Soybean Aphids Populations Rising in Some Fields

More soybean fields across the province are being discovered to have at least some soybean aphids in them. Though in most fields so far populations have been at about 5 to 10 aphids per plant, a few fields have seen aphids increase from 30 aphids per plant to 100 aphids or more per plant. Scouting is recommended every 3 to 4 days in fields not planted with CruiserMaxx, to determine if the aphids are present and whether natural enemies are starting to colonize and lower the aphid populations. This week in particular we are starting to see ladybeetle adults and syrphid fly larvae move into soybean fields to feed on the aphids. And once the winter wheat crop starts to dry down for harvest, any natural enemies feeding on cereal aphids in that crop, should start migrating into the neighbouring soybean fields to join their predator and parasitoid friends. Cruiser seed treatment will be effective on these early season aphid populations up to about 40 days after planting, though these larger and more widespread early season aphid infestations are not the norm for Ontario. In most years these natural enemies can take down this first batch of soybean aphids that are attempting to colonize but I can't promise they will keep up this early in the game with such a widespread distribution of these aphids.

We do not have thresholds for the vegetative stage of soybeans. Spraying at this time can easily kill off the natural enemies that are moving in and can keep them from establishing again in those fields for several weeks, while the aphids can build up their numbers in a matter of days. We experienced that in some trials set up in 2007. Fields that we sprayed knocked the aphid numbers down for only a few days and were back up above 300 per plant in 5 to 10 days, while the natural enemies struggled to get going again and took weeks to a month to make a comeback. And I can't promise a yield response to spraying this early, when the crop has a long ways to go, with many other factors including weather at play before harvest.

The good thing is that with this heat, the soybeans are going to grow quickly and reach R1 (first flowering) soon so we will then have the more rigorously tested and proven R stage thresholds to rely on. That threshold again for those who may be a little rusty on their soybean aphid information is: 250 aphids per plant and increasing, on 80% of the plants in the field during the R1 to R5 stage soybeans.

We will keep you up to date on the aphid situation in a number of ways. More CropPest articles will be coming out next week. We also upload our weekly aphid scouting results (displayed as maps) onto the US IPM PIPE Network at http://sba.ipmpipe.org and onto the Ontario Soybean Growers website at http://www.soybean.on.ca/aphids.php . And I will continue to put up to date information on my blog at www.bautebugblog.com

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