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.