A Mighty Mite : Newly Arrived Avocado Pest Threatens Hard-Pressed Growers
It’s the last thing North County growers need: a destructive littlebug that spreads rapidly among groves as it sucks the life out of what were once highly profitable avocado trees.
And, because this critter is a newcomer to the county--and the United States for that matter--scientists know little about its life cycle or how to combat it.
Already hard-pressed to squeeze a profit out of their fruit in the wake of ever-higher water prices, many avocado growers must now dig deeper into their pockets to pay for chemical sprays or biological controls to combat their new foe: the “persea,” or avocado mite, which growers say is perhaps their most formidable insect foe ever.
To date, the avocado mites have been documented on about 10,000 acres--roughly a third of all avocado acreage in the county, agricultural officials say.
“Countywide it hasn’t done a lot of damage yet, but it certainly has the potential to be very destructive,” said one North County pest management adviser.
Rick Opel, vice president and farm manager for Henry Avocado Co. in Escondido, which manages about 2,000 acres, said: “I’ve got some really productive groves that have just been knocked out of production by this mite. I’d say that, economically, it’s severely affected about 100 of our acres.”
Though it has on rare occasion been seen on the leaves of weeds and peach trees, the avocado mites savor avocado leaves, which the mites kill by sucking out the chlorophyll. That causes yellowish spots to form on the leaves, which eventually drop off the tree and expose the fruit to harmful rays from the sun.
In the worst-case scenario, the fruit gets sunburned and drops prematurely. The tree is weakened and may produce less fruit next season.
There has been no official effort to assess the economic damage of the mite so far and, because so little is known about it, agricultural experts aren’t willing to estimate what its economic effect might be next year, when mite populations will be significantly higher.
“We’re considering it a serious pest, but we don’t know how devastating it can be, and we don’t at this time have a chemical to adequately control it,” said Bill Snodgrass, the county’s assistant agricultural commissioner. “It’s just about anybody’s guess on what the economic impact might be.”
Populations of the avocado mite, which is believed to hail from Mexico or Central America, exploded this spring in southern Escondido and devastated some areas there, while severely cutting productivity in others, according to grove manager Steve Olson of Olson Avocado Management in Escondido.
Until two years ago, the 1-millimeter-long pest had only been spotted on avocado trees in South Bay back yards. At first, it was widely misidentified as the coastal six-spotted mite, and was correctly identified this summer. Today it is found in groves as far north as south Riverside County, officials say.
Olson, who manages about 800 acres of avocado trees, said about 500 acres in Highland Valley were severely affected this year and last by the avocado mite, and he estimates productivity there dropped 30% this year from last.
“When the mite hit last year nobody sprayed, and it just took over,” Olson explained.
Entomologist James McMurtry of UC Riverside said the avocado mite populations soared as soon as temperatures rose in the early spring. And no resident predators could stop the avocado mite, which is difficult to kill with a standard aerial spray because it attaches itself to the underside of leaves.
And natural predators that lurk in avocado groves were unsuccessful in their hunt for the avocado mite. That’s because the mite protects itself under the leaf by spinning a thick web that acts as a barrier. It’s under this barrier that the mite lays its eggs.
Entomologists and growers figure the avocado mites have spread locally in the wind or by attaching themselves to fruit pickers or their equipment. They surmise that the mites reached the county from Mexico or South America by hitching a ride on cargo containers.
Most growers are worried about the pest’s almost inevitable adverse effect on their profit margins, but they have optimistically pinned their hopes on the predator “helveolus” mite. The mite has, in an experiment in an Escondido avocado grove, fed on the avocado mite and kept that pest’s population under control.
“Until a few weeks ago, we didn’t know what to do, but this beneficial (predator being developed in a laboratory) is really helping the situation,” said Avi Crane, vice president of the California Avocado Commission.
The helveolus mite is being raised now in two Southern California labs and will be available commercially in a couple of weeks. Also, some growers have noticed a new, naturally occurring predator, called the “annectens” mite, in their groves. It also appears to be feeding on the insidious avocado mite.
At stake in this battle of the bugs is the already questionable health of San Diego County’s avocado crop, which at 76,254 tons last year made this county the most productive for avocados in the country last year. The ’91 crop was valued at $132 million, second locally only to decorative plants, according to the county Department of Agriculture-Weights and Measures. There are about 32,000 acres of avocado trees in the county.
The mite, agricultural officials say, has hit the avocado industry at the worst possible time. The county, which was last year’s leader for agriculture in Southern California, rests at the distant end of the state’s water pipeline, and farmers are paying dearly in the drought, many even selling off their farms.
Clearly, economically strapped growers must act now to control the avocado mite before its population explodes again when temperatures rise in the spring, pest management experts say. If infestations are too heavy in a given grove, they say, several costly applications of chemical sprays applied from helicopters would probably be necessary.
Explaining the challenge in controlling the avocado mite in the county, McMurtry, one of the leading entomologists working on the problem, said “there’s still so much we don’t know about this mite.
“These native and introduced (predator) species appear to be promising,” he said.
But he cautioned that, even with widespread introduction of the predacious mite being raised in the insectories, it may take two or three years for the predators to spread adequately in the groves and for growers to see substantial reductions in avocado mite populations.
“This is the most threatening bug in years for avocados, but I’m optimistic that, with the (predators) being developed--assuming they survive the winter in the groves--we’ll be in good shape,” said Kevin Grangetto, who helps manage about 80 acres of his family’s North County avocado groves. He is also a sales representative at, and co-owner of, Grangetto Agricultural Supply.
“Everyone thinks the predacious mite they’re rearing in the lab is the answer, but whether it will make (the avocado mite) go away to the point it won’t cause economic damage has yet to be seen,” said Vincent Lazaneo, a farm adviser with the University of California Cooperative Extension in San Diego.
Theoretically, the lab-grown mites will take a huge bite out of the avocado mite population here. Many of the county’s major growers, those with hundreds or thousands of acres at stake, say they will begin buying the beneficial bugs as soon as they are available. The first batch will come from a Riverside insectory called Biotactics Inc.
“We’re going to be buying millions of them,” said Opel of Henry Avocado.
Anything, he and other growers say, to avoid spraying.
Avocado trees have historically been one of the county’s least sprayed and most pest-free crops. Problems in the industry have mostly stemmed from disease and from exorbitant water prices.
For the first time in several years, North County avocado growers this year had to resort to spraying their crops--at a cost of about $150 an acre, Opel said.
The cost of fighting the mites “could be the straw that breaks the camel’s back,” said Jim Davis, an independent pest control adviser and owner of American Insectories Inc. in Escondido.
“When water prices went up 50%, most farmers saw their profits go right out the door. Now they get hit with this.”
Barring another water rate increase, heavy winter damage or a widespread explosion in the avocado mite population this spring, growers still might come out ahead.
Despite the looming threat of the avocado mite, the current crop, which will be picked from December to August, is a relatively heavy and healthy one, growers say.
And Hurricane Andrew, which wiped out Florida’s avocado business, has caused higher prices for consumers but bigger profits for California growers, who last year produced 87% of the nation’s avocado crop. The storm should raise prices paid to San Diego County farmers, whose ranches produce 45% of the state’s annual avocado harvest, according to a spokesman for the California Avocado Commission.
