- Author: Mark Bolda
Previous posts have written about the use of chemical sprays or baits to control the recent onslaught of spotted wing drosophila, but growers should know that they have another very powerful tool in managing this pest: enhanced field sanitation.
Since spotted wing drosophila lays eggs in and develops in fruit (see photo #1 below), physical removal of the fruit from the field, either by harvest or discarding, will also remove the ability of the fly to reproduce and increase in number. Furthermore, fruit, among other things such as fungal spores and pollen, is a source of food for spotted wing drosophila. Males do not lay eggs, yet are constantly seen wandering around on fruit, and the flies are frequently seen with bodies filled with red fluid obviously extracted from fruit.
While we are not certain whether spotted wing drosophila females will oviposit on rotten fruit, we know that one, they complete their lifecycle and pupate there, and two, that spotted wing drosophila feeds quite well on rotten fruit (see photos #2 and 3 below). Therefore, removing overripe and rotten fruit fulfills two goals of fly management: removing the source of food and removing the source of reproduction.
Sanitation for strawberry growers consists of the same strategies as those used for limiting Rhizopus rot and Botrytis grey mold. Overripe and rotten fruit are removed from under the plant canopy and deposited in the furrow. Passing over the fruit with the tractor from time to time will smash it, but more importantly will foment rapid desiccation of the fruit, which is fatal to spotted wing drosophila.
Caneberry sanitation is more complicated than for strawberries simply because of the great difficult of introducing machinery into the field once harvest has begun. Up until the recent infestation of spotted wing drosophila, it has been common for pickers to deposit rotten or overripe fruit at the foot of the hedgerow (see photo # 4 below). However, this situation is very advantageous to vinegar flies, since it allows fruit to fester for long periods in the shade and high humidity of the caneberry hedgerow. The lifecycle of spotted wing drosophila is very easily completed here, as well as providing an abundance of food. A change in how raspberry and blackberry harvest is conducted which includes the removal from the field and destruction of cull fruit will result in a dramatic reduction of vinegar flies including spotted wing drosophila.