Many people have indicated that alfalfa maturity is a week or two ahead of "normal" this spring. Consequently, early cutting of this first crop of alfalfa is a potential management tactic for alfalfa weevils. Harvesting reduces numbers of alfalfa weevils, saves on the cost of insecticide application, and is less disruptive to the parasitic wasps and the fungus discussed in previous issues of the Bulletin. However, cutting too early can sacrifice some yield. Therefore, growers must determine the stage of alfalfa growth before they make the decision to cut early.
If alfalfa is cut early, we urge you to watch the regrowth carefully for several days. Although harvesting alfalfa reduces weevil numbers, enough of them can survive to feed on the regrowing buds after harvest. Look for mature larvae and new adults in the stubble for several days after cutting. If 50 percent of the stubble is defoliated for 3 to 5 days, an insecticide may be warranted. The numbers of larvae and adults required to cause this level of damage are four to eight per square foot.
As a reminder, if an insecticide application is warranted, check the harvest intervals on the labels of suggested insecticides before deciding which product is appropriate. An insecticide with a short harvest interval is recommended if the alfalfa will be harvested relatively soon. Suggested insecticides along with their harvest intervals were published in issue no. 6 (April 30, 1999) of the Bulletin.--Kevin Steffey