An upside to the extraordinary cold temperatures South Dakota experienced this winter is the negative impact the extreme temperatures may have had on egg mortality of corn rootworms (CRW).
"Combined with lower than expected CRW numbers last year and increased mortality of overwintering eggs caused by long spells of cold temperatures this winter, we may see even lower numbers of CRW this year," said Ada Szczepaniec, SDSU Extension Entomology Specialist.
Szczepaniec explained that the lifecycle of 2014 CRW began when mated females laid their eggs in the soil last summer. Laid deep in the soil the eggs depend upon residue and snow cover to insulate them from cold. Western CRW lay eggs at 4-inches or deeper in the soil, while northern CRW lay their eggs in upper 4-inches of the soil. After spending winter in an egg stage, CRW will hatch in late spring/early summer. Larvae feed on the roots, complete their development in the soil, and start emerging as adults from about mid-July in our area. These insects are well adapted to our winters, and can do reasonably well if winter temperatures hover around normal lows.
"What happened to them this winter? Because we had long periods of below normal temperatures, it is possible that a lot of eggs did not make it through the winter," she said. "This is especially likely in areas with little to no snow cover."
How do these predictions affect management decisions?
Because winter mortality is likely to be higher than normal Szczepaniec said extreme measures, such as combining insecticide treatments with Bt corn with anti-CRW traits, may not be necessary.
"Specific factors such as field history and previous levels of CRW populations will affect management strategies in a particular field. A good management practice is to use crop rotation, and to avoid using the same anti-CRW Bt corn for more than three years," she said.
Managing CRW will always be more challenging in a continuous corn production because nothing controls CRW as powerfully as rotating to a non-host crop such as soybean.