Western corn rootworm egg survival and winter
The following is a direct quote from Eric’s thesis (page 42): “The results of this study indicate that tillage environment and location in the soil profile can have profound effect on western corn rootworm egg survival if the soil is cold enough for negative degree-day accumulation to exceed approximately 100.”
Many locations in the Corn Belt experienced below zero degree Fahrenheit temperatures for extended periods of time this past winter. Zero degree Fahrenheit is equal to -17.78 Celsius. If this temperature was sustained for approximately 1 week, we could infer that significant mortality to western corn rootworm eggs could begin to take place. Eggs that are buried more deeply in the soil profile, especially with snow cover, would be less likely to see these extreme temperatures.
However, eggs near the surface of the soil in fields that lack snow cover or much crop residue will be far less likely to survive the winter temperatures this past season. Research that I conducted as a graduate student at Iowa State University revealed that western corn rootworm eggs can be found at various levels in the soil profile. In 1985, in a study near Ames, Iowa, I found 21.2% of western corn rootworm eggs occurred in the upper 4 inches of the soil profile. Approximately 44.5% and 34.4% of the eggs were found from 4 to 8 inches and 8 to 12 inches in the soil profile, respectively. Eggs that are laid as deep as 12 inches, or deeper, will be less affected by the extreme cold this past winter, especially if they are in fields with plentiful crop residue and snow cover.
Because the rotation resistant (variant) western corn rootworm lays at least a portion of its eggs in the soil of soybean fields, fields with less residue cover as compared with cornfields, egg mortality may be greater in soybean fields. Soybean residue is also less likely to trap and retain snow cover, perhaps leading to greater egg mortality.
Many questions will continue to linger regarding the influence of this winter on a wide spectrum of insect pests, especially those that overwinter in the Midwest.
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