Did this winter kill all the corn rootworm eggs?

Insect mortality happens every winter, even under ideal conditions. However, this winter was the 9th coldest in 121 years and I’ve been getting questions about how the harsh conditions may have impacted overwintering corn rootworm eggs. Maybe we don’t need to care rootworms this year if all the eggs froze to death? We’re probably not so lucky. Many factors besides cold air temperatures influence successful overwintering of insects in Iowa, including our most important field crop pest.

Cold temperatures can kill rootworm eggs; temperatures below 18.5°F can be lethal to eggs (Woodson and Gustin 1993). We know the eggs are deposited into soil cracks and crevices and are somewhat protected to air temperatures. Mike Gray (University of Illinois) provided a nice blog summary of research involving soil temperatures, egg depth and survivorship. To see how cold it really was this winter, I was able to extract a graph showing soil temperatures at three depths near Ames (1 November 2013 – 1 May 2014). There were several dates where the temperature was cold enough to kill eggs in the top 12ʺ of the soil in central Iowa and likely other places throughout the state. 

Soil temperature data courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.

Egg deposition is highly variable within and between species, but in general eggs have a better chance of surviving if they are placed deeper in the soil. Western corn rootworms tend to lay most eggs 4-8ʺ below the soil surface, compared to northern corn rootworms that tend to lay most eggs in the top 4ʺ (Gray and Tollefson 1988). So the odds are in favor of more westerns surviving the winter just because of where females put the eggs.

Crop residue and snow cover can significantly improve egg survivorship (Godfrey et al. 1995). However, just how much residue/snow cover is needed is not fully understood. Tillage and tillage timing does not significantly reduce egg populations (Gray and Tollefson 1988). Soil texture did not appear to influence egg mortality in a Nebraska study (Godfrey et al. 1995).

Saturated soils do not kill corn rootworm eggs, but they can negatively impact larvae. When soil is saturated, oxygen can be limited and cause suffocation. About 50% of third instar western corn rootworm larvae die in saturated soils after 24 hours (77°F); survivorship is increased in saturated soil with decreasing temperatures (Hoback et al. 2002). So later this summer, saturated soils could reduce larval populations but don’t count on it for eggs.

The bottom line is all these factors had some impact on overwintering egg mortality. There was probably more egg death this winter compared to more normal winter temperatures. I do think some corn rootworm eggs survived in Iowa. In a recent ICM News article, I estimated corn rootworm egg hatch is happening now if they survived. This prediction is solely based on growing degree days in the soil. Research has demonstrated about 50% of the eggs hatch when they accumulate 684-767 degrees (base 52°F, soil). It makes sense that egg hatch starts in southern Iowa every year, with the average hatching date for the state around 6 June. Predicted egg hatch is important because larvae will feed on corn roots for about 3 weeks. I encourage everyone to assess corn root injury as larvae finish feeding. Remember, one node of injured roots means a 15% yield loss (Tinsley et al. 2012). It's called the billion dollar pest for a reason!

Map data courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.

References

Godfrey, L.D., L.J. Meinke, R.J. Wright, and G.L. Hein. 1995. Environmental and edaphic effects on 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.

Hoback, W.W., T.L. Clark, L.J. Meinke, L.G. Higley, and J.M. Scalzitti. 2002. Immersion survival differs among three Diabrotica species. Entomologia Experimentalis et Applicata 105: 29-34.

Tinsley, N.A., R.E. Estes, and M.E. Gray. 2012. Validation of a nested error component model to estimate damage caused by corn rootworm larvae. Journal of Applied Entomology 137: 161-169.

Woodson, W.D., and R.D. Gustin. 1993. Low temperature effects on hatch of western corn rootworm eggs. Journal of the Kansas Entomological Society 66: 104-107.