Simulated host potential index (HPI) values (30 simulated HPI values) for five northern half-high blueberry (Vaccinium sp.) cultivars. Red dashed lines indicate the mean HPI value for the cultivar. Red dotted lines illustrate the 95% confidence intervals (2 * SEM). Higher HPI values correspond to a greater preference by spotted-wing drosophila.
The effect of blueberry (Vaccinium sp.) fruit quality on spotted-wing drosophila (SWD), Drosophila suzukii (Matsumura) host suitability. (A) The amount of SWD oviposition (number of eggs/berry) as a function of blueberry fruit firmness (g force/mm2) (adjusted R2 = 0.2572; P = 0.007). Berries were harvested from the Sand Plains Research Farm in Becker, MN, and fruit firmness was measured on five berries of each cultivar. Other berries from the same harvests were used for host choice assays in which mated pairs of SWD were introduced to individual berries. (B) The proportion of SWD eggs that reached adulthood as a function of blueberry fruit pH (adjusted R2 = 0.1961; P = 0.030). Fruit pH was measured on 20 mL juice samples obtained from 250 g of berries. Mated pairs of SWD adults were introduced to individual berries, removed after 24 h, and counted after 14 d.
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Spotted-wing drosophila (Drosophila suzukii; SWD) is an invasive pest in the United States that is responsible for significant economic damage to soft-skinned fruit and berries worldwide. SWD uses a wide variety of cultivated and wild fruit for reproduction. Host suitability may depend on physical and chemical factors of the fruit, with a positive correlation of SWD oviposition and larval development generally associated with soluble sugar content, softer fruit, and higher pH, and a negative correlation of oviposition with fruit firmness. Variety selection is an important tool for integrated pest management, but few studies have reported host suitability across varieties within a single cultivated crop species for SWD. In this study, we investigated SWD oviposition and larval development in five half-high blueberry cultivars, Chippewa, Northblue, Northland, Patriot, and Polaris, using no-choice and two-choice laboratory bioassays. Using a host potential index, our results showed that Chippewa was the most preferred cultivar for oviposition as measured in the number of eggs laid per fruit, and Polaris was the least preferred. The inverse was true for larval development, with a higher survival rate and adult emergence in ‘Polaris’ than in ‘Chippewa’. There was a negative relationship between fruit firmness and oviposition and a positive correlation between pH and larval development. The results of this study indicate that cultivar selection for half-high blueberries may be a promising integrated pest management (IPM) tool, although further research under field conditions is needed for validation.
Spotted-wing drosophila (Drosophila suzukii; SWD) is an invasive insect in North America, South America, and Europe, and it is a significant pest to soft-skinned fruit and berries in these regions (Calabria et al. 2012; Deprá et al. 2014; Hauser 2011). Female SWD possess a serrated ovipositor, allowing them to deposit eggs inside ripe and ripening fruit (Atallah et al. 2014). After hatching from eggs, the larvae proceed to feed on the interior of the fruit, rendering it unmarketable (Walsh et al. 2011). SWD exhibit host use plasticity and use a wide variety of cultivated and wild fruit for reproduction (Bellamy et al. 2013; Lee et al. 2011).
Since SWD was first detected in the United States in 2008, numerous management tactics have been investigated and can contribute to an integrated pest management program for SWD (Tait et al. 2021). To combat SWD infestations, insecticide application (Gullickson et al. 2019; Haviland and Beers 2012; Sial et al. 2019; Van Timmeren and Isaacs 2013), increasing harvest frequency (Diepenbrock et al. 2017; Leach et al. 2018), exclusion netting (Cormier et al. 2015; Ebbenga et al. 2019; Leach et al. 2016; Rogers et al. 2016), and field sanitation and trapping programs (Hampton et al. 2014; Leach et al. 2018) have been researched and implemented, thus contributing to increased production costs (Farnsworth et al. 2017).
The economic impact of SWD on fruit crops varies by crop and region (Tait et al. 2021). Early research estimated that revenue losses from SWD infestations on raspberries, blackberries, blueberries, and strawberries are more than $500 million per year in California, Oregon, and Washington combined (Bolda et al. 2010; Walsh et al. 2011). However, more recent research found that SWD caused only $39.8 million in revenue losses for California raspberries between 2009 and 2014 (Farnsworth et al. 2017). For wild blueberries in Maine, which are likely more comparable in market size to that of the Upper Midwest, SWD infestations were found to cause revenue losses ranging from $1.1 to $6.9 million per year (Yeh et al. 2020). In Minnesota alone, raspberry losses caused by SWD in 2017 ranged from 2% to 100% for different farms, with the median yield loss equal to 20% and valued at approximately $2.36 million in sales (Digiacomo et al. 2019). The economic impact of SWD on blueberries in the Upper Midwest may be lower than that on raspberries because SWD prefers to oviposit in raspberries (Bellamy et al. 2013).
