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A Natural History of Change in Native Bees Associated with Lowbush Blueberry in Maine
Francis A. Drummond, Alison C. Dibble, Constance Stubbs, Sara L. Bushmann, John S. Ascher, and Jennifer Ryan

Northeastern Naturalist, Volume 24, Monograph 15 (2017): 49–68

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49 2017 NORTHEASTERN NATURALIST 24(Monograph 15):49–68 A Natural History of Change in Native Bees Associated with Lowbush Blueberry in Maine Francis A. Drummond1,2,*, Alison C. Dibble1, Constance Stubbs1, Sara L. Bushmann1,3, John S. Ascher4, and Jennifer Ryan5 Abstract - More than 120 native bee species have been documented in Maine since 1930 in association with the native plant Vaccinium angustifolium (Lowbush Blueberry). We report 3 studies in commercial Lowbush Blueberry fields: (1) a survey of diversity in Osmia (mason bees) and closely related Megachile (leaf-cutter bees) using trap nests in 93 fields from 1990 to 2012, (2) a 29-year study of a native bee community, and (3) an examination of climate-change effects on bee-foraging periods during blueberry bloom. Osmia appeared to be more stable over a 22- year period in their species richness and relative abundances in Lowbush Blueberry fields when compared to Megachile over a similar 17-year period. The native bee community in a single location in Winterport was observed to fluctuate in abundance 2 to 3 times annually. Modeling of the total bee community and taxa-specific group abundances (Bombus, Megachilids, Andrenids, and Halictids and other bees) suggest that while stochastic density-independent processes such as weather can play a role in determining their annual oscillations, density-dependent lags of 1 and 2 years appear to be the main driving forces. Estimation of fruit set over the same 29-year period, based upon native bee abundance, suggests that pollination is more buffered than community bee abundance, resulting in a lesser degree of fluctuation over time. We speculate that this finding is due to redundancy in floral preferences, multiple floral visitations, and differing pollination efficiencies by the highly diverse native bee community associated with Lowbush Blueberry. Effects of climate change in Maine Lowbush Blueberry fields during May bloom was investigated using a historic weather database. Since the early 1990s, precipitation has, to a large degree, reduced the number of optimal bee foraging days during bloom, with implications for pollination and bee species abundances. This new information reinforces the need for provision of pollinator gardens to support native pollinators of Lowbush Blueberry. Introduction In Maine, native bees have received attention due to their role as pollinators, in particular of native, insect-dependent Vaccinium angustifolium Aiton (Lowbush Blueberry; Bell et al. 2009, Jones et al. 2014). No other plant species that grows in Maine has received the extent of research on pollinators that was directed to Lowbush Blueberry. From this body of research on the natural history of native bees and this particular host plant, there has emerged an understanding of the associations 1University of Maine, School of Biology and Ecology, 5722 Deering Hall, Orono, ME 04469. 2Cooperative Extension, 305 Deering Hall, Orono, ME 04469. 3Current address - George Stevens Academy, 23 Union Street, Blue Hill, ME 04614. 4Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Singapore 117543. 5Trustees of Reservations, 200 High Street, Boston, MA 02110. *Corresponding author - fdrummond@maine.edu. Manuscript Editor: David Halliwell Northeastern Naturalist F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 50 between as many as 120 bee species (Bushmann and Drummond 2015). None of these are known to specialize on Lowbush Blueberry, but they may rely on its flowers for pollen and nectar in spring before many other plants ar e flowering. We lack an understanding of the temporal dynamics of bee abundance over decadal time intervals, and ways in which climate change might affect future bee abundance. Lowbush Blueberry is a wild shrub that is found throughout Maine, including in closed-canopy forest (Dibble et al. 1999). The nutritious fruits are consumed by many species of birds and mammals in Maine and throughout its range. Pre-contact fields managed by Native Americans using slash and burn culture (Moore 1994) were later burned and harvested by European colonists. At the time of the Civil War, more than 80,000 ha were managed and harvested, and the fruit was shipped to Boston and New York by train (Phipps 1930). Today this native, perennial plant, designated in the industry as Wild Blueberry and also known as Low Sweet Blueberry, is unusual in that large monocultural stands are managed commercially in Maine and eastern Canada (Hall et al. 1979). Management has become more intensive over time. Since the 1940s, pest management of weeds and insects was incorporated (Drummond and Collins 1999). Current production can comprise some or all of the following practices: fertilization, pest management, soil acidification, rock removal, mowing or burning for pruning, mechanical harvesting, and importation of Apis mellifera L. (Honey Bee) or Bombus impatiens Cresson (Common Eastern Bumble Bee) for pollination (Yarborough 2009, Yarborough et al. 2017). Current management typically involves a 2-year cycle with crop flowering in May of the first year, when the number of flowers in a field may exceed millions (~8 x 107 flowers/ha; Bajcz et al. 2017), followed