Following another week of warm temperatures, almost the entire state has accumulated 300 degree-days. Figure 5 illustrates the accumulation of degree-days (base 48°F), from January 1 to May 5, 2003. At approximately 300 degree-days, first-instar weevils can be found in the terminal leaves of the plants. Growers in the northern portion of the state should start looking for pinhole feeding in alfalfa fields. As more degree-days accumulate (see Figure 6 for projected degree-days) and larvae mature in these areas, feeding will result in the skeletonization of leaves. Much of southern Illinois should be in the midst of a second larval peak, so continue to monitor alfalfa fields!
As you scout your alfalfa fields, keep your eyes peeled for brown or discolored larvae on leaves at the top of plants. These larvae have been infected by a fungus, Zoophthora phytonomi. This fungus favors a warm and humid environment. With all the rain received over the past week, an environment may be present in which the fungus can develop and cause a dramatic decline in alfalfa weevil populations.
Alfalfa weevils may also be naturally controlled by two parasitic wasps, Bathylplectes anurus and B. curculionis. Both wasps are very small, about 1/8 inch, and deposit eggs in weevil larvae. Adult female wasps lay their eggs in weevil larvae. The egg then hatches and feeds on the weevil larva. The parasitoid larva kills the weevil as it completes its cocoon. A brown cocoon with a white band is formed by the parasitoid. The parasitoid cocoon may be found in the cocoons of the alfalfa weevil, and these parasitoid cocoons appear to "jump" several centimeters when disturbed.
Bathlyplectes cucurlionis adult and cocoon.
Alfalfa weevil populations can be kept in check by these natural control agents. Be sure to look for these as well when scouting your fields, and keep us posted on any findings.--Kelly Cook