It was a cold day in Iowa, but did the soybean aphids freeze out?

February was cold--really cold. The Climatology Bureau of the Iowa Department of Agriculture and Land Stewardship reports that "February began very cold with below normal temperatures recorded on 16 of the first 18 days of the month. Decorah reported the lowest official temperature of the month with a -26º reading on the morning of the 5th while Estherville had the lowest wind chill reading with -36º on the morning of the 15th. Temperatures averaged 15.8º or 8.4º below normal. This ranks as the 14th coldest and 15th wettest February among 135 years of state records. This was also Iowa's coldest month since December 2000."

Soybean aphids overwinter in the egg stage on common buckthorn. The eggs are laid near leaf buds on young plants and are therefore exposed to cold winter air unless they are covered by snow. So what does this cold February mean for the survival of soybean aphid eggs during the winter? Should we expect these really cold temperatures to freeze and kill some or all of our soybean aphid eggs, thereby reducing the potential pest problem next summer? We believe the answers to these questions can be found in recently published research on the overwintering survival of soybean aphids.

Two soybean aphid eggs laid next to the bud scales of buckthorn. (Marlin E. Rice)Two soybean aphid eggs laid next to the bud scales of buckthorn. (Marlin E. Rice)

University of Minnesota entomologists David Ragsdale, Brian McCornack, and their research team investigated soybean aphid survival during simulated winter conditions. They found that the capacity for soybean aphid eggs to survive may depend on their ability to supercool and tolerate winter temperatures. When insects supercool, they lower the freezing point of their body fluids to prevent the formation of ice crystals. This behavior protects them against low temperatures. However, even aphids have their limits as to what they can tolerate. The temperature at which ice crystals will form in an insect and cause instantaneous death is known as the supercooling point.

The Minnesota researchers found that the average supercooling point of soybean aphid eggs was a very cold -29 °F (that's minus 29!). This is the temperature at which eggs would rupture from ice crystal formation and die. Then they determined the annual probability that winter temperatures would equal or fall below the average supercooling point. The map shows a 10 to 25 percent probability that extreme low air temperatures are likely to reach or exceed the mean supercooling point for soybean aphid eggs in the northern one or two tiers of Iowa counties. There is one small area in southwestern Iowa near Atlantic that suggests temperatures may get cold enough to kill soybean aphid eggs. Because this area is not topographically unique with respect to surrounding counties, this predicted "cold spot" may be a function of incorrectly reported weather station data and can probably be ignored.

So how do these research findings help us interpret what may have happened to soybean aphids in Iowa? February temperatures in northeastern Iowa approached the supercooling point of -29 °F, and it is certainly possible that some areas were this cold or colder.

If cold winter temperatures did have an impact on overwintering survival of soybean aphid eggs, then it would probably be in extreme northeastern Iowa this winter. For most of the state of Iowa, in spite of the cold February, the soybean aphid should have experienced widespread success at overwintering during the winter of 2006-2007.

Probability of air temperatures reaching or exceeding the mean supercooling point for soybean aphids. (Map provided by Brian McCornack and David Ragsdale, University of Minnesota.)Probability of air temperatures reaching or exceeding the mean supercooling point for soybean aphids. (Map provided by Brian McCornack and David Ragsdale, University of Minnesota.)

This research was published in the journal Environmental Entomology. The complete article is Physiological Constraints on the Overwintering Potential of the Soybean Aphid (Homoptera: Aphididae).

Marlin E. Rice is a professor of entomology with extension and research responsibilities in field and forage crops. Matt O'Neal is an assistant professor of entomology with research and extension responsibilities in field crops.

This article originally appeared on page 82 of the IC-498 (3) -- March 26, 2007 issue.

Updated 03/26/2007 - 2:57pm