By Vicky Boyd

Asian citrus psyllid are growing less sensitive to commercial pesticides compared to a highly susceptible psyllid population that never has been exposed to the chemicals.

Although the findings shouldn’t panic growers, it should nonetheless serve as a reminder to follow strong resistance-management strategies, including using full label rates and rotating modes of action.

"You simply can’t put the same mode of action out back to back multiple times,” says Lukasz Stelinski, an entomologist based at the University of Florida’s Citrus Research and Education Center in Lake Alfred. “The more complicated you make that rotation and the more distance you can have between each mode of action, the better off you’re going to be.”

What complicates the effort is a growing number of products are premixes that contain two different modes of action, and many contain a neonicotinoid as one of the ingredients.

"If you spray [that premix] and follow with just a neonicotinoid alone, essentially you have applied the same mode of action twice,” Stelinski says. “We’re especially concerned about those because right now, [neonicotinoids] are the only type of chemistry that is a soil-applied systemic. And they’re an important tool to protect young trees through soil application.”

Part of Stelinski’s ongoing research involves examining the best rotations. Insecticide A, for example, may be better rotated with product C than product B. He’s also looking at optimum spray timing during the year to help growers get the most bang for their buck and possibly reduce the overall number of sprays.

Since 2000, scientists at CREC have maintained a colony of Asian citrus psyllids that is highly suscpetible to insecticides, having never been exposed to them.

To determine baseline susceptibility, they treated some of the insects with different pesticides and calculated the LD50 rate. An LD 50 is the amount of insecticide needed to kill 50 percent of an insect population.

Since 2008, they have collected psyllids from the field and exposed them to the same dosages of more than a dozen registered insecticides to determine the LD50. They perform the same tests on psyllids from the lab colony.

By dividing the field insects’ LD50 by the lab insects’ LD50, Stelinski obtains a resistance ratio. The results range from a 3-fold increase in the resistance ratio for many of the pesticides up to a 30-fold increase for imidacloprid.

The ratios also vary among locations, depending on what products have been used. Stelinski emphasizes that the findings are based on laboratory tests intended to monitor changes in baseline susceptibility.

Many researchers have found that when similar tests show a 100-fold increase in the LD50 ratio of an insecticide, growers begin to notice a decline in a product’s performance. But Stelinski says he doesn’t know if that also will be the case with the Asian citrus psyllid.