Cultivar selection has not been fully evaluated as an IPM strategy for SWD despite evidence of its efficacy against arthropod pests in several horticultural production settings. For example, Hampton et al. (2014) conducted a study of high-bush blueberry (V. corymbosum L.) cultivar susceptibility to SWD and found that rates of SWD infestation differed among cultivars (Hampton et al. 2014). Specifically, earlier-ripening cultivars had lower SWD infestations than later-ripening cultivars (Hampton et al. 2014). Similarly, Kamiyama and Geudot (2019) investigated the susceptibility of different tart cherry (Prunus cerasus L.) cultivars and found that the number of eggs and adults that emerged from tart cherry depended on the ripening state and cultivar interactions. Fruit of some cherry cultivars were more susceptible at the ripening or ripe stage compared with the unripe stage. Intriguingly, one cultivar, Kántorjánosi, had the most SWD oviposition, but it was among the lowest for adult emergence (Kamiyama and Guedot 2019). Finally, among mapping populations of strawberries, research has shown a positive correlation between methyl anthranilate and SWD oviposition preference (Bräcker et al. 2020).
Cultivar differences in host fruit suitability may depend on the physical and chemical factors of the fruit. Lee et al. (2011) reported positive correlations for SWD oviposition and larval development associated with soluble sugar content, softer fruit, and higher pH, and a negative correlation between oviposition and fruit firmness. Regarding table grapes (Vitis sp.), cultivars with higher sugar and lower acidity were more susceptible to SWD infestation (Ioriatti et al. 2015). In contrast, Ebbenga et al. (2021) observed successful SWD larval establishment in only six of the 34 cold-hardy grape varieties assessed in Minnesota, with some having less than 1% of berries infested. The diversity of SWD infestation results among highbush blueberries, tart cherries, and grapes suggests that there are differences among other fruit crops as well. Bellamy et al. (2013) developed a host potential index (HPI) to evaluate SWD preference and suitability among seven cultivated fruits. The HPI ranks fruits based on how often SWD selected them when compared with different fruit options. The number of times SWD selects a potential host compared with an alternative host is used to determine the preference proportions for that choice. The preference proportion values are averaged when the potential host has been tested against all the other hosts. This allows researchers to compare one cultivar to all others, rather than as an individual pairwise comparison, for example, to determine the most preferred host when host A is weakly preferred over host B, host B is weakly preferred over host C, but host C is strongly preferred over host A. Although Bellamy et al. (2013) did not investigate cultivar differences, this method may provide a valuable tool for determining SWD preferences among cultivars of a single fruit species.
Blueberries can be infested by SWD, but several studies suggest that SWD prefers other fruits, such as strawberries and raspberries over blueberries (Abraham et al. 2015; Bellamy et al. 2013; Lee et al. 2011b). Half-high blueberries (Vaccinium corymbosum/V. angustifolium), which are hybrid varieties of high-bush and low-bush blueberries (V. angustifolium Ait.), are popular in cold-climate regions because of their increased winterhardiness and greater yields compared with low-bush plants (Finn and Luby 1992). Early-ripening cultivars of high-bush blueberry can be harvested before SWD begin to oviposit, thus avoiding fruit damage (Hampton et al. 2014). The amount of SWD found on a Minnesota farm in half-high ‘Polaris’ was low for the first 3 weeks of harvest before 17 July, and significantly higher for the last 2 weeks of harvest (24 Jul and 30 Jul) (Gullickson, unpublished data). These data are similar to what was observed during previous research (Gullickson et al. 2020), but data for cultivar preference, phenological overlap of fruit development, and SWD oviposition timing for other half-high cultivars are lacking.
The objectives of this research were to determine the host preferences and suitability of different cultivars of half-high blueberries for SWD and whether SWD preference or suitability was correlated to the chemical (pH, total soluble solids) and physical (firmness) properties of the fruit. We hypothesized that there would be differences in SWD host preference and quality among the cultivars, and that differences may be explained by fruit physical and chemical properties. This information may contribute to the body of IPM research of SWD and provide growers with information about half-high blueberry cultivar preference and suitability for SWD.
