Cold winter temperatures this past winter have caused a lot of people to reflect on that age-old question, "What effect is all this cold weather going to have on the bugs?"
A corn rootworm survival study was conducted by Mike Gray and Jon Tollefson at Iowa State University during the winters of 1983-1985 may help to answer part of this question. The study examined the influence of different tillage methods and winter weather on rootworm egg survival. During the winter of 1983-1984, December through February were characterized by below normal snowfall and temperatures. The mean temperature for December was 9°F, or approximately 15°F colder than average. These conditions were very similar to December 1989 conditions when the mean temperature for Iowa was 14.2°F (the fourth coldest on record), or 10.1°F colder than usual. Central Iowa also had either no snow cover or less than an inch in spots during December 1989. The following winter (1984-1985), precipitation and temperatures were near average.
The study indicated that corn rootworm egg survival was affected by tillage during a severe winter. Egg survivorship was reduced by about 63-75 percent in plots that had been para-plowed or mold-board plowed during the fall. There was no significant reduction in egg survivorship in plots that were no tilled or chisel plowed. The influence of tillage type on corn rootworm egg populations during moderate winters (1984-1985) seems to be negligible; there were no significant differences among the four tillage methods.
If a rootworm population reduction was to occur, then three conditions would have to be met:
- severe cold
- absence of an insulating snow cover, and
- para-plow or mold-board tillage in the field.
Now, to answer the original question--I would anticipate that fields that were plowed (which is probably very few fields) will have greatly reduced populations of corn rootworms, whereas fields with minimum tillage or no tillage should not expect a significant reduction in rootworms. The practical application of this information is that if you scouted your fields for adult beetles last summer and arrived at counts that were borderline regarding the need for a soil insecticide in 2003, then this research may be that added piece of information you needed to refine or adjust your management decision this spring.
This article originally appeared on page 29 of the IC-490 (4) -- April 14, 2003 issue.