by harvest of the berries in July–August, then pruning in the fall followed by a year of vegetative growth with no blueberry flowers at all in the second year (Yarborough 2009). Extensive studies of Lowbush Blueberry since the 1960s (Boulanger et al. 1967; Bushmann and Drummond 2015; Drummond 2016, Drummond and Stubbs 1997a, 1997b, 2003; Stubbs et al. 1992) have identified native bees as important pollinators including Bombus (bumble bees), Andrena (mining bees), Halictus and Lasioglossum (sweat bees), Megachile (leaf-cutter bees), and Osmia (mason or orchard bees). Of particular interest to researchers and blueberry farmers are the alternate forage plants visited by bees for pollen and nectar before and after the bloom period for the Lowbush Blueberry crop (Bushmann and Drummond 2015, Drummond et al. 2017, Stubbs et al. 1992, Venturini et al. 2017a). Other research foci within the Lowbush Blueberry pollination system are the effects of pesticides, pests, and diseases on native bees (Bushmann et al. 2012, Drummond 2012, Yarborough et al. 2017). Some species of native bees, especially bumble bees, are known to be effective pollinators of the Lowbush Blueberry crop (Asare et al. 2017; Drummond 2012, 2016; Javorek et al. 2002). Despite this, Honey Bees are moved by truck from distant earlier-blooming crops, especially California almonds, and set out in Maine blueberry fields that are in the crop year as opposed to the alternate, or prune year for any given field (Asare et al. 2017, Drummond 2002). Approximately 75,000 colonies are brought to Maine for the pollination of Lowbush Blueberry, the second Northeastern Naturalist 51 F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 largest number of migratory Honey Bee hives annually of any crop; only almond pollination in California uses more hives (Drummond 2012). After crop bloom, the Honey Bees are removed to other parts of North America to pollinate other crops. Managed non-native bees such as Honey Bees and Megachile rotundata (Fabricius) (Alfalfa Leaf-cutter Bee), while economical, have been shown to be less effective pollinators of Lowbush Blueberry on a per-individual-bee basis than some of the more common native bees (Drummond 2016; Javorek et al. 2002; Stubbs and Drummond 1997a, 1997b, 1997c). For the sake of risk management, growers rely on Honey Bees, given the super-abundance of flowers during the bloom period (~8000 flowers/m2; Bajcz et al. 2017). Hive rental costs for Honey Bees continue to escalate across the US (Rucker et al. 2012, Sumner and Boriss 2006) because of severe losses due to Colony Collapse Disorder and other causes of Honey Bee colony losses (Drummond et al. 2012, Lee et al. 2015, Ratnieks and Carreck 2010). Therefore, pollination strategies are shifting toward providing more habitat and floral resources for native bees adjacent to the blueberry crop (Drummond et al. 2017; Venturini et al. 2015, 2017a, 2017b). Due to experimental evidence confirming the importance of insect pollination for fruit production, Phipps (1930) began to document wild bee species associated with Lowbush Blueberry flowers. From these early times, a high priority was given to documenting bee fauna and obtaining expert identification of specimens, as correct determination to the level of species is a crucial aspect in understanding ecological patterns (see discussion in Cane 2001). In 1961–1965, bees and other insect floral visitors to Lowbush Blueberry were documented in 3 Maine counties and 4 Canadian provinces (Boulanger et al. 1967). Eben A. Osgood (1972, 1989) examined the nesting biology of Andrena and contributed to the identification of 2 Osmia species (Rust and Osgood 1993). His students and others extended this research by investigating native plants as floral resources and the response of the bee communities to pesticides applied to control Choristoneura fumiferana (Clemens) (Spruce Budworm) outbreaks (Hansen and Osgood 1983; Miliczky and Osgood 1979a, 1979b; Stubbs et al. 1992, 1996). Building on this historical perspective of research on native bees in Lowbush Blueberry, we report here on 3 previously unpublished studies, each involving native bees associated with Lowbush Blueberry and their temporal dynamics. These studies were conducted in Maine between 1989 and 2017 and focused on native bees, not Honey Bees, because the prospects of Lowbush Blueberry pollination in the absence of Honey Bees was our interest. We sought to take what is known of the natural history of a native pollination system, but under commercial management, and search for patterns indicative of change in bee abundance over time, and to identify weather factors that could influence such change. The first study was designed to assess the diversity of Osmia and Megachile species associated with Lowbush Blueberry fields in Maine in 1990, 1997 and 1998, and 2010–2012. In the second, we examined the long-term annual fluctuations in the bee community associated with a Lowbush Blueberry field in Winterport, ME, and consequences for fruit set. The purpose of the third study was to estimate current climate-change Northeastern Naturalist F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 52 effects on the number of days available for bee foraging during the 3-4-week bloom period for the Lowbush Blueberry crop in Maine. Field-Site Description A total of 103 commercial Lowbush Blueberry fields were sampled for bee abundance and species diversity between 1989 and 2017 in Maine. Lowbush Blueberry fields were located in Knox, Lincoln, Waldo, and Washington