Blueberries were harvested from the Sand Plain Research Farm of the University of Minnesota in Becker, MN (45°23′36.42″N, 93°52′36.3″W) and included five cultivars, Chippewa, Northblue, Northland, Patriot, and Polaris, with three replications or bushes of each cultivar chosen. Branches with berry clusters in the green–pink stage were netted for protection from SWD infestation and left to ripen in the field. Mature, bagged fruit was harvested from each plant. This was repeated twice for two harvests, 23 Jun and 6 Jul 2018. Individual berries were placed in lidded 37-mL containers (Frontier Agricultural Sciences, Newark, DE, USA) along with mated 3- to 5-day-old adult SWD (five male and five female) and kept in a growth chamber at 23 to 25 °C with a 16-h light and 8-h dark photoperiod and 47% relative humidity. Adults were removed after 24 h, and the number of eggs per fruit was counted. Individual berries were returned to their 37-mL containers and the growth chamber for an additional 14 d; thereafter, the numbers of larvae, pupae, and adult flies were counted and recorded for each sample. The survival rate was calculated based on the number of adult SWD at the end of 14 d divided by the number of eggs counted at 24 h, similar to the methods described by Aly et al. (2017).
In 2019, a series of two-choice tests between the aforementioned blueberry cultivars was conducted to determine relative preference in a choice setting. Individual berries were weighed to control for berry size, and only berries of the same weight were used for comparisons. Choice arenas consisted of two three-berry samples. Three berries of the same cultivar were placed in a clear 36-mL container (Frontier Scientific, Newark, DE, USA) labeled with the sample identifier, and two of these samples were placed in 946-mL semi-transparent plastic deli containers with lids (Choice Paper Company, Brooklyn, New York, NY, USA). Five mated female and five male SWD were introduced to the choice arenas, and flies had equal access to the three-berry samples. Each pairwise comparison combination of cultivars was replicated five times on two dates, providing choice comparisons between all berry cultivars (111 pairings per cultivar, except for Northland, which had 84 pairings because of lower fruit availability). After 24 h, adult fly preference (i.e., flies observed at 24 h after introduction as physically present on berries of a particular cultivar) was recorded, and adults were removed from the containers. The number of eggs was counted in individual berries.
A modification of the host potential index created by Bellamy et al. (2013) was used to determine SWD host preferences and developmental suitability of different blueberry cultivars in both no-choice and two-choice settings. An example of calculating choice scores (C-scores) for SWD oviposition preference among five blueberry cultivars is provided in the Supplemental Methods. Preference proportions were determined, and then C-scores were calculated (Supplemental Table S1 and Supplemental Methods) to assign ranks (1–5 in order of most to least preferred) (Supplemental Table S2) to each cultivar following the methods described previously (Bellamy et al. 2013).
Ranks were assigned for oviposition in a no-choice setting (Eq. [1], no-choiceHPI), oviposition preference in a two-choice setting (Eq. [1], ovipositionHPI), adult preference in a two-choice setting (Eq. [1], adultHPI), and larval survival in a no-choice setting. After ranks were assigned, the HPI value for each measurement was determined for each of the five cultivars (values ranged from 232 to 303). Larval survival HPI was omitted from the averageHPI values because a statistically significant negative relationship was observed between the amount of oviposition and the survival rate for the larvae, perhaps because of intraspecific competition for larval feeding resources. For the averageHPI measurements, it was assumed that all measurements were equally important for assessing SWD host preferences, and the overall HPI was calculated according to Eq. [1]: [1] in which β is a weighting factor for each measurement.
Fruit firmness, measured as force in grams required to puncture the exocarp, used in host choice assays was measured in 2018–19 using a handheld FDK-32 penetrometer (Wagner Instruments, Greenwich, CT, USA) equipped with a 3-mm probe. Five different berries from each cultivar were tested at each harvest. A subsample of 250 g of harvested berries was homogenized and filtered through cheesecloth to obtain blueberry fruit juice for up to five samples for the total soluble solids (SS) and pH measurements. Three SS measurements were performed for each sample using a PAL-1 digital refractometer (Atago USA, Inc., Bellevue, WA, USA). The pH was also measured in 20 mL of this same juice for each subsample using an electronic pH and electrical conductivity meter (model mw802; Milwaukee Electronics, Milwaukee, WI, USA).
