Soybean Aphids or Bean Leaf Beetles - Which is Your Pest?

Depending on your location, soybean aphids may barely be present in your fields. Other locations, particularly those in Perth, Wellington and Dufferin Counties, have been dealing with them for over a month now, with some fields requiring more than one application of insecticide. We really only have 2- 3 weeks left for the aphids to start doing something to reach thresholds before the crop is in the later R5, early R6 stages. So cross your fingers and hope that the natural enemies keep up and let's hope that a mass immigration of winged adults does not take place before then.

We have also been fortunate that spider mites have not been as big of a problem, given how dry it has been in many locations. Why they haven't been is still a bit of a guess. But there is still some risk of them causing problems in some fields so continue to monitor for signs of activity until the soybeans are in the R6 stage.

But the next concern is bean leaf beetle. The first generation started up a few weeks ago and with the lack of rain, some fields have not been putting out new leaves quickly to compensate for defoliation. A small number of fields had to be sprayed in the last week as they did reach the 15% defoliation threshold during the R1-R4 stage. The concern is that there is still one more generation of adults to come. This second generation typically starts up in August and starts to feed on the pods. Given the higher number of first generation adults present and their further geographical range this year, more fields are at risk before the season is finished. For IP, food grade and seed fields, if 10% of the pods on the plants have feeding injury AND the beetles are still active in the field, a spray is warranted. Keep in mind however, the closer you get to harvest you need to consider days to harvest intervals.

Some are asking… "If I spray for bean leaf beetles, does it kill off the natural enemies that are keeping the aphid populations down so it causes them to become a problem too?" This is a risk, but the closer we get to the R6 stage, the less likely the aphids will be able to build up to threshold levels. Also, the fewer aphids you have at the time of the spray, the less there are to rebound. In fields with very low aphid populations, the natural enemies have done their job already, and most likely have moved on to find more aphids to feed on in more populated fields. Best recommendation is to deal with the pest at the time, following the thresholds and watch closely to make sure another pest does not take its place before the season is done.