Features Agronomy Soybeans
Soybean aphid experience in eastern Ontario

Soybean aphids were a widespread problem again in many soybean fields through
eastern Ontario in 2005. This followed 2004 when eastern Ontario was the main
location in North America where soybean aphid numbers were high enough to reduce
crop yield.

In 2004, high soybean aphid numbers precipitated field scale side-by-side trials
between growers, ag industry and extension staff. The comparisons were conducted
to determine spray efficacy, timing of application and to verify the established
threshold level of 250 aphids per plant with the number of aphids increasing.

The data generated by these plots recommends spraying time be moved past the
R4 stage of soybean development into the R5 stage of soybean development. The
average difference between spraying was five bushels per acre. There was economical
control more than 75 percent of the time when soybean aphid numbers were above
250 per plant. The area had plenty of rainfall and limited plant stress from
weather and the response to aphid control was better than anticipated.

In 2005, the soybean aphid situation was different than it was in 2004. Soybean
aphids were present in almost all soybean fields in early July, but at very
low and scattered numbers. The numbers gradually increased through the month
and exploded in many fields after the long weekend in early August. Aphid numbers
increased because of winged movement to other fields and rapid multiplication.
Infestations continued to increase in many fields until about mid-August and
then in some fields where threshold numbers had not yet been reached, aphid
numbers began to decline.

By mid-August, aphids moved off stems and new plant growth and were distributed
throughout the crop canopy. These were very small, white aphids; not the regular
size 'Mountain Dew' coloured version. These 'white dwarfs' may have resulted
from a change in one or more factors such as hot temperatures, higher humidity,
shorter day-length, advancing soybean maturity, or changing predator populations.

There was less yield response on some fields sprayed after August 10 as aphid
feeding had already stressed the crop. The dry, hot weather that persisted through
August kept soybeans from recovering, even soybean fields that were at the late
R4 stage of development. In some fields, spider mites further affected the soybean
crop.

The aphid outbreak again in 2005 provided an opportunity for co- operative
side-by-side trials to evaluate the impact of spraying. More trials were conducted
across eastern Ontario than in 2004, according to Gilles Quesnel, OMAFRA integrated
pest management lead. Across more than 25 comparisons, the average yield response
was five bushels per acre. The largest yield response was in situations where
soybean aphid numbers increased to at least 750 per plant when the soybean growth
stage was at R4 or early R5.

I had considered the threshold levels for spraying to be quite conservative
until witnessing how fast aphid numbers climbed in early August. It became quite
clear that if there was a base number of more than 250 aphids per plant, then
there was an underlying potential for a big explosion in numbers. The threshold
of 250 aphids per plant provides some built-in lead time to get the spraying
done before aphid numbers escalate. This did not happen in all fields, but it
emphasizes the need to scout for numbers and re-scout to monitor the aphid numbers.
In most fields, aphid numbers climbed but not in all fields.

Managing soybean aphids requires diligence and scouting the field to monitor
soybean development stage and aphid numbers. The key time period coinciding
with the most critical control requirements is mid-July to mid-August. Like
other insects, activity and numbers are unpredictable. Plant sucking insects
like soybean aphids and leafhoppers do not seem to garner the attention they
should. For example, at one time this summer we had more growers concerned about
flea beetle feeding than soybean aphids because they were putting holes in the
soybean leaf even though the yield impact was minimal.

Spraying results were quite acceptable from our experience in both years. Hot
weather in 2005 was less than ideal for the application of Matador, yet in most
situations aphid control was good.

Our experience for both years was that soybean aphids tended to infest thick,
lush canopies before thinner stands or wide row fields. This is another reason
not to over-plant soybeans. Thinner stands are also easier to walk through,
making scouting more effective and timely.

The future may hold some genetic control. Until then, we need to stay abreast
of soybean aphid movement every summer.

Paul Sullivan is an independent crop consultant,
operating Sullivan Agro at Kinburn, Ontario. Certified crop advisors across
Ontario assist growers with crop management decisions. Since 1996, individuals
from most growing regions in Ontario have entered this intensive program.
They maintain their designation with continuing education credits by attending
courses and workshops. Look for the CCA emblem!