Host no-choice oviposition data were assessed for normality of residuals before analysis using R statistical software (R Core Team 2020). Host no-choice oviposition data were analyzed with a negative binomial distribution, followed by an analysis of variance (ANOVA) and Tukey post hoc test using the MASS, car, and emmeans R packages (Fox and Weisberg 2019; Lenth 2022; Venables and Ripley 2002). The host choice data were only used to calculate C-scores and determine host preference rankings for the HPI equation (Eq. [1]).
The average HPI was determined based on mean HPI values (Eq. [1]) from our no-choice and two-choice measurements, excluding larval survival. The averageHPI assumes that oviposition preference, adult preference, and no-choice measurements are equally important for SWD host-finding decisions. To address this assumption, 30-iteration bootstrap-simulated weights for each HPI value for each cultivar were used to consider variations in weighting for each of the measurements (i.e., in case oviposition preference is a greater driver of behavior than adult preference). Three weighting values from a uniform distribution where the sum of the weighting values equaled 1 were randomly selected for Eq. [1]. Then, the mean HPI and 95% confidence interval for the simulated data were calculated. Data were analyzed using R Statistical Software (R Core Team 2020).
For blueberry fruit quality (firmness, SS, and pH) measurements, data fit the assumptions of normality; therefore, a one-way ANOVA followed by Tukey post hoc tests were used to determine differences in firmness among cultivars and between years. The 2018 data for SS was the only data measurement that was not normally distributed; therefore, a nonparametric Kruskal-Wallis test was used to determine whether there were differences in SS between years. A linear model was used to analyze the relationship between blueberry fruit firmness and the amount of SWD oviposition (number of eggs per berry), as well as the relationship between blueberry fruit pH and larval survival (proportion of eggs that reached adulthood).
In the no-choice study, the number of SWD eggs laid in blueberries at 24 h after adult exposure was significantly different among cultivars (χ2 = 10.83; degrees of freedom = 4; P = 0.028). However, the Tukey post hoc analysis revealed no significant difference in the number of eggs at α = 0.05 (Table 1). ‘Northblue’ berries had the greatest number of eggs and ‘Polaris’ berries had the fewest number of eggs, but these differences were not significant. The five cultivars differed significantly in numbers of larvae, pupae, and adults 14 d after SWD exposure (χ2 = 13.76; degrees of freedom = 4; P = 0.008). ‘Patriot’ berries had the greatest number of SWD; its number of SWD was significantly different from that of ‘Northland’ berries, which had the lowest number of SWD. Berries of the other three cultivars had intermediate numbers of SWD; however, their numbers of SWD were not significantly different from those of Northland or Patriot (Table 1).
Oviposition and adult preference proportions among cultivars, C-scores, and calculated HPI values for each cultivar from the two-choice study are reported in Table 1. Patriot berries had the highest proportion of oviposition; however, based on the performance of all cultivars in a two-choice pairing (i.e., the C-score), SWD preferred to oviposit the most in Chippewa berries and the least in Polaris berries. In contrast, based on C-score results, adult SWD preferred ‘Polaris’ berries the most and ‘Patriot’ berries the least. ‘Chippewa’ and ‘Northland’ berries had lower adult SWD preference rates than ‘Polaris’ and ‘Patriot’ berries. Polaris berries had the greatest proportion (HPI = 303) of larval survival in the two-choice assay compared with the other cultivars, and Chippewa berries had the lowest proportion of larvae surviving to adulthood (HPI = 232).
Regarding the observed data, SWD least preferred ‘Northland’ berries (averageHPI = 246), whereas ‘Chippewa’ berries had the highest averageHPI score of 274.3. Regarding the simulated data, ‘Northland’ berries remained the least preferred by SWD and had an HPI 95% confidence interval of 245.55 to 248.3, and ‘Chippewa’ berries remained the most preferred, with an HPI 95% confidence interval of 271.26 to 277.95 (Fig. 1).
Citation: HortScience 58, 6; 10.21273/HORTSCI17097-23
The fruit characteristics of blueberry cultivars used in host choice assays were evaluated in 2018 and 2019. Year had a significant effect on fruit firmness, with fruit being firmer in 2018 than in 2019 (P < 0.0001) (Table 2). Firmness was also significantly different among cultivars (P = 0.009). ‘Polaris’ had the firmest fruit and was significantly different from the least firm ‘Northblue’. Fruit of the three remaining cultivars were not different from those of either Polaris or Northblue. Year did not influence fruit SS (P = 0.333). Additionally, fruit SS were not significantly different among cultivars (P = 0.792). Year had a significant effect on fruit pH, with fruit being less acidic in 2019 than in 2018 (P = 0.026). pH was also significantly different among cultivars (P < 0.0001), with Polaris fruit being significantly less acidic than those of the other four cultivars.
