Soybean Aphids Not as
Abundant as Expected

Reports from my US colleagues came in last week that the aphid populations on buckthorn in the US are much lower than expected, given the high number of eggs that were there in the fall. There could be a few reasons for this decline. Research done at the University of Guelph (Welsman et al. 2007) has found that there can be up to a 70% decline in soybean aphid egg numbers over the winter, even before they hatch, possibly due to predation, exposure to extreme winter temperatures, heavy rain that may knock off eggs not placed along the seam of the buds or degradation of unfertilized eggs. The US researchers conducting the buckthorn survey last week speculate that the cause of the aphid crash this spring was the winter storm that went through the Midwest in April just after the nymphs emerged from the eggs. The nymphs are very vulnerable to cold temperatures and would not have been able to deal with the cold conditions.

Here in Ontario, we had a much lower number of soybean aphid eggs going into the winter compared with the US and compared to our higher fall counts in 2004. I have yet to find aphid colonies at my buckthorn sites, but colonies have been found at a few other locations.

So what does this all mean? In terms of recommendations for this summer, nothing has changed. Though fewer aphids starting out on buckthorn may reduce the likelihood of very early season infestations in June, it does not guarantee that we won't get infestations in July and August that reach threshold. Unfortunately Ontario is geographically positioned to receive aphids from any location in the Midwest anytime in the season. What determines if they will reach threshold when they arrive are the weather conditions at the time, the stage and condition of the crop and whether the natural enemies are up for the task of eating them all. So far, our natural enemies have done a great job in keeping initial infestations down, as we have not seen aphids reach thresholds in the month of June when they first migrate from buckthorn.

We will be scouting many fields across Ontario again this year and will report our results on the USDA rust/aphid PIPE website (www.sbrusa.net) as well as the Ontario Soybean Growers website (www.soybean.on.ca). And of course, we will continue to provide updates and management recommendations in future CropPest articles.

Related Links

• Ontario Soybean Growers

• United States Department of Agriculture

| Top of Page |