Where are corn rootworm eggs right now?

I am "all done" with winter and I'm probably not alone. I'm looking forward to buds bursting, better vegetables and sandals. Insect-related questions coming my way have all been focused on cold winter temperatures that could be killing pests. The main pests of concern are corn rootworms (really, what else is there to talk about in Iowa?). I had to do a little digging in old literature, but found some interesting research on survivorship of corn rootworm eggs (the overwintering life stage).

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. 

References:

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.

How do insects survive the winter?

We sure have experienced very cold air temperatures and even colder windchills this winter. I’ve been asked several times “how cold does it have to get to kill insects?” Perhaps it is important to understand why cold temperatures kill insects. Insects are unlike mammals and birds because they must generate their own heat (called ectotherms). Insects die with they are exposed to temperatures below the melting point of their body fluids. If they want to survive our cold Iowa winters, they must avoid freezing or tolerate freezing. Over time, insects have developed several strategies to survive cold temperatures and none of them involve wearing fleece.

Some insects just move into human structures in the fall and keep warm until spring. Think about boxelder bugs and multicolored Asian ladybeetles aggregating on houses every year. Even if they are protected inside, they will likely die before spring if they don’t get food and water. Some insects also migrate to warmer climates to avoid freezing. A classic example is monarch butterflies moving from Canada to Mexico every year. Sounds pretty good about now!

Multicolored Asian ladybeetles mass on structures every fall. 

Photos by Robert Koch. 

But most of our persistent insects in Iowa have to overwinter outside, and two strategies have evolved to survive extreme conditions: freeze avoidance and freeze tolerance. Freeze-avoidant insects keep body fluids liquid and freeze-tolerant insects can handle the formation of internal ice. Wait a minute, what? I know…either strategy seems fantastical.

The main strategy for insects living in the northern hemisphere, where we have cold temperature for long period of time, is freeze avoidance. Freeze avoidance can be achieved a few ways. Sometimes insects enter a “dry” hibernation by getting rid of all the food and water in their body. That way, ice can’t form inside the body and kill them. Water needs food or dust particles in order to crystallize; water can cool down to -42C without freezing if particles are absent. Other insects have a super waxy coating on the exoskeleton that protects against ice formation on the body. Amazingly, some freeze-avoidant insects also produce cryoprotectants, such as glycerol and sugar, to reduce the lethal freezing temperature of the body. So yes, cryoprotectants act like the antifreeze in your car. I can’t make this stuff up! 

Most insects living in the southern hemisphere, where the climate is more variable, employ freeze tolerance. These insects can stand ice formation in the body. Some will actually initiate freezing their body at relatively high temperatures in order to prepare for a longer hibernation. An example of a freeze-tolerant insect is the woolly bear. [Sidenote: Several winter festivals celebrate the woolly bear kinda like Groundhog's Day.]

Woolly bears overwinter as cold-hardy caterpillars. 

Photo by IronChris, Wiki. 

No matter the overwintering strategy, all insects will eventually die if it gets cold enough. However, the lower lethal temperature is different for each species. Insects can overwinter in any life stage - some are belowground and some aboveground. It gets complicated quickly, and so I will save that for another time. 

Find out more about how insects survive the winter from this Wiki page.