There was a statistically significant negative relationship between blueberry fruit firmness and the amount of SWD oviposition (Fig. 2A) (adjusted R2 = 0.26; P = 0.0067). The number of eggs laid per blueberry decreased by a factor of 0.066 per unit increase of fruit firmness. Additionally, there was a statistically significant relationship between blueberry fruit pH and larval survival (proportion of eggs that reached adulthood) (Fig. 2B) (adjusted R2 = 0.20; P = 0.030). The proportion of eggs that survived to adulthood increased by a factor of 0.287 for every unit increase in blueberry fruit pH.
Citation: HortScience 58, 6; 10.21273/HORTSCI17097-23
Cultivar selection is a useful tool for IPM, but few studies have reported host quality and preference across cultivars within a single cultivated crop species for SWD. In this study, we investigated SWD oviposition and larval development in five half-high blueberry cultivars, Chippewa, Northblue, Northland, Patriot, and Polaris, using no-choice and two-choice laboratory bioassays. We used the HPI developed by Bellamy et al. (2013) because it allows for comparisons across multiple hosts. Additionally, the HPI can include multiple variables of preference, such as oviposition and adult preference. Based on HPI, our results showed that Chippewa was the most preferred cultivar for oviposition measured in the number of eggs laid per fruit, and that Polaris was the least preferred in the two-choice tests. The inverse was true for larval development, with a higher survival rate and adult emergence in fruit of ‘Polaris’ than ‘Chippewa’, possibly indicating that intraspecific competition for host resources was occurring in ‘Chippewa’ fruit (Bezerra Da Silva et al. 2019).
The HPI values and preference ranks were generally consistent with direct measures of SWD cultivar preference, but there were some exceptions. For instance, ‘Patriot’ had a lower ranking (ranking of 2) than ‘Chippewa’ (ranking of 1) for the two-choice oviposition measurement, even though it had a higher preference probability than the direct measure (0.57 vs. 0.50). This was because SWD strongly preferred to oviposit in Patriot than in some cultivars, whereas SWD only moderately preferred Chippewa to all of the cultivars besides Patriot (see Supplemental Methods). Another aspect of the HPI that influenced the final result was the adult preference; Patriot was the least preferred cultivar and Chippewa was moderately preferred. These examples highlight the limitations of relying solely on pairwise comparisons and suggest that multichoice tests including all cultivars could provide a more comprehensive understanding of SWD preferences, although controlling for berry size and harvest date may be more challenging as the number of cultivars increase. The difference in SWD behavior preferences may be attributable to factors such as fruit volatiles (Keesey et al. 2015), microorganisms (Hamby et al. 2012), or macronutrient content (Silva-Soares et al. 2017), which could indicate whether a host is more suitable for ovipositing or foraging. Future studies could investigate whether differences exist among these blueberry cultivars, and whether they are correlated with SWD host suitability.
Blueberry characteristics that have been reported to affect SWD preferences include fruit flesh firmness and SS content (Lee et al. 2016). In fact, we observed a negative relationship between fruit firmness and oviposition and a positive correlation between pH and larval development. This supports previous laboratory and field experiment results observed by Lee et al. (2016), who found that SWD were more likely to oviposit as penetration force values decreased, and that pH increased in high-bush and rabbiteye blueberry fruit. The total SS content was less reliably related to oviposition probability than penetration force and pH (Lee et al. 2016). In two-choice assays involving blueberries, chokecherries, lingonberries, raspberries, and sea buckthorn fruit, Little et al. (2017) found that, considering all types of fruit and their characteristics, SWD preferred fruit with lower total SS content and lower pH. Of the half-high blueberries used in our comparisons, ‘Northland’ was the least and ‘Chippewa’ the most preferred by SWD (Table 1). These cultivars did not differ in firmness, total SS, or pH in 2018–19 (Table 2), suggesting that some other fruit characteristics, such as penetration force (not measured during this study), may influence SWD oviposition preference in half-high blueberries.
