At the outset of another season of swarming ash whiteflies, agriculture officials said Thursday that, for the first time in a frustrating three-year battle against the tree-damaging insect, they have encouraging evidence of progress.
California Department of Food and Agriculture officials said it appears that a strategy started last year to combat the pest by releasing stingless wasps to prey on it is working.
"It's still too soon to say for sure, but it's definitely looking great at this point," said department spokeswoman Veda Federighi.
Federighi said it may take one or two more years before most residents notice a decline in the clouds of ash whiteflies. But she said the ash whitefly population apparently has been reduced in a test area in Riverside County, which was the site of an early release of the wasp parasites in late 1989.
"It has had a noticeable impact," she said. "In some places you just can't find them anymore."
She said scientists are finding that the parasite wasps establish themselves in every area they have been released in, indicating that the state's control efforts are working.
The state was infested with the tiny Mediterranean insect in 1988 and it quickly spread from Southern California to Northern California. The thickest season for ash whiteflies starts in late summer and continues through early fall.
The pest can attack 100 varieties of trees and shrubs, sucking the vital juices from the leaves. Most residents know the pest by its habit of swarming in dense, annoying clouds.
Although the ash whitefly is native to Europe and North Africa, it is not considered a pest in those regions because natural predators keep it in check. But when the insect was somehow introduced to California, it encountered an environment free of any enemies.
"With no natural enemies in California, it just went to town," Federighi said.
Scientists began looking for a predator they could import to California. They found a tiny Mediterranean wasp that was virtually unnoticeable to humans, but deadly to the ash whitefly.
Federighi said this method of insect control, which is widely used by farmers, cannot eradicate the ash whitefly. But it can reduce its number to a level where it is no longer noticeable by the public.
Starter colonies of parasite wasps were released last year in 12 Southern California counties. This year, the state will release wasps in 13 more counties. The program will continue until all 36 infested counties are covered.