counties over the course of the 3 studies reported here. Lowbush Blueberry fields are embedded within a range of different landscapes across Maine, from upland deciduous forest to glacial outwash plains along the Downeast coast (Drummond et al. 2009). Most fields are a result of forest clearing and management through burning or herbicide use to minimize continuous colonization of competitive vegetation (Yarborough 2009). Plant species diversity typically found within Lowbush Blueberry fields is listed in Bushmann and Drummond (2015) and Drummond et al. (2017). Invertebrate species diversity in these managed habitats is discussed by Jones et al. (2014). Methods Osmia and Megachile diversity in Lowbush Blueberry fields We estimated the species richness and relative abundances of Osmia and Megachile species in Maine in 3 time periods and varous sites. We surveyed bees in Knox, Lincoln, Waldo, and Washington counties in 1990; Washington County in 1997 and 1998; and Hancock, Waldo, and Washington counties in 2010–2012. In the first 2 samples (1990 and then 1997–1998), we used wooden trap-nest blocks with holes measuring 6.4 mm and 8.5 mm in diameter (16 holes, 8 per diam size, per 51 x 102 x 254 mm kiln-dried pine block). We deployed 20 blocks along forested edges in each of 30 wild blueberry fields in 1990 and 20 blocks along forested edges in each of 18 and 15 Lowbush Blueberry fields, respectively, in 1997 and 1998. We placed the blocks 1.5–2 m above ground oriented south–southeast. The trap nests were deployed in March well before Osmia and Megachile species emergences and collected in August and September after adult activity ceased. Trap nests were overwintered in a non-heated utility shed in Winterport and moved in March to the laboratory at the University of Maine Orono, individually enclosed in metal screen cages and incubated at room temperature. Emerged bees were collected, pinned and sent to T. Griswold and W.E. LaBerge for identification. Due to the focus of the 1990 survey, only Osmia, not Megachile specimens, were identified to species. We calculated relative abundance for each species by genus and by year and compared these data to that of richness and relative abundance of species from the same genera captured by hand on Lowbush Blueberry flowers or bowl traps in 40 fields in Hancock, Waldo, and Washington counties during 2010–2012 (detailed methods described in Bushmann and Drummond 2015). Northeastern Naturalist 53 F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 A long-term temporal study of a native bee community in Lowbush Blueberry Starting in 1989, we sampled the bee community annually in a Lowbush Blueberry field in Winterport, ME, over a 29-year period, although 7 years were missed. This crop flowers in mid-May for as long as 3–4 weeks. We did not sample the bee community in 1998, 2001, 2005, 2010, 2012, 2014, and 2016. Each year at peak bloom (50–80% open flowers), 20 1-m2 quadrats placed in flowering Lowbush Blueberry were observed for 1 minute. The quadrats were approached slowly, and the observer (F.A. Drummond) waited 1 minute without moving and then proceeded to count the number of bees within and entering the quadrats for 1 minute. This sampling was conducted 2–3 times during peak bloom and performed on only sunny days between the times of 1000 and 1400 hrs when air temperatures were >15.6 ºC and winds were less than 24 kph. These conditions are conducive to native bee foraging activity (Drummond 2016). We recorded bees as belonging to the following groups: Bombus spp. (bumble bees), Megachilids (mostly Osmia spp. with a few Megachile spp.), Andrenids (Andrena spp., [mining bees]), and Halictids (sweat bees) and Other Native Bees (Colletidae and non-social Apidae). Honey Bees were recorded, but not included in our analysis of native bee densities. Bushmann and Drummond (2015) and Drummond et al. (2017) list representative species identifications of bees that were captured and identified during the time period of this study. We converted bee density per minute of each taxa group to a per-hectare basis for graphical analysis. We estimated the fruit set contribution by the bee community using the formula we had derived for estimating fruit set in Lowbush Blueberry based upon native wild bees per m2 per minute (Asare et al. 2017). We used time-series analysis (autoregressive moving average regression [ARIMA]) to assess periodicity in abundance fluctuations from 1989 to 2015 and serial cross-correlation to test temporal synchrony among pairs of taxa groups (Shumway 1988). We utilized linear interpolation to estimate the bee community densities in the non-sampled years (Wei 2006) and sample data from 2017 to assess or validate future predictions of the models. Pollination level was not measured annually in the Winterport field where bee densities were measured. Using a predictive formula of fruit set based upon native bee densities per m2 per minute (Asare et al. 2017), we estimated percent fruit set over the 29-year time period. Statistical analysis was performed by using JMP statistical software (SAS Institute, Inc. 2015). Estimated current climate-change effects on bee foraging periods during Lowbush Blueberry bloom For a single site in Blue Hill, Hancock County, where an automated weather station records hourly conditions, we used a degree-day model developed and validated in Nova Scotia by White et al. (2012) to estimate the bloom “window”, or the period between the beginning and end of bloom from 1960 to 2015. We generated estimates of the number