It has been almost a decade since desert vegetable and melon producers in Southern California and Western Arizona were on the ropes from an onslaught of silverleaf whitefly.
During a two years span of 1991-93, whitefly damage cost producers in that winter fresh market vegetable growing area $500 million.
The farming's future in that area was in jeopardy, said University of Arizona research entomologist John Palumbo, stationed at the UA Yuma research center.
About that same time, Palumbo told a group of California Central Coast pest control advisers, the widely used aphid control pesticide Phosdrin was taken off the market, threatening the area with a one-two pest punch of unparalleled devastation.
About the same time a new class of insecticides, chloronicotinyl neo nicotinoids, was reaching field development stages. One of those was imidacloprid, sold today as the systemic Admire and foliar Provado.
Saved the day
It saved the day in the desert, Palumbo told consultants at a seminar sponsored by Bayer. Coastal vegetable producers are facing a threat similar to the whitefly from the lettuce aphid, which burrows deep into the head of lettuce and is difficult to control topically.
Palumbo said the silverleaf whitefly is also difficult to control not only because of its high mobility, but immatures prefer the underside of leaves where they are hard to control with foliar pesticides.
As a result, the systemic formulation of imidacloprid became the standard for whitefly and aphid control in the desert. It is the mainstay in an area wide suppression program where 80 percent to 85 percent of the desert fresh market crops in Yuma County are treated with Admire, according to Palumbo. It remains viable and with no signs of pest resistance after almost a decade of widespread use.
Growers have little choice but to apply Admire prior to planting or run it through a drip irrigation system. Without it, it would be difficult to survive economically. Its control is so dramatic Palumbo has had field trials where rows of untreated melons were completely devastated in 60 days only feet away from healthy Admire plots.
However, Admire will not provide control unless it is placed in the soil where plants will take it up. It also requires moisture for update.
Palumbo recommends a depth of one to two inches before the surface for cole crops and two to three inches for melons. Vince Wavra, Bayer regional rep in the desert, prefers three to four inches deep.
It can be applied in a band and sprinkled in or injected at bed shaping.
“When growers apply Admire properly, they can expect to get a consistent 50 to 60 days of residual control of silverleaf whitefly with no impact on beneficials,” said Palumbo. “There is also no adverse affect on bee pollination of melons,” he added.
Drip irrigation is growing in the desert and with it chemigation, which Palumbo said may be the most optimal way to apply a systemic like Admire. Drip lines are buried 15 to 18 inches below the soil surface. When Admire is applied through the lines, it moves up to the plant with the water where it is taken up by the plant.
While the lettuce aphid has appeared in the desert, it has not caused serious problems like it has on California's central coast. However, desert growers battle aphids, specifically green peach, potato, cabbage and turnip aphids.
Again, Admire provides season long control, up to 110 days for peach aphid and 75 days for spinach, a crop aphids prefer.
With the success of Admire has come other neo nicotinoids. One of those is thiamethoxam, marketed as Platinum.
Palumbo has tested it in comparison with Admire.
“We tested Platinum at the 11 ounce rate and were surprised when it was labeled at the maximum rate of eight ounces,” said Palumbo.
“That is a 25 percent reduction in the label rate and if you were to reduce the maximum Admire label rate (16 ounces) by that same percentage, I think you can expect to reduce residual control,” said Palumbo. He saw that with the 11-ounce rate in his trials.
Amy Wyman, a product development specialist for Bayer, pointed out that thiamethoxam kills insects with the same mode of action as imidacloprid and it is “counter productive and irresponsible” to suggest the new insecticide be used in a resistance management program.
With heavy use of any pesticide, resistance is a major concern. However, Wyman said imidacloprid remains “extremely robust” after 10 years of worldwide use, often under very heavy use in greenhouses.
Scientists also have discovered that imidacloprid use stops insect feeding and reduces the number of offspring from those surviving treatments. It also prevents virus transmission from surviving vectors.
“We have observed glassy-winged sharpshooters not feeding on imidaclolprid treated vines and in lab tests where we recorded only 15 percent mortality, honeydew production was reduced by 95 percent,” she said.
Palumbo said he has seen the same effect in commercial vegetable fields.