Soybean Aphid Thresholds Have
Been Reached for Some!

Lots of winged adults blew in from the U.S. on the Canada Day weekend, so even though your fields may not have had aphids in them before, they could be there now. Most fields (4 out of 5) from Windsor to Ottawa have some aphids in them and a few fields have reached thresholds this week. The pleasant (25-27oC) temperatures that we experienced last week allowed populations to build in some fields. 3-4 fields in Stratford, 1 field near St. Paul's and 1 field near Durham had over 250 aphids per plant on 90% of the plants, with the Durham field reaching 1000's per plant on nearly 90% of the plants. These fields seem to be the exception though as fields directly next to these ones and within the area still only have 1-20 plant though aphids can be found now on 50-90% of the plants in each field. These hot temperatures this week should slow their reproduction down a bit but scouting is still very important as some fields may be nearing thresholds now. Bottom line is that the fields that have reached thresholds already are so scattered and unpredictable that everyone needs to be diligent and scout each and every field in case at least one of theirs has higher numbers.

OMAFRA, partnered with the Precision Ag. Specialists of Laresco (London), are keeping track of soybean aphid populations on a provincial map (Figure 1) which can be found on the Ontario Soybean Growers website. Data coming in from our scouting efforts, including input from OMAFRA scouts, industry agronomists, scouting groups and keen ag. reps will be used to update the map on a weekly basis (every Friday) until we are far into the R6 stage of soybeans when the threat is over. Also provided on the OSG website are management guidelines and protocols on how to set up on-farm trials for soybean aphid control.

Figure 1. Current Ontario Soybean Aphid Scouting Observations as of July 6th, 2005

Though the map will give you an indication of what kind of populations are happening in your area, I do strongly recommend you keep checking your fields at least once a week. As you get closer to 200 aphids per plant, start to scout your field every 3-4 days. Ensure that your populations have reached thresholds and are increasing instead of remaining stable or even dropping down in numbers. Spraying too early, before reaching the "250 aphids per plant with increasing populations on 90% of the plants in the field" can cause more problems. Insecticide applications will kill off the beneficials like minute pirate bugs which have been faithfully helping to keep aphid numbers down so far. Without the natural enemies there, the aphid populations can build up very quickly. And they have at least another 5 weeks to do it in! Also, the applications of a pyrethroid in this hot dry weather may promote spider mite activity, as pyrethoid will again kill the natural enemies, but will not harm spider mites that may be present. This will give the spider mites free reign in your field.

If you do have to spray, keep in mind that Matador starts to break down around 27oC or higher so spray in the evenings when temperatures are cooler.
Keep alert and stay informed!

Related Links

• Ontario Soybean Growers

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