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.