Mite vs. Mite : Avocado Growers Hope ‘Good’ Insect Will Kill the One Ruining Crop

Ed Anderson stands in his 28-acre avocado grove and hears the sound of a crop dying.

It is the thudding sound of sunburned and shriveled fruit dropping to the ground, victimized by a microscopic but voracious mite that has infected the avocado crop of San Diego County and is spreading to avocado groves in Riverside, Ventura and Santa Barbara counties.

“I don’t think I’m going to salvage much fruit this year,” said Anderson, a veterinarian who has grown avocados in northern San Diego County since the 1960s.

This year may be a financial bust but Anderson has hopes for next year’s crop because of a biological warfare battle plan in which growers are banking on a good mite to kill the bad mite that is devouring their industry.

The mite vs. mite war in the avocado groves is the latest example of a trend among California farmers: fighting off pests not with pesticides but by recruiting the pest’s natural enemies.

It is not a new tactic--citrus growers last century beat back a scale invasion by importing a beetle from Australia. Still, the use of what is called integrated pest management has increased greatly in the past two decades as pesticides have come under mounting environmental criticism and governmental regulation.

“For every pest, there is some natural enemy that is controlling them,” said Frank Zalom, a UC Davis entomologist and director of the Statewide Integrated Pest Management Project. “We need to understand what those factors are and how to manage them.”

Each week a million or so of the good mites, the Galendromus helveolus, are being released into the avocado groves of San Diego and Riverside counties to battle their archenemy: the avocado-loving Olygonichus perseae mite.

The two mites have a genetic blood feud and, luckily for beleaguered avocado growers, the helveolus is meaner and faster than the perseae.

“It’s a predator by nature,” said Jim Davis, whose Escondido-based firm, American Insectaries, is one of two retailers of helveolus. “It feeds on all stages of the perseae . It likes the eggs best because they don’t fight back, but it feeds on eggs, juveniles and the adults.”

Some growers have surrendered. “I talked to a grower in Highland Valley (near Poway) who says you can hear chain saws every day as growers cut down their trees,” Davis said.

But other growers are cautiously optimistic that the good mite in a year or two will triumph over the bad mite and rescue an industry battered by increasing water costs and shifting prices.

The dreaded perseae is thought to have sneaked into the United States from Peru or Mexico in 1990, landing in back-yard trees in Coronado and La Jolla and then high-tailing for the avocado groves. Up to 3,000 have been found on a single leaf.

For two years it munched happily until James McMurtry, an entomology professor at UC Riverside, harnessed the animosity of the helveolus in a counteroffensive that began in November. McMurtry’s research was funded by the California Avocado Commission, the trade group for the $180-million-a-year industry.

In the meantime, the perseae migrated to the Temecula groves in Riverside County and has been spotted recently in Moorpark in Ventura County and Goleta in Santa Barbara County.

It attacks the underside of the leaves, sucking out the life, killing the leaves and leaving the fruit unprotected from the sun. The fruit withers and drops to the ground.

The helveolus, which are being bred by the firm Biotactics in San Bernardino County, are fearsome predators but need sufficient numbers to be effective. Unfortunately, the helveolus has a tendency to be a slow breeder and drops off some trees without doing its job.

Rick Morrocco runs Fallbrook Ag Lab, which, like American Insectaries, buys helveolus from Biotactics and resells them to farmers at a rate of $15 per thousand. He says he could sell twice as many mites if he could get them.

The first battalions of helveolus were laboratory-grown mites, the product of generations of culturing. Replacements will be crossbred from field-hardy helveolus caught in the wild.

“Culturing is like Club Med for insects,” Davis said. “They get plenty of food, plenty of water, plenty of mates and an ideal temperature. It tends to make the predator weak over time because it’s not stressed.”

Some growers in northern San Diego County, where more than half the statewide crop is grown, are using pesticides. But Ed Anderson is not among them; he is leery of the long-term ecological effects.

He withstood the inchworm attack that struck the groves several years ago. And he just dug a 400-foot well to escape the threefold increase in the cost of water.

He even manages a small laugh at the sound of his fruit going kerplunk on the ground. “That’s a nervous laugh you hear,” he said.

Times researcher Kate McCarthy assisted with this story.