Drosophila suzukii

Wild-fruit diversity and use by Drosophila suzukii In total, 11 wild berry fruit species were collected in 2016 (Fig. 1A), and 16 species were collected in 2017 (Fig. 1B). The availability of these species’ fruits for sampling varied with year and field site, as expected. Among sampled fruit species in 2016 and 2017, 5 and 11, respectively, were recorded as suitable hosts for full development up to fly emergence of D. suzukii (Table 1; Supplementary material, Table S2). Wild-fruit species Aralia hispida (Araliaceae), Prunus pensylvanica (Rosaceae), and Sorbus americana (Rosaceae) are reported here for the first time as suitable hosts for D. suzukii (Table 1). Additional wild hosts, Viburnum nudum var. cassinoides (Adoxaceae), Ilex mucronata (Aquifoliaceae), Cornus canadensis, C. sericea (Cornaceae), Vaccinium angustifolium, Aronia melanocarpa (Rosaceae), Prunus virginiana (Rosaceae), and Rubus idaeus (Rosaceae), are also recorded in the region of this study. The five wild-fruit species from which no D. suzukii flies emerged (Table 1) were Maianthemum canadense (Asparagaceae), Arctostaphylos uva-ursi (Ericaceae), Gaultheria procumbens (Ericaceae), Amelanchier bartramiana (Rosaceae), and wild strawberry, Fragaria virginiana (Rosaceae). (Continued) 1 All native in eastern North America. 2 Emerged from fruits in Nova Scotia, Canada 2017. 3 Not infested in Maine, United States of America in 2015 – 2016 (four sites). 4 Infested in Maine, 2015 (one site). 5 Not infested in Newfoundland, Canada, 2017. 6 Also known as Chamaepericlymenum canadense (Linnaeus) Ascherson and Graebner. 7 Infested in Maine, 2015 – 2016 (12 sites). 8 Also known as Swida sericea (Linnaeus) Holub. 9 Infested in Oregon, United States of America, 2010 – 2013 (five counties). 10 Not infested in 2017 (cultivated endemic). 11 Sampled in Atlantic provinces, Canada in 2017, no infestation reported. 12 Not infested in Okanagan Valley, British Columbia, Canada (many sites, 2010 – 2016). 13 Tested as (European) exotic host (Poyet et al. 2015); eggs fail to hatch (Poyet 2014). 14 Blueberry infested in forests bordering cultivated blueberry, 2015 – 2016 (5 – 6 sites). 15 Not infested in Atlantic Canada, 2017, citizen science project; other Amelanchier spp. not infested in Michigan, United States of America (2011) and Oregon (2012); suitable congeneric hosts known in Europe (Thistlewood et al. 2019). 16 Infested in Wisconsin, United States of America, 2015, suboptimality revealed in bioassays. 17 Not infested in Atlantic Canada, 2017, citizen science project. 18 Fruits support full development (Gong et al. 2016). 19 Not infested in Atlantic provinces, 2015. 20 Infested in Okanagan, British Columbia, four years in early 2010s. 21 Fruits infested in New York State, United States of America in 2013 (16 sites). 22 Negative in Maine, 2015 – 2016 (9 – 15 sites). 23 Rubus spp. sampled in 2015 – 2016 (15 and 9 sites). 24 Not infested in Oregon, 2012 (one site). Seasonal variation Of the five berry fruit species sampled that were not used as hosts by D. suzukii in the present study, bearberry, A. uva-ursi, and Canada mayflower, M. canadense, were sampled only in 2017, and eastern wintergreen, G. procumbens (2016 and 2017), was sampled at both the start and end of the growing season. In 2016, the species infested the earliest, in the third week of August, were C. canadensis and V. angustifolium, followed in the next two weeks by A. hispida, A. melanocarpa, and V. nudum var. cassinoides (Fig. 1A). In 2017, with sampling extended to 16 species, the earliest infestation occurred on P. pensylvanica during the first week of August (Fig. 1B), about one week earlier than on A. hispida, C. canadensis, and R. idaeus and at the same time as infestation of lowbush blueberry. Infestation of C. sericea and P. virginiana fruits, and then of S. americana and A. melanocarpa, followed in late August to early September. In 2016, wild-fruit species with the longest period of infestation were C. canadensis (six weeks, up to the third week of September) and wild V. angustifolium (10 weeks, up to the third week of October; Fig. 1A). In 2017, fruit species that were infested the longest were S. americana and V. nudum var. cassinoides, infested at the same time for seven weeks up to the second week of October, and lowbush blueberry V. angustifolium, infested for 11 consecutive weeks up to the third week of October (Fig. 1B). Seasonal fruit suitability of A. hispida also lasted