Reports from southern Illinois, southern Indiana, and southern Missouri indicate that alfalfa weevil damage is severe in many fields. Kevin Black, with Growmark in Bloomington, reported that many fields south of Route 50 were devastated, with newly hatched (first instars) feeding shoulder-to-shoulder with third instars. In one field in Effingham County, he observed 80% tip feeding in 12-inch-tall alfalfa. Kevin also observed pupation beginning in some fields in southern counties.|
In central Illinois, alfalfa weevil activity is beginning to pick up steam. Jerry Harbour and Tom Harms, with Lincoln Land FS, have been finding all sizes of alfalfa weevil larvae (1/8 to 3/8 inch long) in virtually all fields they have examined in Logan, Menard, and Sangamon counties. They found a range of two to five larvae per stem, and feeding injury was obvious.
The warning signs were in place for a "bumper year" for alfalfa weevil problemshigh numbers of overwintering eggs in southern areas, mild winter weather. So now it's a matter of staying on top of it and preventing serious yield losses. If an insecticide is warranted (see Table 5 in issue no. 3, April 12, 2002, of the Bulletin), make certain you make note of preharvest intervals (i.e., the number of days between insecticide application and harvest). Preharvest intervals for the insecticides suggested for control of alfalfa weevils are presented in Table 3.
Figure 1 shows actual degree-day accumulations (base 48 deg F), from January 1 through April 22, 2002. Figure 2 shows projected degree-day accumulations (base 48 deg F), from January 1 through May 6, 2002. The warm temperatures during the week of April 15 accelerated alfalfa weevil development, and the more recent cool temperatures have slowed them down. Remember that an early peak of third-stage larvae from overwintering eggs occurs after an accumulation of 325 degree-days, which has already occurred throughout the southern half of Illinois. A second major peak of third-stage larvae from spring-deposited eggs occurs after an accumulation of 575 degree-days.
As you scout for alfalfa weevil larvae, look for larvae that are infected with Zoophthora phytonomi. This fungal organism causes larvae to turn slightly yellow, and it eventually kills the weevil larvae. Dead larvae are brown and look like miniature loaves of bread. This fungus can cause epizootics that cause populations of alfalfa weevils to "crash" in a few days. Cool, damp weather promotes the spread of Z. phytonomi.
Alfalfa weevil larva killed by Zoophthora phytonomi.
Bathyplectes curculionis adult and cocoon.
The parasitic wasps Bathyplectes anurus and B. curculionis also may help regulate alfalfa weevil populations. However, signs of their presence are not as obvious. Unfortunately, these natural control agents (the wasps and the fungus) usually don't exert their influence until after alfalfa weevil larvae have caused economic damage. Nonetheless, they could have an impact on the next generation of alfalfa weevils (the one that begins when adults lay eggs this fall and next spring), so their presence is worthy of note.--Kevin Steffey