One physical characteristic that may have influenced SWD oviposition preference in the 2018 no-choice assay was berry size. Although fruits with similar weights were selected in the two-choice tests to minimize the influence of size between the two cultivars, berries from each cultivar in the no-choice test were randomly selected. A shortcoming of this study is that berry diameter was not controlled because we prioritized maximizing the number of replications and controlling for harvest date rather than diameter. This could have resulted in differences in SWD oviposition based on fruit size (e.g., larger fruits could have had more eggs) and potentially biased our results in the no-choice test. Studies suggest that fruit size is likely to influence oviposition, but reported data are lacking (Lee et al. 2016; Young et al. 2018).
SWD adults were observed on Polaris more than other cultivars despite this cultivar having the lowest amount of oviposition in the two-choice experiment. This may be because of differences in the quality of the fruits or volatile cues associated with the fruit. Cloonan et al. (2018) reported that female SWD attraction cues differ based on whether the individual is seeking food, mates, or an oviposition substrate. Future research could investigate microbial communities on the exterior of the fruit or metabolite differences among the berries with gas chromatography–mass spectroscopy to determine what role these characteristics may have in host-seeking behavior or larval nutrition (Deans and Hutchison 2021).
Our results showed that Chippewa was the most preferred cultivar for SWD oviposition as measured by the number of eggs laid per fruit, and Polaris was the least preferred. The inverse was true for larval development, with a higher survival rate and adult emergence in ‘Polaris’ than ‘Chippewa’, indicating that competition for host resources was occurring in ‘Chippewa’. As expected, there was an overall negative relationship between fruit firmness and oviposition and a positive correlation between pH and larval development. The results of this study indicate that cultivar selection for half-high blueberries may be a promising IPM tool; however, further research under field conditions is needed for validation.
Simulated host potential index (HPI) values (30 simulated HPI values) for five northern half-high blueberry (Vaccinium sp.) cultivars. Red dashed lines indicate the mean HPI value for the cultivar. Red dotted lines illustrate the 95% confidence intervals (2 * SEM). Higher HPI values correspond to a greater preference by spotted-wing drosophila.
The effect of blueberry (Vaccinium sp.) fruit quality on spotted-wing drosophila (SWD), Drosophila suzukii (Matsumura) host suitability. (A) The amount of SWD oviposition (number of eggs/berry) as a function of blueberry fruit firmness (g force/mm2) (adjusted R2 = 0.2572; P = 0.007). Berries were harvested from the Sand Plains Research Farm in Becker, MN, and fruit firmness was measured on five berries of each cultivar. Other berries from the same harvests were used for host choice assays in which mated pairs of SWD were introduced to individual berries. (B) The proportion of SWD eggs that reached adulthood as a function of blueberry fruit pH (adjusted R2 = 0.1961; P = 0.030). Fruit pH was measured on 20 mL juice samples obtained from 250 g of berries. Mated pairs of SWD adults were introduced to individual berries, removed after 24 h, and counted after 14 d.
Contributor Notes
We thank Dave Bellamy, Mark Sisterson, and Naxo Riera Vila for technical assistance, James Luby for blueberries, and Matt Clark, Robert Koch, and Leah Worth for manuscript preparation. Funding was provided by the Minnesota Experiment Station for project MN21-043.
M.R. is the corresponding author. E-mail: roge0168@umn.edu.
Simulated host potential index (HPI) values (30 simulated HPI values) for five northern half-high blueberry (Vaccinium sp.) cultivars. Red dashed lines indicate the mean HPI value for the cultivar. Red dotted lines illustrate the 95% confidence intervals (2 * SEM). Higher HPI values correspond to a greater preference by spotted-wing drosophila.
The effect of blueberry (Vaccinium sp.) fruit quality on spotted-wing drosophila (SWD), Drosophila suzukii (Matsumura) host suitability. (A) The amount of SWD oviposition (number of eggs/berry) as a function of blueberry fruit firmness (g force/mm2) (adjusted R2 = 0.2572; P = 0.007). Berries were harvested from the Sand Plains Research Farm in Becker, MN, and fruit firmness was measured on five berries of each cultivar. Other berries from the same harvests were used for host choice assays in which mated pairs of SWD were introduced to individual berries. (B) The proportion of SWD eggs that reached adulthood as a function of blueberry fruit pH (adjusted R2 = 0.1961; P = 0.030). Fruit pH was measured on 20 mL juice samples obtained from 250 g of berries. Mated pairs of SWD adults were introduced to individual berries, removed after 24 h, and counted after 14 d.
