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How to scout for and manage soybean aphids

Soybean aphid infestations have been sporadic since they showed up in high numbers in Manitoba in 2006, says John Gavloski, provincial entomologist with Manitoba Agriculture, Food and Rural Initiatives at Carman, Manitoba. He cautions, though, that growers need to stay on top of scouting to ensure they keep tabs on the damaging pest.

March 18, 2010  By Bruce Barker

Soybean aphid infestations have been sporadic since they showed up in
high numbers in Manitoba in 2006, says John Gavloski, provincial
entomologist with Manitoba Agriculture, Food and Rural Initiatives at
Carman, Manitoba. He cautions, though, that growers need to stay on top
of scouting to ensure they keep tabs on the damaging pest.

Scouting is an important activity for aphid management. (Photo by Bruce Barker)

Soybean aphids cause damage by sucking sap from the plants. Infested
leaves may wilt or curl when infestations are large. Other symptoms
include plant stunting, reduced pod and seed count, and yellowing of
leaves. “The economic injury level (where expected yield loss equals
the cost of control) for aphids on soybeans is actually about 670
aphids per plant on average, but you don’t want to wait that long
because populations can increase quickly under the right conditions. If
natural enemies are not present at levels high enough to provide good
regulation of aphid numbers, those numbers can increase quite rapidly.
They can double their numbers in about seven days if conditions are
favourable,” says Gavloski. 


Because there is often some lag time between when a field is scouted
and when an insecticide can be applied, economic thresholds (the level
at which control is recommended) are often set lower than the economic
injury level, so aphid numbers do not have time to go above the
economic injury level before an insecticide is applied. Gavloski
explains that in soybeans, the economic threshold for soybean aphid has
been set at 250 aphids per plant “and the population is increasing.”
The reason “and the population is increasing” has been included in the
economic threshold is that 250 is not the damaging level of aphids, but
means the field is in danger of reaching damaging levels and needs to
be watched carefully.

If natural enemy populations progress to the point where they are able
to keep the soybean aphids at or under 250 aphids per plant, then
purchasing and applying the insecticide would cost more than the damage
that the aphids would have caused. “When scouting for soybean aphids,
keep track of whether the population seems to be growing, or is
stabilizing below a level where control would be economical,” advises

The Canadian Soybean Aphid Working Group has a Soybean Aphid Scouting Card to help estimate numbers. (Photo courtesy of John Gavloski, MAFRI)


In addition to threshold numbers, research has also shown that economic
yield benefits from controlling aphids are also related to crop stage.
When soybeans are in the early reproductive stage (when there is one
open flower on a plant) to R5 (beginning seed, when there is a tiny
bump that can be felt when running a thumb or finger over the top pods)
controlling populations above the economic threshold would be
economical. Once the crop is in the R6 stage (full seed, when the top
pods have seeds in them that are swollen and filling the entire pod
cavity), aphid numbers need to be much higher to see a yield response.
“Spray if the economic threshold is reached in the R1 to R5 stage. Do
not spray before flowering,” says Gavloski. 

To help estimate soybean aphid numbers on a plant, Gavloski refers
growers to a Soybean Aphid Scouting Card put out by the Canadian
Soybean Aphid Working Group. It has photographs of aphids on a plant
leaf at 20, 50 and 250 aphids per leaf. The card is available through
agriculture offices or online at; click on
the link near the bottom of the page that indicates “Soybean Aphid
Scouting Card.” 

Soybean aphids may be anywhere on the aboveground parts of the plant,
and when numbers are high it would not be practical or possible to try
counting them. Estimating levels on the entire plant is the optimum
goal. Visual aids, such as cards showing what different aphid densities
look like, can be helpful. Often it is obvious that, on average, plants
are either above or below the economic threshold, and decisions can be
made quite easily. It is when levels are close to the economic
threshold that deciding on the appropriate course of action becomes
difficult. To determine a field average, estimate the number of aphids
on about 30 plants/field. Estimating aphid numbers on about six plants
in each of five areas of the field will provide a good average.

Gavloski says to be careful when selecting plants to do counts on, so
as not to bias estimates of average aphid numbers per plant by only
selecting plants where aphid clusters are highly visible. If this is
done, then the estimates do not reflect the average number of aphids
per plant, since only plants with higher aphid levels had populations
estimated. Plants need to be selected on a random basis. “This is a
common concern when field scouting for many insects. Our eyes are
naturally drawn to those plants where the insect levels or their damage
are highly visible,” explains Gavloski. “So some method of ensuring
that plants for doing estimates on are selected randomly is necessary.”

Gavloski recommends that when a grower gets to an area to do counts, he
may want to select a plant without first glancing at aphid levels, or
have an object he can toss and assess insects on the plant nearest the
object. These are just a couple of methods to try to make counts
random. Do not avoid counting insects on the randomly selected plant
because levels appear low and there is a plant nearby with many more
aphids. Following this pattern will not result in an estimate of the
average number of aphids per plant.

If the field really does have an economical aphid problem, random
scouting of levels per plant will provide a total above the economic
threshold provided there are enough plants sampled, says Gavloski.
Biased scouting can result in populations below economic threshold
being sprayed, which means investing more money into controlling aphids
than they are causing in yield loss. From a business standpoint, this
is bad decision making. Learning to scout fields properly is one way
farmers and agronomists can ensure that net returns from the crop are
maximized. When scouting the field, if it is very obvious early on that
a field is well below or well above an economic threshold, then doing
the more intensive scouting techniques for a pest may not be necessary.
But if it is near the economic threshold, the extra effort can be well
worth it.

If insecticidal treatment is required, Gavloski says the registered
products Matador, Silencer, Cygon or Lagon can provide good control. He
recommends using the higher end of the recommended water volume to
ensure good coverage of the plants. 
Note, however, that soybeans also can be a source of nectar for
honeybees, so the general guidelines for protecting honeybees also
should be followed if soybean fields are still flowering when being
treated for soybean aphids.

Enlist natural controls to help

Gavloski says that although 250 aphids per plant on average seems a
high threshold, and it would be tempting to some to apply an
insecticide at lower numbers, this would not be wise. Not only would
treatment at these lower aphid levels result in more money spent than
yield saved, it may do harm by removing the natural enemies that are
helping to stabilize the population.

He says that aphids have many natural enemies, including lady beetles,
pirate bug, damsel bug and lacewings. “Learn how to identify the
natural enemies so you know if you have help in controlling the pests.”

The Soybean Aphid Scouting Card has pictures of some common natural enemies of soybean aphids.


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