Every year at about this time, we spray clay on the olives to stop the olive fruit fly – dacus oleae or bactrocera oleae – laying her eggs and ruining the fruit. The olive fruit fly is a major pest for olive growers and can wreck a crop: the larvae hatch, eat the fruit which rots and, of course, this ruins the taste of the oil, which is why the AOC only allows a maximum of 10% damaged fruit. Untreated, a full on attack by la mouche will easily ruin more than that.
The spraying start-date depends on weather, temperature and altitude, all of which affect when the fruit fly hatches and flies off looking to lay eggs in our olives. This year, a friend on the coast sprayed on Friday 8 July, while we started on Tuesday 26 July.
The clay is a very fine powder. We put 7+ kgs in 300 litres of water in a sprayer fitted to the tractor’s 3-point linkage, and use a lance to spray the mixture onto the fruit on the trees.
The water suspension dries and leaves a fine film of clay on the olives. When dacus oleae – la mouche – lands to pierce the fruit and lay her eggs, she finds things are not as she likes… and flies away to lay elsewhere.
We estimate the treatment is 90 – 95% effective, which means it is as effective as chemical insecticides, but clay is an inert substance so the treatment doesn’t harm any insects and is biologically sound.