of cumulative degree-days from the historical maximum and minimum air temperatures. Historical weather data for the site (1960–2015) were downloaded from archived weather reports (NOAA 2017). Most bees are not actively foraging in weather conditions such as cold, precipitation including mist Northeastern Naturalist F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 54 or rain, or brisk winds. To predict the number of potential pollination days during the bloom window, we subtracted days that were below a high of 4.4 ºC, had 2.54 cm or more of rain in a day, or had winds greater than 32.2 kph. These conditions have been reported as thresholds for some native and exotic bee species in Maine that visit wild blueberry during bloom (Drummond 2016). This subtraction yields the number of days during bloom that bees pollinated flowers or “pollination days”. Use of daily weather parameters did not allow us to develop a high-resolution, hourly prediction of foraging activity. We assume that our predictions based upon the use of 3 weather metrics (air temperature, precipitation, and wind velocity) aggregated over a 24-hr period will underestimate foraging activity, but should be consistently biased over the ~1 month-long period of bloom in Maine and should also average out annually. We employed piecewise linear regression (Oosterbaan et al. 1990) to fit the number of bee foraging days during the bloom window over time (years), and multiple linear regression with mean daily air temperature, mean daily precipitation, and mean daily wind velocity to determine explantory power of weather factors during the segment exhibiting a negative slope (SAS Institute, Inc. 2015). We collected actual bloom data in 2016 and 2017 and calculated the number of pollination days based upon the bee flight-activity parameter s described above. Results Osmia and Megachile diversity in wild blueberry fields The relative abundances of Osmia and Megachile species associated with Maine Lowbush Blueberry (collected in 1990, 1997–1998, and 2010–2012) are shown in Figure 1. Osmia atriventris Cresson and O. inspergens Lovell & Cockerell, were the 2 most abundant species in all 3 collection periods (Fig. 1A). In 1997–1998 and more than 20 years later in the same geographic areas, 8 Megachile species were observed (Fig. 1B), of which Megachile brevis Say, M. centuncularis L., and M. latimanus Say were each found in only 1997–1998, or 2010–2012. The Megachile species with greater relative abundance appear to fluctuate more dramatically among sampling periods (Fig. 1B) than did the more common Osmia species (Fig. 1A). A long-term temporal study of a native bee community in Lowbush Blueberry. Continuous sampling over a 29-year period in a Lowbush Blueberry field in Winterport, Waldo County, shows that total community bee abundance varies 2–3 fold from one year to the next (Fig. 2). Bee taxa groups (Bombus, Andrenids, Megachilids, and Halictids and Others) were correlated, showing some degree of dependence. Halictids and Others are highly correlated with Andrenids (r = 0.704, P = 0.0004), Bombus were correlated with both Halictids and Others (r = 0.489, P = 0.024) and Andrenids (r = 0.588, P = 0.005). However, Megachilids are not correlated with any of the other taxa (P > 0.05). For instance, Megachilids exhibited a continuing decline in numbers since 2007, although in 2017 a slight increase in Megachilid density did occur, while Bombus abundance increased during the same time period, except for 2011 where Bombus density decreased. (Fig. 2). Northeastern Naturalist 55 F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 The fluctuations in the entire bee community (all inidivduals in all taxa groups pooled), modeled using time-series methods, resulted in a significant fit (P < 0.05), but poor overall predictability (Table 1). The time-series second-order model explains only 7.3% of the variation (adjusted r2 = 0.019) in bee numbers over the 29- year period (Table 1). The significant factors are the numbers of bees the previous year (P = 0.049) and a stochastic random-walk factor (P = 0.029). The statistical model suggests that the dynamics of the entire native bee community is driven Figure 1. Relative abundance of (A) Osmia species and (B) Megachile species collected in Maine Lowbush Blueberry fields in 1990 (n = 30 fields), 1997 (n = 18), 1998 (n = 15), and 2010– 2012 (n = 40) (data from Bushmann and Drummond 2015). Northeastern Naturalist F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 56 by density-dependenct factors, possibly disease, competition for nesting sites, or flowers, but fluctuations in density from year to year could also be due to weather and other stochastic events as suggested by the significant random-walk parameter (Table 1). Modeling individual taxa did not result in a significant predictive model for Bombus (P > 0.05), but significant time-series predictive models were developed for Andrenids, Megachilids, and the Halictids and Others (Table 1). The Figure 2. Data from a 29-year bee community survey in a Lowbush Blueberry field at Winterport, Waldo County, with bee abundance by group in timed quadrat surveys. The symbol “*” denotes years when sampling was not conducted. The taxa group “others” refers to observed individuals assigned to the families Colletidae, Apidae (other than Bombus), or unidentified individuals. Table 1. Time-series predictive models (ARIMA) used to predict taxon density at time = t + 1. Taxa Coefficient Coefficent value ± s.e. P-value r2 (adjusted r2) Andrenids 0.208 (0.059) Intercept -46.795 ± 50.13 0.373 Andrenid density (t) -1.026 ± 0.148 less than 0.0001 Andrenid density (t - 1) -0.786 ± 0.166 0.001 Random walk (t) -0.999 ± 0.247 0.002 Megachilids 0.339 (0.215) Intercept -21.109 ± 21.202 0.343 Megachilid density (t) -0.727 ± 0.292 0.032 Megachilid density (t - 1) -0.736 ± 0.202 0.005 Halictids and others 0.237 (0.196) Intercept 241.52 6± 77.120 0.006 Halictid and others density (t) 0.549 ± 0.193 0.010 Total native bee community 0.073 (0.019) Intercept -126.607 ± 151.429 0.421 Total native bee density (t) 0.624 ± 0.283 0.049 Random walk (t) 1.002 ± 0.401 0.029 Northeastern Naturalist 57 F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 model for the Andrenids provided evidence that both 1- and 2-yr time lags (Andrenid bee densities at time t and time t - 1 for prediction of Andrenid densities at time t + 1) and a stochastic random-walk (possibly an abiotic effect) were significant in the community temporal dynamics. The Megachilid community dynamics were also described by 1- and 2-yr time lags, but not a random-walk component, whereas the temporal variance in the Halictid and Others community was only explained by the densities in the previous year (year [t]). The 2017 samples from the same field in Winterport suggest that the time-series models for Halictids and Others, Megachilids, and the total bee community described the future predictions for 2017 quite well. The Andrenid time-series model over-predicted density substantially (~27%). We also found that fruit set predicted by the abundances of the wild bee community varies by nearly a 2-fold amount over the study time period (Fig. 3). The coefficient of variation for the total bee community compared to fruit set was 47.9% versus 15.3%, respectively. Estimated current climate change effects on bee foraging periods during Lowbush Blueberry bloom Historical weather data from Maine indicate that bee activity and foraging during the spring bloom period of Lowbush Blueberry is already affected by climate change (Fig. 4). Our estimates of the average number of pollination days, or days in which weather conditions are conducive for bees to visit Lowbush Blueberry Figure 3. Predicted percent fruit set in Winterport, ME, from 1989 to 2017. The symbol “*” denotes years when sampling was not conducted. The taxa group “others” refers to observed individuals assigned to the families: Colletidae, Apidae (other than Bombus), or unidentified individuals. Northeastern Naturalist F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 58 flowers, for Blue Hill in Hancock County between 1960 and 2015 had no significant change (P = 0.993) from 1960 until 1990 (30 years). On any given year, the number of pollination days was estimated from the models to vary from 7 to 25 days. Starting in the early 1990s, average number of pollination days declined at a linear rate (P = 0.047) through 2015. The major cause of this trend is the increased spring rainfall during bloom. Validation data (2016 and 2017) were used to assess the mean regression-model prediction for the future years in 2016 and 2017. It can be seen in Figure 4 that the regression model based upon the 1960 to 2015 data, underestimates the 2016 bee foraging days by 12 days, but the 2017 prediction is within 2 days. This result is not unexpected since year-to-year variation in climate change has been demonstrated to be high with only a mean trend being representative of field observations. Discussion Osmia and Megachile diversity in Lowbush Blueberry fields In Maine, deployment of nest blocks has been shown to be both an effective sampling method and a good conservation technique for enhancing Osmia abundance (Stubbs et al. 1997a) when combined with a reduction in pesticide exposure (Drummond and Stubbs 1997b). Osmia are mostly vernal cavity nesters (Michener 2007), and many Maine species occur in blueberry field habitats, including those documented to pollinate Vaccinium (Drummond and Stubbs 1997a). These species nest in pre-existing cavities such as old borer holes or galleries in trees, and use either mud or disks of leaf material that they have cut with their large mandibles Figure 4. Estimation of the number of bee foraging days during Lowbush Blueberry bloom in Blue Hill, Hancock County, from 1960 to 2015 (piecewise regression used to estimate decline in bee foraging-days over time). Square symbols represent observed validation data from 2016 and 2017. Northeastern Naturalist 59 F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 to construct cell partitions and closures. A total of 16 Osmia species have been recorded in Maine (Dibble et al. 2017). Bushmann and Drummond (2015) found that the 4 most common Osmia in blueberry fields sampled are: O. atriventris, O. pumila Cresson, O. tersula Cockerell, and O. inermis (Zetterstedt) sensu Mitchell (1962), although see Rightmeyer et al. (2010). Currently, Osmia specimens collected that have suspect identifications or are in poor condition have been categorized in Figure 2 as “Osmia spp.”