Corn rootworm larvae usually begin hatching from overwintering eggs at about this time of year throughout central Illinois. We can use accumulations of degree-days to predict larval hatch, or we can look for the appearance of fireflies, which often coincides with rootworm larval hatch. Kevin Black, with Growmark, reported observations of fireflies in the Bloomington-Normal area as early as May 5. Based on accumulated degree-days (base 52 deg F), at the 4-inch level in soil (Figure 1), the occurrence of fireflies and rootworm larval hatch may not occur at the same time this year. However, it's always wise to keep an open mind--rootworms have fooled us before.|
Approximately 380 to 426 accumulated degree-days are required for 50% of the larvae to hatch. Figure 1 suggests that lots of rootworm larvae could have hatched throughout the southern one-quarter of the state. And larvae probably are hatching throughout central Illinois. So, how will all of the wet weather affect rootworm larvae?
Many people believe that rootworm eggs are killed by flooding. However, rootworm eggs are pretty tough, and there is no evidence to indicate that excess water has any effect on their ability to survive. First instars are another story. Among the most significant population-regulating factors among rootworms are (1) water-saturated soils and (2) lack of a food source. As soon as first instars hatch, they begin seeking a host (primarily corn). If the soil into which the larvae hatch is flooded or saturated with water, many of the larvae will either drown or be unable to locate their host. Larvae are attracted to CO2 from growing roots; water inhibits the rootworms' ability to sense CO2. Obviously if the host is not present (i.e., corn has not been planted), the larvae will starve to death.
The small size of the corn root systems in a lot of fields also may have an impact on survival of rootworm larvae. When (if) the larvae find corn, they tunnel into the roots to begin feeding. However, a small root system will support only so many larvae. It's possible that a lot of rootworm larvae could "gang up" on small root systems and wreak some havoc, but it's more likely that small root systems will not support large numbers of rootworm larvae. Consequently, late planting probably will result in considerable rootworm mortality in some areas of Illinois (and elsewhere in the Midwest) this year. Only time will tell. As reports of observations of rootworm larvae come in, we'll keep you posted.--Kevin Steffey