Driving along Highway 45 two Saturdays ago, I saw something that sparked an "Aha!" moment. It led me to new information about the prevention of olive fly infestation, which is quite timely just now. If you grow olives for canning or oil in Napa, you must beware of the olive fly. It is found throughout the county, and almost certain to spoil your crop if you don't take aggressive steps.

What caught my attention, among miles of lush green orchards, were whole groves of walnut trees covered with white dust. This odd and surprising sight reminded me of a picture I'd seen in an organic garden supply catalogue, of an apple tree coated with kaolin, a finely powdered white clay, which is used to prevent codling moth infestation in apples. The non-toxic clay coating effectively disguises the fruit, so the adult moths just don't recognize it as a viable site on which to lay their eggs.

I assumed that organic walnut growers are using kaolin to prevent codling moth or husk fly infestation. Knowing that husk flies are somewhat related to olive flies and that they have similarities in their life cycles, I realized the kaolin treatment might have promise for non-toxic prevention of olive fly infestation.

When I returned home, I found that my Aha! moment was way behind others who have already researched the idea and shown that it works.

My first link, through ucdavis.edu led me to Mitchell King, who has been working on the olive fly problem. In a phone interview he told me that research done in Syria showed that the kaolin treatment is effective, and further trials in California have shown some "great results." "It's every bit as effective as dimethoate, and a kaolin treatment lasts longer" he said. Dimethoate is a toxic pesticide that has proved effective in controlling the pest. King added that kaolin is registered for use in California, it's a food grade material and it presents no problems for canning and olive oil production.

King directed me to UC Extension farm advisor Paul Vossen, in Sonoma County, who has the data on it.

"Surround (a trade name for kaolin) works extremely well!" Vossen said. He added that treatments should be repeated at 5 to 6 week intervals and the effectiveness may or may not last that long.

What makes this all so timely is that the application must be made when pit-hardening occurs, which is right about now in Napa Valley.

To check this at home, pick a young olive and slice through it with a sharp blade. If you feel the resistance of the young pit, it's time to act.

Another call, this time to the Napa County Agriculture Commissioners office, put me in touch with Craig Graffin, who is working on the problem locally. "Kaolin is great but homeowners are going to have to love those olives." Effective treatment requires thorough coverage of all surfaces of the fruit every seven to 14 days, he said, and the material may not spread well without power spray equipment. Craig added that users must beware of the residue, which is a glaring white, and tends to coat everything nearby.

He went on to explain that spinosad, another effective material that is registered for use on organic farms, has now been registered for use by homeowners, if they obtain a special use permit. It's sold as GF120, a combination of spinosad with a bait-attractant that is simply sprayed into a small portion of each tree's canopy.

Whatever you choose, don't wait. Timing is critical.

Even though my inspiration was a little bit behind the growers and researches, it turned out to be good thinking. I love when that happens.

Bill Pramuk is a registered consulting arborist. Please e-mail questions to bpramuk@pacbell.net