Issue No. 6, Article 5/April 30, 2004
What Weevils Are in Your Alfalfa?
Kevin Black, with Growmark in Bloomington, observed tip feeding by alfalfa weevils as far north as the Princeton area (I-80) on April 22. He also noted leaf feeding by clover leaf weevil larvae, so confusion between these weevils may continue. Following is a refresher.
Tip feeding can be caused by both alfalfa weevils and clover leaf weevils, so it's important to differentiate between the two when scouting alfalfa fields. In general, tip feeding by clover leaf weevil larvae is more ragged in appearance. Instead of chewing pinholes like early-instar alfalfa weevils do, young clover leaf weevils chew larger chunks from the leaves.
The article "Alfalfa Weevil or Clover Leaf Weevil," published in issue no. 3 (April 9, 2004) of the Bulletin, provides a comparison of these two alfalfa insects. Briefly, an alfalfa weevil larva is green, with a white stripe on its back, and has a black head capsule. A clover leaf weevil larva also is green, with a white stripe on its back, but the white stripe has a pink/red border. Clover leaf weevil larvae have brown head capsules and are slightly larger than alfalfa weevil larvae. Additional information about these insects can be found in new fact sheets on the IPM Web site (http://www.ipm.uiuc.edu/fieldcrops/insects/alfalfa_weevil.pdf and http://www.ipm.uiuc.edu/fieldcrops/insects/clover_leaf_weevil.pdf).
Matt Montgomery, Extension unit educator in crop systems in Springfield, reported that alfalfa weevil feeding in some fields in his area is at or near threshold levels. As degree-days continue to accumulate throughout the state, observations of alfalfa weevil feeding injury will escalate.
Table 4 shows the accumulated degree-days for alfalfa weevils through April 25, as well as projections for accumulated degree-days 1 and 2 weeks after April 25. Also listed in the table are historic degree-day accumulations. This is the 11-year average of accumulated degree-days at the specified locations. You'll notice that accumulated degree-days in 2004 at all locations are ahead of the 11-year average. An Illinois map with degree-day accumulations can be viewed at the degree-day calculator (http://www.sws.uiuc.edu/warm/pestdata/choosemap.asp?plc=). All the more reason to take a trip through your alfalfa field to see what's going on. Scouting and threshold information for alfalfa weevils was discussed in issue no. 5 (April 23, 2004) of the Bulletin.--Kelly Cook and Kevin Steffey