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Whitefly Resistance Management Program Holding Pest at Bay

Luck will only go so far in farming and poker. Skill must play a role eventually to consistently draw to winning hands.

Arizona farmers have deftly marshaled a decade of winning hands against a tiny insect called silverleaf whitefly that had them down for an eight-count in the mid-1990s.

Growers teamed with University of Arizona entomologists and the agrichemical industry to devise an artful defense against the tiny insect that turned vegetables into mush and cotton lint into a black sticky mess textile mills did not want at almost any price.

Successfully controlling whitefly for most of the past decade is only a third of the story. It has been accomplished without a single documented case of field-resistance to a highly effective, but limited arsenal of insecticides.

And in the process, Arizona cotton farmers have reduced pesticide sprays on cotton by as much as 85 percent. Where the average cotton grower sprayed six to 12 times in 1995, the average is now less than two.

It is a remarkable success story detailed recently in a report from University of Arizona entomologist Tim Dennehy, Benjamin DeGain, Virginia Harpold and Sarah Bring and Cotton Incorporated entomologist Robert Nichols.

When the silverleaf whitefly turned Arizona agriculture on its ear in the mid-90s, it was not the first pest to hammer Arizona cotton. Producers had successfully survived pests in the past, but it came with a big price tag. That price usually was quick resistance buildup to pesticides brought on by bombarding pests into submission.

That was the scenario growers found themselves when whitefly resistance to synthetic pyrethroids and other conventional insecticides reached crisis proportion in 1995.

Growth regulators

Fortunately, there were two insect growth regulators in development then. Farmers, the IGR owners and UA entomologists devised a three-stage resistance management program that included a scouting/threshold scenario along with a strict label that limits use of the two IGRs to only once per season each.

“This program has been highly successful for eight years,” report the entomologists. “Success has been fostered by intensive investment into improved whitefly sampling and treatment decisions, coupled with conservation of natural enemies.”

Maintaining natural predators has been accomplished with strategic use of the IGRs (Knack and Courier/Applaud), use of the neonicotinoid insecticide, imidacloprid (Admire/Provado) in vegetables and melons and “tactical deployment” of non-pyrethroid and pyrethroid chemicals.

UA entomologists have monitored cotton and vegetable fields statewide since the program began. In the past two seasons after several years of treatments for whiteflies, they found “no major problems” in field performance of insecticides against whiteflies.

Just because Dennehy and others have not found resistance, it does not mean it cannot happen. Up to an 82-fold resistance has been identified in laboratory conditions and field or greenhouse resistance to the IGR compounds and imidacloprid has been confirmed in Span and Israel.

Dennehy and his team will continue to monitor for resistance in Arizona with the goal of identify potential, emerging resistance problems and formulate potential solutions before they reach an economic crisis level.

Less susceptible

While there has been no reported field failures with Knack, Dennehy said he found whiteflies “substantially less susceptible” to the active ingredient in Knack than before.

“This finding does not mean that Knack has failed under field conditions or that failure is imminent,” said Dennehy, who noted that the Arizona Whitefly Resistance Working Groups continues to recommend Knack or Courier/Applaud as the first treatment against whiteflies in Arizona.

The UA entomologists encourage growers to be alert to any signs of possible resistance. If they are identified early, Dennehy said “the more likely we will be to have the needed time to isolate and manage the problem.”

Another added bonus to this whitefly resistance management scheme is sharp decline in whitefly resistance levels statewide to pyrethroid insecticides.

However, the entomologists added the yearly percentage of individual cotton fields with greater than 20 percent resistant whiteflies varied widely from as high as 60 percent to as low as 10 percent. Some growers did not get adequate whitefly control from pyrethroids.

Nevertheless, the pyrethroids continue to be an important, late-season tool for controlling whiteflies. However, if they are used too early or too frequently, they can disrupt the overall whitefly management scheme and increase grower production costs, warned Dennehy.

These non-IGR products are lower cost than Knack or Courier and it is tempting to use them earlier and more often.

‘Could slip back’

“Without appropriate constraint, Arizona cotton producers could slip back into insecticide use patterns that triggered the resistance-related whitefly outbreaks in 1994 and 1995,” said Dennehy.

UA entomologists recommend no more than two pyrethroid applications per season and only late in the year.

Whiteflies in cotton continue to show high levels of susceptibility to Admire/Provado, even though this product has been widely used in vegetables and melons for more than a decade. All Arizona whitefly collections also show high susceptibility to Intruder (acetamiprid) and Actara/Centric/Platinum (thiamethoxam).

It is so high that Dennehy said he rarely finds more than 10 percent survivors after an Admire/Provado treatment.

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