Although the recent cold temperatures have slowed down development of alfalfa weevils, the preceding warmer temperatures had them chugging along just fine. An onset of warmer temperatures will speed up their development again. Thus far we have received no confirmed reports of alfalfa weevil activity, although accumulated degree-days (base 48°F) from January 1 through April 7 suggest that larvae probably have hatched in alfalfa fields in the southern one-fourth to one-third of Illinois (Figure 1). We expect larval hatch when about 200 degree-days accumulate. As we have stated in the past, check alfalfa fields on south-facing slopes or lighter soils first. These fields tend to warm up more quickly in the spring, so the presence of alfalfa weevil larvae in these fields might provide an early warning for other fields in the vicinity.
Figure 2 shows projected degree-day accumulations (base 48°F) from January 1 through April 21. By the third week in April, we can expect larval hatch in fields throughout much of Illinois. Remember, these projections are based on historical weather data. Unusually warm or cold temperatures could speed up or slow down weevil development, respectively. If the temperatures warm up and stay warm, alfalfa weevil activity could be in full swing in a couple of weeks in the southern half of the state.
Keep watching, and let us know what you find.--Kevin Steffey