Drosophila suzukii is a highly polyphagous pest species that can also attack grape. To prevent economic damage in vineyards, it is important to detect D. suzukii infestation early in order to protect grapes efficiently. Here, we firstly examined if peripheral zones adjacent to natural habitats and central zones within a vineyard differ in infestation levels, and, secondly, where within a grape cluster most eggs are laid. Among the eight infested vineyards studied, seven had higher D. suzukii infestation levels in peripheral zones adjacent to hedgerows, forests or gardens. Females laid 2.0-fold more eggs on berries in the interior of grape clusters than on berries on the exterior of the cluster. Based on the finding that berries in the interior of grape clusters are more vulnerable, we developed a novel sampling method to estimate D. suzukii infestation level in vineyards more effectively. The so-called ‘whole cluster method’ consists of a representative but random collection of five grape clusters per plot and a visual inspection of five berries each from the inner and outer part of a cluster totalling 50 checked berries per plot. We then compared our newly developed method to two established sampling methods to estimate D. suzukii infestations (i.e. ‘single berry method’ and ‘cluster fragment method’), which rarely collect and inspect berries from the interior of clusters. In 87 comparisons conducted in 35 different plots, the ‘whole cluster method’ was the most sensitive sampling method as the calculated mean infestation rate was highest and it identified eggs earlier and in more samples than the other two methods. We therefore believe that the ‘whole cluster method’ is currently the most effective method to assess D. suzukii infestations in commercial vineyards.