. Some, or possibly all, Maine specimens housed in the Maine State Museum should be checked regarding O. laticeps Thomson, a species overlooked by regional workers prior to Rightmyer et al. (2010). Inspection of Figure 1A suggests that overall there is little evidence of significant decline in the relative abundances of the most common species of Osmia over the past 25 years, with the exception of the less abundant O. albiventris Cresson and O. virga Sandhouse. This finding bodes well for Maine blueberry growers because Osmia atriventris and other Osmia spp. are effective pollinators of Lowbush Blueberry (Drummond 2016, Javorek et al. 2002) and any major decline in their numbers might have implications for the Lowbush Blueberry farming community. However, if the absolute abundance of native Osmia spp. is in decline (see Fig. 2), we have evidence from only a single Lowbush Blueberry field, and so at this point assessment of mason bee health would need to be conducted over a broader geographic range. Non-native Osmia species were not found in Maine during the sampling years, although Osmia cornifrons (Radoszkowski) is now abundant farther south in New England and was recently reported from New Hampshire (Tucker and Rehan 2016). Osmia cornifrons was introduced into the Northeast from East Asia for pollination (Yamada et al. 1971). Information on the diversity and abundance of Megachile is not as available for Maine as for Osmia in the region. In large part this lack of data might be due to phenology. The most intensive sampling for Megachilidae in Maine has been in Lowbush Blueberry fields (Dibble et al. 2017). It is our experience that Megachile species occur toward the end of Lowbush Blueberry bloom and increase during the summer (Bushmann and Drummond 2015, Chandler et al. 2012). Thirteen Megachile species have been reported from Maine (Dibble et al. 2017), including 2 species, Megachile brevis Say and M. latimanus Say, that were absent from our collection in the 1990s, possibly reflecting population abundance oscillations over time. Megachile mucida Cresson was found abundantly in the 1990s, but was not found in 2010–2012 (Fig. 1B). This finding might suggest that this species is in decline in Maine. We are not aware of any recent reports on its occurrence, although it has been reported in southern New England recently (Ascher and Pickering 2016). Therefore, M. mucida may not be in decline throughout New England. The non-native M. rotundata was not found in 2010–2012, even though it was introduced in the early 1990s (Stubbs and Drummond 1997a, 1997b). This finding suggests that this species might not have become established in Maine or if it was, has since disappeared. This is understandable as M. rotundata is not a cold-tolerant bee species, based upon studies conducted in Orono, ME, during the early 1990s (Stubbs and Drummond 1997b). Northeastern Naturalist F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 60 If we were to speculate a cause for a potential decline in the Megachilidae as a whole or for select species, 2 factors other than climate change come to mind. The increase in the importation of migratory Honey Bee colonies for blueberry pollination in Maine could have resulted in increased competition for floral resources or spillover of pathogens that might have been virulent to Megachildae. Another factor might have been the brief period of time in the early 1990s when the Alfalfa Leafcutting Bee was introduced for blueberry pollination. This might have resulted in undetected introduction of pathogens adapted to native Megachilidae or resulted in competition for forage or nest sites in the wild. None of these speculative causes, however, can be supported by evidence derived from data. A long-term temporal study of a native bee community in Lowbush Blueberry Annual fluctuations in bee communities have not been well studied in Maine (Bushmann and Drummond 2015, Venturini et al. 2017b). It is not surprising that the individual taxa groups result in better model fits considering the diverse mixture of life-history patterns (e.g., bumble bees are eusocial, living in colonies, whereas all others are solitary or might have connecting tunnels, as is the case for Andrena spp.; Dibble et al., in press). The dynamics of the individual taxa groups show strong evidence of density dependence, especially in both Andrenids and Megachilids with a second-order model (t [1-year] and t - 1 [2-year] lags; Table 1). Royama (1992) has shown that these dynamics are typical in insect populations that are regulated by parasitoids and pathogens. This finding, therefore, in the Maine bee community is not surprising as disease and parasites are common in the bee taxa we observed (Batra et al. 1973, Brown and Paxton 2009, Hedtke et al. 2015) and kleptoparasites can make up 10–20% of the bee fauna in Maine (Dibble et al. 2017). Floral abundance, represented both as managed pollinator reservoirs (Venturini et al. 2017b) and natural wildflower communities along Lowbush Blueberry field edges (Drummond et al. 2017, Stubbs et al. 1992), has been shown to increase native bee abundance. Venturini and Drummond (in press) have provided evidence of farm-management effects: biennial fluctuations in Andrena spp. in blueberry fields that are geographically isolated and managed on a single cycle (i.e., the entire field is either fruit bearing or vegetative in a given year) have been demonstrated to result in lower Andrena spp. abundances. Figure 5 shows the model predictions for the taxa groups and forward predictions up to and including the year 2020. In general, the model predictions represent the fluctuations in the observed abundances well for the specific taxa groups, except for the total native bee community model predictions (Fig. 5D). The model predictions suggest that long-term and future abundances are stable for Andrenids as well as Halictid and Others. The Megachilids show a decline without recovery since 2006. This decline over time of an entire taxon group is supported by the findings of Bartomeus et al. (2013), who