Efficacy on the decline

11/01/2009 01:00:00 AM
Vicky Boyd, Editor

Kevin Seitzinger, the crop protection manager for a large tomato grower/packer/shipper near Immokalee, Fla., battles whiteflies— and the viruses they carry—in his tomato fields on several fronts.

He starts with clean virus-free transplants, lays reflective metallic mulch over his beds, applies an insecticide at planting, scouts his fields twice a week and performs field sanitation shortly after harvest.

But imidacloprid, his traditional insecticide at planting, wasn’t providing the same length of control as it did when it was first registered about 14 years ago. So Seitzinger had to return sooner than he used to with a foliar insecticide treatment.

He has since switched to Venom, a neonicotinoid applied at planting that is marketed by Valent USA.

“This is not the silver bullet either, but it does definitely help us,” Seitzinger says.

Justin Hood, a farm manager with Collier Pacific Grower Partnership near Immokalee, says he’s lucky now to get a couple weeks’ control out of imidacloprid.

“Five years ago, the imidacloprid was a really good product for the whiteflies,” Hood says. “Now it just doesn’t have the same get up and go that it used to.”

Seitzinger and Hood aren’t alone in their observations. Researchers from universities as well as Bayer CropScience, the chemical’s original registrant, say whiteflies in some locations have become more tolerant to the chemical.

Bayer markets its imidacloprid as the soil-applied Admire Pro and the foliar Provado. Several other companies have generic or off-patent imidacloprid products.

“In some areas, there’s been a gradual decline with imidacloprid control of whiteflies, particularly adults,” says David Rogers, insecticide product development manager for Bayer CropScience in Research Triangle Park, N.C. “There have been no field failures, but the level of activity is less today as it was in former times.

Photo courtesy of the Agricultural Research Service

“This is perhaps not completely unexpected as Admire and Admire Pro have been the backbone for whitefly management programs for many, many years­—longer than the other neonicotinoids currently on the market.”

Researchers also have found whiteflies are more tolerant of thiamethoxam, a neonicotinoid marketed as the soil-applied Platinum and the foliar Actara by Syngenta Crop Protection.

A wake-up call

Even if these insecticides are still providing you with long residual control, the documented cases of resistance should serve as a wake-up call to adhere to strict insecticide resistance-management recommendations, Rogers says.

“Make sure you put out a good, robust rate at planting,” he says of Admire Pro. “When the residual begins to wear off, switch to a foliar compound with an alternate mode of action.”

Photo courtesy of the Natural Resources Conservation Service
Apply a neonicotinoid at planting, then switch to products that do not belong to the Insecticide Resistance Action Committee’s Group 4A for the remainder of the season. This helps manage whitefly insecticide resistance by rotating to different modes of action.

Imidacloprid, thiamethoxam, dinotefuran (Venom) and acetamiprid (Assail) are neonicotinoids that all belong to the Insecticide Resistance Action Committee’s Group 4A.

Most of the major manufacturers prominently list the IRAC group number or numbers on the pesticide label, Rogers says. This can simplify resistance management because users can simply rotate to a compound with another IRAC group number.

Gradual decline in residual activity

John Palumbo, a research entomologist with the University of Arizona in Yuma, has been tracking whitefly sensitivity to imidacloprid for 11 years in broccoli plots and 16 in lettuce. He’s seen a steady increase in imidacloprid resistance in the past five to six years.

“It’s not the same product as it was when it [first] came out,” he says. “It doesn’t have the residual effects in broccoli and cole crops.” In his broccoli plots, for example, Palumbo says he sees a steady decline in control after 20 days.

“By 40 days, it’s like it’s not there any more. It’s no different than the untreated control,” he says. “Down here, it’s just we’re not seeing the control we used to get of 45 to 50 days. We’re lucky [now] if we get 25 days control.”

In fact, one of Palumbo’s broccoli-imidacloprid trials this year was “pretty much blown out,” so the pest control adviser had to come over the top with Movento to rescue the crop, he says.

Arizona lab tests remain inconclusive

Xianchun Li, an associate entomology professor at the University of Arizona in Tucson, took over entomologist Tim Dennehy’s whitefly resistance monitoring program this year.

Dennehy had been conducting laboratory bioassays to gauge whitelfy sensitivity to imidacloprid since 1996. Only one year—1998—did the bioassays show a decrease in sensitivity, Li says.

That was until this year. Li says initial tests show some whitefly populations have a decreased sensitivity to both imidacloprid and thiamethoxam. The drop in sensitivity to imidacloprid appears greater than to thiamethoxam.

“I’m not comfortable right now saying it’s resistance,” Li says, adding he needs to conduct additional tests to verify the results.

Florida lab tests point to resistance  

David Schuster, a University of Florida Institute of Food and Agricultural Sciences entomology professor, has been monitoring adult whitefly sensitivity to imidacloprid since 2000. In 2003, he added thiamethoxam to his testing.

“We have definitely documented decreased sensitivity or increased tolerance of whitefly adults to both imidacloprid and thiamethoxam,” says Schuster, who’s based at the Gulf Coast Research and Education Center in Balm.

He has not been testing the nymphs or immature whiteflies.

Schuster maintains a colony of whiteflies that are susceptible to the insecticides. He has established baseline insecticide doses that will kill 50 percent of the population—the LC50—and 95 percent of the population—LC95.

The bioassay involves putting the petiole of a cotton leaf in solutions of neonicotinoid insecticides at the LC50 and LC95 doses for 24 hours. Then whitefly adults are confined on the leaf for another 24 hours.

Schuster compares the results when the susceptible adults are placed on the leaves with those of field-collected whiteflies placed on the leaves.

Between 2000 and 2006, whitefly resistance to imidacloprid increased about eight-fold. Between 2003 and 2006, resistance to thiamethoxam increased about 15 fold, Schuster says.

Nevertheless, Schuster says he continues to recommend a neonicotinoid at planting because it still controls nymphs.

Growers should then follow up with another mode of action applied as foliar sprays to control adults, he says.



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