Kevin Seitzinger, the crop protection manager for a large tomato grower/packer/shipper near Immokalee, 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 four 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. A handful of other companies have generic or off-patent imidacloprid products.
“There’s been a gradual decline with imidacloprid control of whiteflies,” says David Rogers, insecticide product development manager for Bayer CropScience in Research Triangle Park, N.C. “There have been no field failures, but clearly the level of activity is no where close to being today as it was in former times.”
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 the insecticides are still providing a long period of 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.”
Imidacloprid, thiamethoxam, dinotefuran (Venom) and acetameprid (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 simplifies resistance management because users can simply rotate to a compound with another IRAC group number.
Gradual decline in activity
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 white fly 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 known to be 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.
Recommendations for resistance management
In 2005, Schuster and a group of Extension researchers and educators, manufacturers, commodity group leaders and scouts drafted guidelines to help growers manage whiteflies, whitefly-vectored viruses and insecticide resistance.
Among the recommendations were using neonicotinoids only during the first six weeks after transplanting and preferably as a soil application.
If growers needed additional adult whitefly control during that early period, they could return with a different mode of action applied as foliar treatments.
The reason was to reduce nymph exposure to neonicotinoids later in the season, Schuster says.
The recommendations also called for crop destruction within five days after the end of harvest to quickly remove host material.
In addition, they recommended a two-month host-free period, preferably between mid-June and mid-August, when adult whiteflies would not be exposed to insecticides.
Whiteflies take two to six generations to regain sensitivity to the insecticides, and each generation takes two to three weeks.
But some growers are not heeding the recommendations, Schuster says.
“The tomato-free period is very short,” he says. “So the whiteflies are coming out of the spring crop, and they’re going right back into the fall exposed. They have a reduced opportunity to revert back to susceptible.”
The host-free period also would help break the virus disease cycle, since it would reduce the number of reservoir hosts from which whiteflies could acquire the pathogens, Schuster says.
Compounding the problem are three whitefly-vectored cucurbit viruses, including watermelon vine decline, he says.
As a result, growers also are applying neonicotinoids to cucurbits, further exposing the whitefly to the products.
Contact Vicky Boyd at firstname.lastname@example.org or (209) 571-0414.
Management of Whiteflies, Whitefly-Vectored Plant Virus, and Insecticide Resistance for Vegetable Production in Southern Florida