suggest that species with shared ecological traits, as many sepcies of the Megachilids do, may decline together. Three of the 4 models showed high similarity for the future prediction of 2017 with sample data collected for the purpose of validation, suggesting that the mechanisms of density dependence and independent stochastic factors, such as weather events, driving community abundance fluctuations might Northeastern Naturalist 61 F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 be a reasonable hypothesis. The decline in Megachilid abundance is addressed below in the section on potential climate change. The rate of fruit set estimated for the 29-year period by use of the formula developed by Asare et al. (2017) is typical for what is expected from background pollination by native bees in the absence of Honey Bees in Lowbush Blueberry fields (Stubbs and Drummond 1997c). Estimated fruit set appears to be highly buffered from major changes in the native bee community. This is an important dynamic because it suggests that the redundancy in the native bee pollination network due to bee species composition (Bushmann and Drummond 2015) and varying foraging behavior and pollination efficiency (Drummond 2016) provides resiliency in pollination level to changes in bee abundance over time. We speculate that this result is due to redundant floral visitation by bees during bloom. Most bee species probably do not recognize previously visited flowers, such has been documented for some species of bumble bees (Goulson et al. 1998), and so Figure 5. Model predictions of taxa group and total native bee community over a 27-year time period in Winterport, ME. (A) andrenids, (B) megachilids, (C) halictids and others, (D) total bee community density observed. Dashed arrow denotes forward prediction of bee abundance from 2017 to 2020, square symbols denote 2017 validat ion samples. Northeastern Naturalist F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 62 multiple visits in excess of what is necessary for fruit set is probably characteristic of the Lowbush Blueberry system. The same may not be true of yield, as higher rates of pollen deposition result in more fertilized ova per flower and higher numbers of seeds per fruit, and lar ger fruit is a consequence of higher numbers of seeds per fruit (Bell et al. 2010). Estimated current climate-change effects on bee foraging periods during Lowbush Blueberry bloom Climate change and in particular hotter summer temperatures may already be resulting in significant changes to bumble bee distributional ranges in Maine and elsewhere (Kerr et al. 2015). Climate change appears to affect not only bee species shifts, but also the flowering phenology for plant communities that they depend upon (Bartomeus et al. 2011). It has been speculated that climate change will result in wetter conditions in northeastern North America (Campbell et al. 2009). Rainy springs and summers could detrimentally affect bees of Maine and other regions of northern New England in several ways: (1) upset a synchrony between bloom period for host plants and active period for native bees (but see Bartomeus et al. 2011), (2) limit good weather conditions for foraging with the result that insects are unable to provision brood for subsequent generations, and (3) increase conditions that enhance fungal infections in soil-nesting bees (Batra et al. 1973). With climate change, our results suggest that in the future Lowbush Blueberry growers will experience fewer days in which pollinators visit flowers in their fields compared to 30 years ago (Fig. 4). This has potential implications for food-resource acquisition by female bees during this time and possibly long-term bee population dynamics because the females are provisioning their brood for the next generation. Even during the decline period, there have been years with good pollination windows, such as 2016 (Fig. 4). It remains to be seen whether a limit to the number of bee-foraging days in the spring will affect bee survival and reproduction. Among Maine bees, we know of relatively few that demonstrate host-plant specificity, although ~15% of northeastern US bee species have been classified as pollen specialists (Fowler 2016, but compare pollen forage with same species in Stubbs et al. 1992). However, most bee species of temperate zones forage on a wide number of plant species depending on what may be in flower. This fact does not diminish the severity of a proposed lack of synchrony between bee emergence and flowering of host plants. In such a scenario, bees emerge from their nests according to influences such as soil temperature that differ from those that trigger phenology in their host plants (e.g., day length, weather and climate cues, depth of frost in winter, rainfall in the previous growing season). Thus, there is potential for bees to have inadequate or suboptimal nectar and pollen resources (Miller-Rushing and Primack 2008). However, it appears that bees respond less variably than do their plant forage species (Bartomeus et al. 2011). In conclusion, the results of our 3 long-term studies in Maine provided evidence for changes in native bee community dynamics as well as the potential influence of climate change on those shifting dynamics. Our first study suggests that Osmia bees might be more stable and resistant to change than Megachile bees. Bartomeus et al. Northeastern Naturalist 63 F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 (2013) showed through a survey of museum specimens that bee species are in flux, with