Top Crop Manager

Features Inoculants Seed & Chemical
Careful management controls beetles and preserves tools

The danger of resistance developing to insecticides is always looming, but using products effectively can minimize the risks.

November 15, 2007
By Top Crop Manager


Potatoes In Canada Special Issue: The danger of resistance developing to insecticides
is always looming, but using products effectively can minimize the risks.

With so few options available for chemical control of Colorado potato beetles,
there is always a risk that the beetles will develop resistance to those products.
In the case of Admire, the only three-way product (in-furrow, seed piece treatment
and foliar) beetle control, resistant insects have been documented in the US.

Fortunately, resistance to Admire has not yet been confirmed in Canada and
experts believe it is the judicious use of the product and careful management
on the part of Canadian growers that may have made the difference. However,
the risk is ever present and entomologists and agronomists believe growers must
never let their guard down and they remind growers of old strategies to reduce
risk and suggest one or two new ones.


"We really believe a high dose applied in-furrow or as a seed treatment
will limit survival pressure from partially resistant insects," says Andrew
Dornan of Bayer CropScience. "But, if there are still beetle problems later
in the season, be sure to apply a foliar product that is a different chemistry
than Admire." He explains that high dose applications in-furrow will ensure
that all partially resistant beetles will be controlled. He adds that the danger
lies in allowing partially resistant beetles mating with other partially resistant
beetles to produce fully resistant progeny. Over the next generations, if left
uncontrolled, those progeny will eventually produce resistant off-spring.

"From what we have learned in the US, multiple foliar applications, with
little or no rotation among chemistries, will select for resistance," Dornan
adds. "Also, a foliar application of Assail or Admire following an in-furrow
or seed treatment with Admire will also select for resistance."

Most experts agree that using 'minimum effective' rates of application, whether
foliar or in-furrow, can lead to more rapid development of resistance. Dr. Jeff
Tolman, a researcher with Agriculture and Agri-Food Canada (AAFC) in London,
Ontario, disagrees with labelled 'minimum effective' rates because they are
only effective when the conditions are perfect. Since perfect conditions do
not happen very often in the real world, using the 'minimum effective' rates
will not offer the most effective control. "We need to label a recommended
rate that is reliable," he says, implying that environmental concerns and
perceptions on the danger of these products may prompt regulators to insist
on a 'minimum effective' rate rather than an effective rate range. While growers
sometimes use the minimum rate to manage costs, the reality is they may be setting
themselves up for pest management problems down the road that will cost even
more to control.

"Minimum rate applications can accelerate the development of resistance,"
says Eugenia Banks, a potato specialist with Ontario Ministry of Agriculture,
Food and Rural Affairs.

The modern reality of chemical controls is that, whether weed, disease or insect,
resistance to products will most likely develop over time and, therefore, the
best strategy to preserve the tools at hand is to implement some form of integrated
pest management. This strategy also buys time while manufacturers discover and
develop new chemistries and researchers determine improved pest management recommendations.

"The number one strategy to manage pests is to monitor your insect population,"
advises Dr. Christine Noronha of AAFC on Prince Edward Island. "You need
to scout your fields and spray only when economic thresholds are reached."
While most growers understand this allows for reduced use of insecticide which,
in turn, can be a cost saving, many find that scouting is time consuming. But,
as Noronha points out, there are crop scouts who will monitor fields for growers
and who will keep track of insect populations. She says after every application
of foliar spray there should be a follow-up walk through the field at the re-entry
period to see if the application was effective. She advises to always keep records
of insect populations and spray conditions because these records can be an indicator
of resistance problems. She is adamant about rotating products as part of resistance

Over the years, numerous other pest management techniques have been tested,
such as flamers and plastic-lined trenches, but most researchers agree these
techniques are more acceptable in organic production, particularly in smaller
fields. In large scale commercial potato production, most agree these methods
are not economically viable and require more management in order to get good
results. However, Noronha and Tolman have variations on a theme that offer another
option for minimizing insect pressure while providing a way to manage populations.

"Consider creating a refuge in your field that is not sprayed," says
Noronha. "This allows a susceptible population to always be present to
mate with the resistant population and minimizes the possibility that the resistant
population will become dominant. This is really useful if you have a highly
effective product that maintains control in the remainder of the field."
She says the refuge only needs to be a few rows or a corner of a field.

Tolman suggests the refuge strategy works well for in-furrow applications,
but he modifies the suggestion by proposing the use of an in-furrow treatment
around the perimeter of the field. As he points out, in the spring, when temperatures
are lower, Colorado potato beetles walk into the field and are killed when they
feed on treated plants at the edge of the field, effectively reducing populations
before temperatures rise high enough to enable them to fly. He says growers
need to know where the insects are migrating from and then provide a minimum
of eight rows of potatoes treated with in-furrow insecticide. "This barrier
works well," he says, "but you have to know where your insects are
migrating from."

All the scientists agree that the best strategy for controlling Colorado potato
beetle is rotation: rotation of chemistry and rotation of crops. While a two
year rotation is the current standard in most potato growing areas of the country,
Prince Edward Island has mandated a three year rotation. Tolman adds that rotating
potatoes out of one field to the adjacent field, while an option for disease
and weed control, may not offer much assistance in insect control. If all the
insects have to do is turn around and walk into the next field, the rotation
has not been truly effective. The same would be true if a neighbour plants potatoes
in an adjacent field. He did stress, however, that even delayed arrival of beetles
can be beneficial in certain weather conditions.

Noronha says there are advantages and disadvantages to in-furrow and foliar
applications. In the case of in-furrow, she explains, the potential for run-off
is low, but the application is made before it is known if there is any serious
threat from insects. In the case of foliar sprays, drift and run-off are always
concerns, but you don't have to use them if the economic thresholds don't require

New chemistry is in development but there is no indication when registration
will be complete. Also, the current new products will be foliar sprays, leaving
Admire as the only option for in-furrow use. "Admire is the only product
registered for in-furrow and gives longer term control," says Banks. "I
believe resistance can be delayed by using in-furrow treatments because the
product is present longer. Foliar sprays are a one-shot application and several
days after the application new insects can arrive." She believes that over-use
of foliar applications is what is leading to the resistance development in the
US. She says Canadian growers embraced the in-furrow option and that could be
why there are no documented cases of resistance north of the border.

Greig Zamecnik of Bayer CropScience agrees there have been no cases of resistance
reported in Canada. "However, we are seeing shifts in tolerance to Admire,"
he admits. "We truly believe we need to be vigilant with crop rotations
and pesticide rotations to minimize the threat. I believe Canadian growers are
much more careful about following label recommendations."

"I know what extremely busy growers need and we can't beat insecticides
for their effectiveness," says Tolman. What is needed, he believes, is
careful use of the products currently available, combined with rotation strategies
that together will provide the best control and the most effective means for
delaying or preventing resistance problems.

The best pest management advice can be summed up simply: rotate crops and chemistry,
consider high dose in-furrow applications and never use minimum foliar application
rates. Even when new chemistry is registered, the threat of resistance will
not disappear. The threat of beetles developing resistance to any product is
always present and new products only offer more options for rotating chemistries;
they are not the solution to the resistance problem. -30-


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