Features Agronomy Insect Pests
Unpredictable aphids make forecasting difficult

Soybean aphids have been under the microscope since they first started making
their way into Ontario in 2001. This intense scrutiny has yielded some management
insight, but researchers and crop experts continue to search for a way to predict
whether aphid populations will reach yield damaging levels.

When it comes to aphids, there is no method for prediction, says David Townsend,
a certified crop advisor and technical services manager for NK Brand, Syngenta
Seeds. "In 2006, the aphid population was much lower compared to 2005.
But we can't predict what the population will be in 2007," explains Townsend.

Tracey Baute, field crop entomologist, Ontario Ministry of Agriculture, Food
and Rural Affairs, agrees that predicting future populations in Ontario is like
trying to predict the weather. She hopes current research that monitors populations
and influencing factors will help experts better predict if, and when, populations
will reach threshold levels within a season.

Baute says whether aphid populations reach threshold boils down to four factors:
overwintering success, wind patterns, temperature and predator populations.

October cold snap expected to impact 2007 population
In Ontario, aphid populations used to be determined by wind patterns that carried
aphids up from the US. This offered potential to miss such patterns and have
an off-year. Since 2003, however, researchers have been noticing aphids overwintering,
meaning there is no such thing as an off-year anymore, says Townsend.

Aphid eggs overwinter and they are incredibly tolerant to the cold, says Baute,
but first, adult aphids must lay the eggs on buckthorn trees. Both adult and
nymph aphids are also very vulnerable to cold weather and egg laying can be
impacted by wet weather, delayed harvest, quick shifts in temperature and aphids'
ability to find their way to buckthorn.

Canadian and US experts start monitoring aphid migration to buckthorn during
early fall using suction traps. In October 2006, Baute says both US and Canadian
traps were catching high numbers of winged adults. During fall scouting, Baute
also recorded a high number of aphid colonies already on buckthorn. "Because
of the fluctuating temperatures, wet weather and the wind storms in early October,
however, we're not sure how it affected egg laying," says Baute. "We'll
have to wait until next spring to see what the populations look like on buckthorn,
but we're hoping egg laying will have dropped due to unfavourable conditions."

Even if overwintering success is poor, populations can still blow in from neighbouring
US regions. Ontario, especially eastern Ontario, tends to be in the path of
most wind patterns, meaning Ontario soybean fields typically have a greater
chance of aphids entering each year.

Insects protect soybeans best against aphids
Natural
predators are the single biggest factor that can keep aphid populations from
reaching thresholds. "If predators overwinter at a population sufficient
enough to control aphids from the start, then they'll have an advantage for
the rest of the season," says Townsend.

According to Baute, there are some predators that are always present on soybeans,
such as minute pirate bugs, damsel bugs and syrphid fly larvae. Lady bugs, on
the other hand, typically enter the field when there is a larger aphid population
on which to feed. In 2004, experts also noticed a parasitic wasp, Aphelinus
varipes, parasitizing soybean aphids. "This is unique to Ontario,"
says Baute. "This parasitoid has not been observed in US regions to control
aphids."

Baute agrees that if predators and parasitoids overwinter successfully, they
can often keep up to aphid growth. "Seasonal temperatures, however, also
play a significant role in repopulation," explains Baute. "Neither
aphids, nor predators like it really hot. But at 27 degrees C, aphids will repopulate
at a much quicker rate than predators, increasing their chances of reaching
threshold."

If aphid populations reach threshold – 250 aphids per plant and increasing
– insecticides can be used for control. To maintain the upper hand on pests,
growers should scout multiple times prior to spraying to accurately assess predator
and pest population levels. For example, if the population jumps from 200 to
500 aphids per plant between the first and third day of scouting, it is clear
predators are struggling and spraying would be warranted. But if the aphid population
drops or fluctuates between days one and three, the predators are doing their
job. In fact, "strip trial data from across Ontario and Michigan State
show that if you spray too early you won't receive a yield response," explains
Baute. "If you time the spray according to threshold, when the aphid populations
are increasing, then you will see an economic benefit."

By monitoring aphid and predator populations in the fall and spring, overwintering
success, wind patterns and temperature, Baute is hoping to better predict when
population levels reach threshold within a season. Both Baute and Townsend agree
the future success of aphid control will be influenced and improved by the ability
to forecast seasonal populations, the development of spray thresholds according
to predator populations, and the availability of aphid resistant soybean traits.

New aphid resistance trait
New soybean variety research holds promising potential for the fight against
aphids. Don McClure, soybean breeder with NK Brand, Syngenta Seeds, says
new aphid resistant soybean varieties will be tested in field trials in
Ontario in 2007, and are expected to be commercially available in the
next two to three years.

"Aphid resistance will have a positive impact for growers,"
says McClure. Resistance will offer soybeans another mode of protection
against aphids, supporting natural predator control from other insects.
Should predator populations drop and become insufficient to control aphid
populations, the plants will be able to protect themselves, reducing the
need for insecticide application. This aphid resistance trait is also
specific to soybean aphids, and will not affect beneficial insects.

To develop aphid resistance, breeders are using marker assisted technology
that allows them to first test whether the new trait is present at the
gene level, rather than having to repeat field research plots and test
traits seasonally by trial and error. This new method is a significant
benefit because aphid populations are very sporadic and unpredictable.

McClure notes that aphid resistance is a native trait found in an old
soybean variety, which has now been incorporated by conventional breeding
methodology into earlier maturity, adapted varieties forÊCanadian conditions.

McClure reports that 2006 field studies in South Dakota, where aphid
infestations were high, provided promising results for the new aphid resistant
varieties.