Soybean
Aphid Populations
Increased Dramatically Last Week
Those who have been watching their fields closely every week noticed a big
change in the aphid activity early last week. Aphid populations built up in many
fields and reached threshold. Other fields that had barely any aphids in them
before were found to have at or above threshold numbers within the same week.
After some consultation with colleagues in neighbouring states and provinces,
we suspect a flush of winged adults were brought from the Midwest via the storm
fronts last week, as Ohio, Michigan, Ontario and Quebec all experienced this sudden
appearance of more aphids. The heat didn't help us, as it was too hot to spray
but it didn't entirely help the aphids either. The aphids don't like it that hot
and their reproduction is slowed down. But now that we are back to cooler nights
and not as hot days, aphids have a chance to build up again if their natural enemies
can't keep up. So go and check your fields again.
How much longer do we
have to worry about them? Once your crop is into the R6 stage of soybeans, you
need more aphids per plant before control is necessary. Plants are in the R6 stage,
also known as the "Full Seed" stage when the seeds in the top pods of
the plant are large and filling the entire pod cavity. Though we do not have thresholds
set yet for the R6 stage, based on past spray trials, you need at least 1000 aphids
per plant to see a yield response. And once the soybeans are beyond the R6 stage,
economic return from an insecticide application is not likely.
Spraying
them has been challenging for some while others have had good success. Spray coverage
has been the biggest problem. Use high water volumes..20 gals..30 gals preferred.
Set your boom and nozzles as though the target pest is at the middle of the plants,
not at the tops. And keep in mind that Matador is heat sensitive so you do not
want to be spraying it in temperatures higher than 27 degrees C. Hold off and
spray it in the evenings or very early mornings when the temperatures are cooler.
The winged adults can also land in edible bean fields. Though they can
vector a virus into the plants, the aphids can not survive beyond the 1st instar
nymphs on edible beans. Spraying them with insecticides will not reduce the incidence
of virus vectoring..if the aphids are on the plant, they've already plugged in
and would have transmitted the virus if they were carrying it. But they do have
to pick it up first from an unhealthy plant so it really depends on where that
aphid has been as to whether it even has the virus in it to vector.
And
finally, the rains that went though should help a bit. It does not knock
aphids off but it will help the plants put out new flowers and deal with the aphids
that are sucking the nutrients out of them. But if you've reached thresholds,
you still have to spray. Rain can help with the spider mite situation so if you
were going to spray for mites just before the rain came but didn't get around
to it, check the field out again before you spray to make sure the mites are still
there. With any luck, the mites will have vanished though the damage they have
done will not recover.
