Biological, Behavioral and Preventative Management of the Invasive Insect Pest, Spotted Wing Drosophila (Drosophila suzukii Matsumura), in Maine Lowbush Blueberry (Vaccinium angustifolium Aiton)

This research was conducted in order to identify the potential for utilization of various management techniques against the invasive Drosophila suzukii Matsumura, commonly referred to as the spotted wing drosophila (SWD), using Maine lowbush blueberry (Vaccinium angustifolium Aiton) as a model crop system. These included evaluations of three prospective approaches often considered when developing agricultural pest management programs for novel insect pests: 1) biological control through the intended release of natural enemies, in this case entomopathogenic fungi; 2) behavioral management through mass trap deployment in order to capture and kill adult SWD, and; 3) prevention through the deployment of insect exclusion netting during the pre-harvest fruit ripening period. The first assessment was accomplished through complementary laboratory and field experiments. Mass-inoculation laboratory assays with four species of fungi resulted in significant mortality of SWD flies over five days post-exposure (P < 0.0001). While both Beauveria bassiana (strain GHA) and Metarhizium anisopliae (strain F-52) were among the most lethal isolates, only B. bassiana mycoses were shown to exert a significant dose-mortality response over a three day period following initial contact with conidia (P < 0.0001); based on the data obtained, the derived LC­­50 value corresponded to a pathogen surface density of approximately 16,000 conidia mm-2. Although no detectable mortality effect was found during the M. anisopliae assay (P = 0.64), the frequency of sporulating fly cadavers increased substantially at elevated conidia doses of either fungal pathogen (P < 0.0001). A sub-lethal assessment of B. bassiana mycosis on reproductive development in immature D. suzukii females also generated support for decreased oocyte maturation rates in individual flies (P = 0.02). Coupled with the observable germination of conidia through SEM imaging, these results provide strong evidence for positive infection under laboratory conditions. Despite these promising results, however, the subsequent field evaluation of a commercially available B. bassiana (strain GHA) containing myco-insecticide yielded no additional evidence that could justify these entomopathogens as being feasible biocontrol agents in SWD management. Spraying blueberry enclosures prior to the introduction of 2,000 adult SWD failed to reduce the quantity of larvae inhabiting fruit samples, with 59 ± 63 (SD) vs 28 ± 19 obtained in sprayed vs unsprayed plots, respectively. Objectives two and three entailed field experimentation only with lowbush blueberry. Mass trapping with volatile semiochemicals was evaluated at different trap concentrations. Varying the spatial arrangement of traps within study grids significantly influenced the quantity of SWD larvae infesting sampled blueberry fruits (P = 0.0003). The trap design and bait tested here were most effective when deployed at the lowest density (0.9 m trap spacing). Fruit samples collected from crops provided this treatment contained mean larval infestations of 1.5 ± 1.8 (SD). For comparison, the deployment of traps with 1.8 and 2.7 m of trap spacing resulted in larval sampling averages of 8.8 ± 11.1 (SD) and 17.3 ± 13.7, respectively. However, there was no detectable treatment effect of trap spacing on the mean number of adults captured in traps (P =0.40). The results of this field investigation, in conjunction with those of other studies, might justify additional research on trap cropping in order to reduce the overall degree of chemical inputs required to adequately suppress fruit infestation. The final objective produced results consistent with those of analogous investigations, which have shown insect-netting to be an effective preventative agent for physical exclusion of SWD flies from contacting viable host fruits prior to harvest intervals. Studies conducted in the lowbush blueberry agroecosystem during summer of 2014 and 2015 provide further support for this conclusion; net-protected fruits contained an average of 0.2 ± 0.2 (SD) larvae, in comparison to uncovered control fruits in which an average of 5.2 ± 3.9 larvae were sampled (P < 0.0001). In order to confidently implement novel management techniques for suppressing SWD infestations, the observations gathered in this assessment cannot justify the immediate utilization of any technique as a replacement for insecticidal treatments. Even the positive results obtained from insect-netting experiments were constrained by limitations of spatial practicality with respect to application in large scale fruit growing operations. Therefore, additional experimentation will be necessary before identifying any of these techniques as viable approaches to incorporate with developing integrated pest management programs.