seven weeks but occurred three weeks earlier (Fig. 1B). Infestation-level variation with fruit species Infestation level (flies emerged per 100 g) strongly varied between fruit species for both 2016 (F 4, 16 = 86.36, P <0.0001; Fig. 2A) and 2017 (F 9, 39 = 5.47, P <0.0001; Fig. 2B). Poor statistical discrimination between species is attributable at least in part to unequal fruit species representation across sites. Infested fruits of Canadian bunchberry (C. canadensis) produced 8–11 times more D. suzukii flies than did lowbush blueberry, clearly making bunchberry the principal wild-fruit host species used in both years. In 2016, C. canadensis and A. hispida were the two most productive species, with more than 100 flies per 100 g of collected fruit on two or three sampling occasions in September. More diverse sampling in 2017 added P. pensylvanica and R. idaeus as highly productive wild-fruit species. Considering trends for both years, wild-fruit infestation levels peaked near mid-September, being greatest on C. canadensis in 2016 (Fig. 3A) and greatest on A. hispida in 2017 (Fig. 3B). Prunus pensylvanica, R. idaeus, and C. canadensis produced abundant D. suzukii flies about two weeks before A. hispida did, but this latter species then reached the highest weekly infestation level recorded, at about 650 flies per 100 g. All other wild fruits, and lowbush blueberry crop fruits, showed moderate (50–100 flies/ 100 g) to low (<50 flies/ 100 g) levels of infestation. Aborted larvae and pupae were rare and were not included in infestation-level estimates. Drosophila suzukii sex ratios on wild fruits The overall sex ratio of adult flies emerging from Canadian bunchberry samples in 2016 was 0.52, not significantly different from 1:1 (Χ 2 1 = 1.44, 0.10 <P <0.25, n = 669). Data for other wild-fruit species were pooled based on nonsignificant heterogeneity chi-square (Χ 2 2 = 1.44; 0.25 <P <0.50, n = 60), the overall sex ratio (0.58) also not being different from 1:1 (Χ 2 1 = 1.67; 0.10 <P <0.25, n = 60). In 2017, the sex ratio of larger numbers of emerging adults from Canadian bunchberry samples was 0.54 and was significantly female biased (Χ 2 1) = 37.56; P <0.001, n = 4652). Pooled data for other wild-fruit species after checking for homogeneity (Χ 2 7 = 8.14, 0.25 <P <0.50, n = 1025) indicated that the sex ratio (0.56) was also female biased (Χ 2 1 = 12.90, P <0.001, n = 1025). When the sex ratio of flies from wild fruits was compared to that from lowbush blueberry crop fruits, no significant difference in either year was shown (2016: Χ 2 1 = 0.38; 0.50 <P <0.75, n = 1789; 2017: Χ 2 1 = 0.21; 0.50 <P <0.75, n = 6741). Field infestation levels in relation to wild-fruit characteristics Supplementary material, Table S2 shows the measured quantitative variables of the 10 fruit species sampled (fruit diameter, sugar content, and colour bands) that were used in modelling field infestation level as a function of fruit characteristics. We also used Family as a taxonomic (nominal) variable in modelling, Family being the sole sufficiently replicated species diversity variable insuring that number of explanatory variables in model did not exceed number of observations. None of the nine variables considered could explain observed field infestation levels, the model as a whole clearly not being significant (F 8,1 = 2.03, P = 0.4971; Table 2). Canadian bunchberry versus lowbush blueberry fruit preference experiments No-choice experiment. The Drosophila suzukii females’ daily egg-laying rate was similar in Canadian bunchberry and lowbush blueberry fruits (F 1, 21 = 0.58, P = 0.4553; Fig. 4), with approximately two eggs laid per day in ripe fruits made available. There were no significant effects of time into the experiment (F 3, 58 = 2.24, P = 0.0935) over the 12-day period of testing or of its interaction with fruit species (F 3, 58 = 2.27, P = 0.0896). Choice experiment. When given choice between a fruit of Canadian bunchberry or one of lowbush blueberry exposed simultaneously, female egg laying significantly differed (F 1, 77 = 9.24, P = 0.0032), with about twice as many eggs being laid on lowbush blueberry than on Canadian bunchberry (Fig. 5A). Time (days) also significantly (F 3, 77 = 3.68, P = 0.0156) affected egg laying (Fig. 5B), with no interaction of time and fruit species (F 3, 77 = 0.47, P = 0.7045), despite females apparently laying slightly more eggs earlier in the experiment.

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