Comparison of extraction methods for quantifying Drosophila suzukii (Diptera: Drosophilidae) larvae in soft- and stone-fruits

Spotted wing drosophila, Drosophila suzukii Matsumura (Diptera: Drosophilidae), is an invasive pest of soft- and stone-fruit, which has become established throughout temperate regions of the world in the last decade. A number of simple methods have been developed in order to detect larvae in fruit so preventative methods may be initiated to reduce damage and prevent the dissemination of the pest from contaminated material. The detection of larvae in fruit may be the first indication to many growers that they have D. suzukii in a crop. Within this project, grower 'friendly' methods were tested and evaluated for time, ease of method and accessibility to equipment. Extraction methods for detecting 1st-2nd and 3rd instar D. suzukii larvae from cherry, blueberry, raspberry and strawberry fruits were compared. Methods including freezing, flotation in sugar (sucrose), salt (NaCl) or detergent solution, dissection of fruit under a microscope and natural emergence were assessed for recovery rates. Natural emergence and/or dissection gave the highest recovery rates of 1st-2nd and 3rd instar larvae. Extraction using a concentrated sugar solution (180 g/L) gave consistent counts of larvae, and was significantly quicker than natural emergence and dissection methods. Salt extraction performed as well as sugar in most cases and freezing was consistently poor and not an effective way of detecting 1st-2nd instar larvae. We discuss the benefits and downfalls of each method from a fruit grower's perspective.