Western corn rootworm egg survival and winter
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.
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