The winter of 2013-14 will long be remembered as one of the snowiest and coldest since the late 1970s for many midwestern states. Not surprisingly, many questions have surfaced regarding the impact of these cold temperatures on the overwintering survival of western corn rootworm eggs. Research on this topic has only sparingly occurred. In my estimation, one of the best field studies on this challenging research area, was undertaken by an Iowa State University graduate student, Eric Lawson, under the direction of Dr. Jon Tollefson in the early 1980s. Eric published his M.S. thesis in 1986 titled: Influence of tillage and depth in the soil on soil temperature and survival of overwintering western corn rootworm eggs. The study was conducted 1.5 miles south of Ames, Iowa, during the winters of 1982-83 and 1983-84. Eric buried western corn rootworm eggs in modified petri dishes that contained soil with stainless steel mesh screening that allowed movement of air and water through the dishes. The petri dishes were buried at three depths (7.5 cm, 2.95 inches; 15 cm, 5.9 inches; and 22.5 cm, 8.85 inches) during the first winter. The second winter, the dishes were buried at one additional depth (2.5 cm, 0.98 inches). During the 1982-83 winter, 240 petri dishes were buried, each containing soil and 15 western corn rootworm eggs. The following winter, 280 dishes were buried, each containing soil and 20 western corn rootworm eggs. At the conclusion of each winter, the eggs were extracted from the soil and viability determined using standard laboratory rearing procedures. The overall experimental design for this research was a split-plot randomized complete block with different tillage systems serving as the whole plots (fall moldboard plow, spring disc; fall chisel plow, spring disc; fall paraplow, spring disc; and no-till) and the different depths at which eggs were buried serving as the split plots. Thermocouples were used to measure the soil temperatures within each tillage treatment at the precise depths at which the petri dishes containing western corn rootworm eggs were buried. Throughout both winters, Eric periodically recorded the thermocouple readings.
Eric was able to analyze the overwintering survival of western corn rootworm eggs by regressing the adjusted percent hatch against negative degree-days that accumulated throughout a winter. He used a – 1 degree Celsius threshold for these calculations. This Celsius threshold is equivalent to 30.2 degrees Fahrenheit. To put the negative degree-day calculation into perspective, Eric provided an example of how 20 negative degree-days could be accumulated: 20 days at -1 Celsius, 4 days at -5 Celsius, or 2 days at – 10 Celsius. 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 0 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.