After a one year hiatus the dreaded Drosophila Suzukii, or spotted-wing drosophila, is set to make a big return to Swiss vineyards in 2016. Early reports from Swiss cherry orchards are not encouraging with levels of drosophila infestation at the end of June matching the August highs from 2014, a particularly bad year (Agroscope Newsletter: Drosophila Suzukii—Nº 141/2016).
Drosophila Suzukii differs from its relative the Melanogaster in the timing of its attack. The Melanogaster is partial to overripe and decaying fruit well past the point of harvest while the Suzukii prefers fruit that is approaching peak ripeness simultaneous with harvest. The Suzukii female, easily distinguished from the male with its long, serrated ovipositor, punctures the skin of its host to deposit eggs within the fruit. From there other microbes have access rendering the host fruit unfit for wine production. The resulting aroma and flavor of vinegar is pronounced and a fatal flaw in the finished wine.
The infestation’s proximity to harvest does not allow much room for synthetic pesticide treatments leaving mostly natural remedies as the most expeditious recourse. Among these are clay-based sprays that disguise the color of the grapes, red grapes are a preferred target; custom traps that contain a solution of apple cider vinegar, sugar and wine that require a grid-like placement around the perimeter of a vineyard; and a silica-based solution spiked with tea tree oil to confuse the scent receptors of the insect. Each of these solutions requires extra work at the busiest and most crucial time of the year and none are fool-proof. Post infection the only remedy is a grape-by-grape triage with the negatives of fallibility and expense.
Early preparation can mitigate some of the risk. Dropping leaves to increase the flow of air through the vineyard can be helpful but with as much moisture as we have seen in the spring and early summer soil humidity is elevated and optimal for insect development. Hot, dry and windy conditions from here on out will be of great help.
To follow the course of this developing story check in with the Drosophila Newsletter at: agroscope.admin.ch