No. 5 Article 4/April 22, 2005

Soybean Aphids: Impact of Soil Potassium and Overwintering Temperatures on Population Dynamics

Since the first observations of soybean aphids in the United States during the summer of 2000, we have continued to learn a great deal about the biology and ecology of this insect pest. Two recent publications shed additional light on this important insect with respect to the influence of two separate abiotic factors on the population dynamics of this aphid species.

Initial field observations by entomologists in Wisconsin during the 2000 growing season suggested that a potential connection might exist between large infestations of soybean aphids in fields with a potassium (K) deficiency. Producers also began to raise concerns that soybean fields with lower potassium levels might actually lead to increased levels of soybean aphids.

In a recent Journal of Economic Entomology article (vol. 98, no. 1, February 2005), "Effect of Soil Potassium Availability on Soybean Aphid Population Dynamics and Soybean Yield," the authors reported the results of a 2-year field experiment that was designed to answer questions related to a potential connection between potassium deficiency and soybean aphid infestations. The authors indicated that other aphid species have been shown to flourish on plants that were grown in soils with lower levels of potassium, such as bird-cherry oat aphids that infest barley. In laboratory studies, the University of Wisconsin entomologists similarly found that soybean aphids displayed a greater net reproductive rate on soybean leaves that were deficient in potassium as compared with nondeficient leaves.

However, the field studies conducted over two growing seasons revealed no significant influence of potassium on soybean aphid densities. The authors concluded that other factors are more likely to contribute to economic infestations in producers' soybean fields. We commend the work of the entomologists who have addressed this important question raised by producers.

Another issue that continues to surface regarding soybean aphids is the influence of winter on their survival. Until recently, very little was known regarding the ability of soybean aphids to survive winter conditions in the north-central region of the United States. A recently published paper (Environmental Entomology, vol. 34, no. 2, April 2005) by entomologists at the University of Minnesota provides excellent information relative to this topic. The specific objective of their research was to determine the supercooling point of the different life stages of soybean aphids, especially eggs, considered the most resilient life stage of this insect. In addition to soybean aphids, other insect species utilize supercooling to survive harsh winter conditions. Through this process, the freezing point of liquids in an insect's body is lowered to avoid the creation of ice crystals--like using antifreeze in your car's radiator during the winter months.

The Minnesota entomologists reported that of the various soybean aphid stages, the eggs had the lowest supercooling point (-34°C). The winged asexual females (gynoparae) and wingless sexual females (oviparae) that deposit the overwintering eggs had the highest supercooling point (-15°C). Once the supercooling point is reached, spontaneous freezing takes place and death ensues quickly. Even though these supercooling points are quite low, as temperatures dip below 0°C aphid mortality begins to increase, thus limiting potential infestations the following season.

The authors indicated that supercooling temperatures leading to aphid mortality are most apt to occur in northern Minnesota, northern Wisconsin, and the upper peninsula of Michigan. They also point out that because soybean aphid eggs are laid along the leaf buds of buckthorn (overwintering host), the amount of snow cover typically has little to do with overwintering success. In essence, the eggs are not insulated and are exposed to raw air temperatures.

Regarding overwintering hosts, David Voegtlin (Illinois Natural History Survey) and fellow investigators reported in 2004 that two buckthorn species may serve as satisfactory hosts: Rhamnus cathartica L. (common buckthorn) and Rhamnus alnifolia L'Héritier (alderleaf buckthorn). Nine other buckthorn species were unable to serve as overwintering hosts for soybean aphids. Winter temperatures in states such as Illinois are likely to be more conducive for overwintering survival of soybean aphids and may serve as sources from which migration occurs to other areas of the region. Availability of winter hosts also will influence overwintering success. Our thanks go to these Minnesota entomologists for increasing our understanding of this very interesting aspect of the soybean aphid life cycle.--Mike Gray

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