Western corn rootworm eggs. Photo by Purdue Extension.
In lab tests, Gustin (1981) showed significant egg mortality at soil depths of 3-6 inches with four constant weeks at -7.5 degrees. But I wonder how often those harsh temperatures might happen in the field? Is that where the majority of eggs are even deposited? And even if those temperatures and depths are realistic, rootworms have survived over 50 years in Iowa and must be relatively cold hardy.
One method for measuring "how cold does it have to be before it kills rootworm eggs" is finding the supercooling point. The supercooling point is defined as the process of lowering the body temperature below its freezing point without becoming solid. I briefly talked about how insects might do this in a previous article. This lower lethal temperature would be species specific. Western and northern corn rootworm both had a supercooling point of -27 degrees, but most eggs did not hatch if they experience -21.5 degrees (Ellsbury and Lee 2004).
Gray and Tollefson (1988) examined the differences of four tillage regimes and corn rootworm egg mortality. In the end, egg density wasn't different in fields that experienced no-till, chisel plowing or moldboard plowing. The timing of cultivation (fall or spring) did not matter either. They also looked at egg deposition, or how deep females might be laying eggs. There was a surprising difference between northern corn rootworm and western corn rootworm. Northern corn rootworm eggs tend to be located closer to the soil surface, while over a third of western corn rootworm eggs were found in the lower 4 inches of the soil. Insects that overwinter deeper in the soil may be further insulated from cold air temperatures.
Some other things that influence corn rootworm egg (and all other soil-dwelling insects) survivorship over the winter include plant residue, snow cover, and soil moisture (Godfrey et al. 1995). Northern corn rootworm tend to handle dry conditions better than the westerns (Ellsbury and Lee 2004).
High residue in cornfields can protect soil-dwelling insects from extreme cold temperatures. Photo by Lori Abendroth, Iowa State University.
The bottom line is this cold winter probably had a negative impact on corn rootworm eggs. I expect there to be higher egg mortality compared to normal winter temperatures. But egg survival is increased if they are deeper in the soil and if the field had snow cover and residue. Egg sampling is difficult and tedious, and not recommended. But I still encourage root injury assessments this summer; that will be the true test of egg mortality in corn.
Ellsbury, M. M. and R. E. Lee Jr. 2004. Supercooling and cold hardiness in eggs of western and northern corn rootworms. Entomologia Experimentalis et Applicata 111: 159-163.
Godfrey, L. D., L. J. Meinke, R. J. Wright, and G. L. Hein. 1995. Environmental adn edaphic effects on the western corn rootworm overwintering egg survival. Journal of Economic Entomology 88: 1445-1454.
Gray, M. E. and J. J. Tollefson. 1988. Influence of tillage systems on egg populations of western and northern corn rootworms. Journal of the Kansas Entomological Society 61: 186-194.
Gustin, R. D. 1981. Soil temperature environment of overwintering western corn rootworm eggs. Environmental Entomology 10: 483-487.