Market Watch: New pest SWD forcing some farmers to turn to pesticides

Last year about this time, as John Tenerelli, a farmers market grower in Littlerock, Calif., was starting to harvest his late varieties of cherries, he noticed little wriggling bugs in the fruit. “Oh, my God, what are these things?” he remembers thinking. “I can’t sell these cherries.”

The pest was the spotted wing drosophila, or SWD, a tiny insect native to Japan and eastern Asia that was first discovered near Watsonville, Calif., in August 2008 and has since spread to many areas from the Mexican to the Canadian borders, and around the country. Aside from the males having spots on their wings, it is similar to the common vinegar flies, popularly called fruit flies, that almost everyone has in their kitchens in the summer, but with one crucial difference: SWD attacks fruit as it ripens on the tree, laying eggs that develop into maggots.

As it turned out, Tenerelli was able to salvage half of last year’s crop by picking only the upper parts of his trees, which were not affected by the pest that has been vexing growers around the state.

But this season he’s taking more extreme measures. Although he had never sprayed his cherries with pesticides before, he is spraying three times to make sure the pest does not reappear.

The sprays — insecticides such as spinosyns, pyrethroids and organophosphates — kill beneficial insects and seem likely to lead to the development of insecticide-resistant pests, requiring even more intensive spraying.

At-risk fruit

But packers, retailers and most consumers have basically zero tolerance for wriggling worms in fruit, so if even a few infested samples are found in a shipment, the whole lot has to be sent to a landfill. Last year many cherry growers were caught by surprise by this new pest and lost their crops.

It is unlikely that consumers will encounter infested fruit at farmers markets or supermarkets, but that doesn’t mean they are entirely unaffected. Farmers, whether conventional or organic —including many who in the past rarely or never applied pesticides — are now being forced to spray if they do not wish to lose their crops.

The upshot is that no one should avoid fruit because of this issue, but it is more important than ever to wash it thoroughly before eating.

The sprays must be applied before the fruit ripens and the females lay their eggs; once they have laid their eggs, there is no way to kill them or save the crop.

Fruits with thin skins, such as cherries, raspberries and blueberries, appear to be most susceptible. Scientists are still learning which other crops may be affected, and to what degree, but the list seems to include peaches (especially if they’re tree-ripe, and their fuzz isn’t too long), nectarines, plums and grapes. Hard fruits, such as apples and pears, and thick-skinned fruits, such as citrus, are unaffected when intact, but almost any fruit can serve as a host if damaged or decaying.

Entomologists have found that SWD males go sterile at temperatures above 85 degrees, so hot areas like the San Joaquin Valley may be spared this problem during the peak summer months, although it may return in autumn; the most seriously affected regions are likely to be cooler coastal districts, especially those where raspberries and other soft fruit are grown. Spray regimes require time, money and careful attention to logistics, so coping with SWD may prove difficult for small, diversified growers, home gardeners and, of course, those who do not choose to spray.

Growing resistance

Teams of scientists from universities, government and industry are searching for better ways to fight SWD. Robert A. Van Steenwyk, an entomologist at UC Berkeley, is looking for more narrowly targeted insecticides and hopes to have some answers for growers by this winter, although he says it will take three to four years to complete his research. “We’ll have some things for organic and conventional growers that are minimally disruptive, and also effective,” he says.

Time is critical because repeated use of insecticides to suppress SWD will cause the pest to become resistant to these substances “within five or so years, maybe 10 years,” Van Steenwyk says.

Other scientists will be traveling to Asia to look for natural methods of control, such as parasites and predators of SWD, but even under the best of circumstances it will take years to import such insects, put them through quarantine, gain the needed approvals and establish effective populations in the U.S., Van Steenwyk says.

Meanwhile, spraying causes additional headaches for growers. Tenerelli appears to have been successful so far in keeping his cherries clean, but he worries that by spraying, “you’re killing a lot of good bugs too,” such as green lacewings and ladybugs needed to prevent mite infestations later on.

When he tells customers that he has had to spray his cherries this year, most understand that he had no choice, but others move on, he says.

food@latimes.com