Abstract

Drosophila suzukii is an invasive pest of soft and stone fruit originating from southeast Asia, which is now present in most northern temperate regions after multiple invasions. D. suzukii has the uncommon ability among Drosophila species to oviposit in ripening fruit and like other Drosophila species is attracted to volatile metabolites produces by individual fruit associated yeast isolates. To aid survival during unfavourable temperate winters, D suzukii enters a reproductive diapause, a behaviour that is associated with a morphologically distinct winter phenotype. The D. suzukii morphs occupy different niches and very in olfactory attraction. Yeast baits have the potential to be used to target phenotypes in attract-and-kill strategies, as liquid baits in traps and as phagostimulant baits with insecticides.

In Chapter 2, two-way laboratory choice tests and field trapping revealed D. suzukii was attracted to volatile metabolites produced by several yeast species and combinations, most notably Metschnikowia pulcherrimaand Hanseniaspora uvarum, seperately and when combined post-fermentation. Chapter 3 used a DNA metabarcoding approach to investigate general fungal and Saccharomycetales yeast communities during ripening on strawberry, cherry, raspberry, and blueberry. On average there was a 5-fold greater difference in fungal communities between fruit types than across maturation stages. Additionally, raspberry, which has a higher susceptibility to D suzukii compared to other fruits, had a greater prevalence of Saccharmycetales yeast attractive to D. suzukii, including H. uvarum. Chapter 4 demonstrated that preference for volatile metabolites from both single yeast species and combinations varies between summer- and winter-morphs. Most notably Candida zemplinina, when fermented in sterile strawberry juice or yeast peptone dextrose media (YPD), was more attractive to winter-morphs. However, co-fermented yeasts were no more attractive than post-ferment mixes of constituent yeasts or single species. Chapter 5 identified a number of effective yeast phagostimulant baits in combination with insecticides. There was some evidence that combinations of yeasts were more effective than single species. M. pulcherrima + H. uvarum, combined with lambda-cyhalothrinwas more effective than H. uvarum for both summer- and winter-morph flies and Candida zemplininia for summer-morphs. As with attraction, the effectiveness of yeast phagostimulant baits varied between D. suzukii summer- and winter-morphs. C.zemplininia or H.uvarum + C.zemplininia were more effective phagostimulant baits against winter-than summer-morphs when combined with lambda-cyhalothrin or cyantraniliprole as well as H. uvarumcombined with cyantraniliprole.

This thesis has added to the growing body of knowledge of D suzukii: yeast interactions and their possible exploitation in pest control. Several yeast species and combinations that are attractive and effective phagostimulant baits in combination with insecticides of novel attractive and selective yeast baits for D suzukii morphs has potentially important implications for season-long control where different baits should be used to optimise attraction to the different phenotypes.