some species increasing in abundance, some in decline, and most maintaining their long-term abundances. Maine has more species of Osmia than Megachile (Dibble et al. 2017), and our data suggest that richness has been more variable in Megachile until recently, but if Osmia abundance is in decline, as seen in Figure 5B, then an intensive survey is required in Maine to determine if Osmia are suffering from climate change. In our second study, our 29-year data from Winterport, ME, indicate that the taxa we measured, except Osmia, appear to be characterized at a single location by stable abundance over time, but can fluctuate dramatically from year to year. Our data suggest that the drivers might be density-dependent factors such as pathogens or parasites in concert with a potential suite of stochastic density-independent factors. However, one density-dependent factor also playing a role in bee community dynamics could be floral resources, which have been shown to be directly and indirectly related to native bee abundance in Maine (Bushmann and Drummond 2015, Drummond et al. 2017, Groff et al. 2016, Venturini et al. 2017b). When bees congregate at a patch of flowers, they might contract pests or diseases from other floral visitors, and this is another factor. Parasites and pathogens are certainly common in native bee communities (Bushman et al. 2012, Cameron et al. 2011, Goulson et al. 2015). Management of parasites and pathogens is not practical in most cases, whereas floral resources can be managed both locally and regionally (Groff et al. 2016; Venturini et al. 2015, 2017a). Response to density of floral resources and to parasites/pathogens is one of the main hypotheses regarding bumble bee species range shifts and decline in abundance in North America and Europe (Kerr et al. 2015). Abiotic factors such as climate should also not be ruled out, as suggested by our time-series modeling and pollinator-day “window” modeling. Is the decline in Osmia abundance since 2005 a result of climate change and wetter springs? This is unknown, but should be put forth as a viable hypothesis. Wet springs might differentially affect Osmia in 2 ways. First, we have observed that Osmia cease foraging on overcast days (Drummond and Stubbs 1997a). This finding suggests that Osmia might not use polarized light for navigation back to the nest as many bees do (Rossel 1993). Second, a significant disease of Osmia bees in Maine can be due to fungi of the genus Ascosphera (Batra et al. 1973, Drummond and Stubbs 1997a). Fungal diseases of insects are often enhanced by cool, wet weather (Tanada and Kaya 2012). Our third study adds evidence that climate change is already occuring in Maine. The number of days for spring bees to forage on Lowbush Blueberry flowers has been declining since the early 1990s. Lowbush Blueberry farmers are already aware of this (F.A. Drummond, unpubl. data). They have continued to import an increasing number of Honey Bees (from about 25,000 colonies in 1990 to more than 74,000 in 2013 and to more than 80,000 in 2015; Drummond 2012, Yarborough 2009). The reason for the increase in Honey Bee importation has been due to Lowbush Blueberry growers attempting to reduce risk of crop loss if a wet spring occurs, resulting in fewer days for bees to visit flowers, but the same number of flowers to set into fruit (F.A. Drummond, unpubl. data). Therefore, increasing rain during bloom is Northeastern Naturalist F.A. Drummond, A.C. Dibble, C. Stubbs, S.L. Bushmann, J.S. Ascher, and J. Ryan 2017 Vol. 24, Monograph 15 64 costing farmers financially, but is it affecting native bees? One might expect that it would because the provisioning of immatures by female bees might be reduced due to less time available for food-resource foraging or potentially a more complex interaction involving competition with Honey Bees. Some of these changes in bee abundances might be addressed through emphasis on habitat improvement. For growers of Lowbush Blueberry, that will include adding floral resources to sustain bees near the crop even in the prune year (when no crop flowers are available) and minimizing pesticide exposure (Goulson et al. 2015, Grixti et al. 2009, Venturin et al. 2017a). Acknowledgments We appreciate funding from the US Department of Agriculture National Institute of Food and Agriculture (Specialty Crops Research Initiative Contract/Grant/Agreement No. 2011-51181-30673), the University of Maine, the Wild Blueberry Commission of Maine, and Stewards LLC (to A.C. Dibble). This project was supported by USDA National Institute of Agriculture, Hatch Project number ME0-21505 through the Maine Agricultural and Forest Experiment Station. We thank Wallace E. LaBerge (posthumously), Terry Griswold, and Sam Droege for identifying the Osmia and Megachile species captured in Lowbush Blueberry fields in 1990, 1997–1998, and 2010–2012; respectively. We also thank Eric Asare for assisting in gathering the historical weather data and calculating pollination days during Lowbush Blueberry bloom. Michael Veit, Lawrence Academy, MA, and 4 anonymous reviewers provided valuable edits and comments on earlier drafts of this manuscript. This is Maine Agricultural and Forestry Experiment Station Publication No. 3565. Literature Cited Asare, E., A.K. Hoshide, F.A. Drummond, X. Chen, and G.K. Criner. 2017. Economic risk of bee pollination in Maine Wild Blueberry, Vaccinium angustifolium Aiton. Journal of Economic Entomology 110:1980–1992. Ascher, J.S., and J. Pickering. 2016. 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