DOI: 10.5061/dryad.xd2547dmn
title: Early spring orchard pollinators spill over from resource-rich adjacent forest patches

description: Pollinator communities are more abundant and diverse in agricultural
 matrices with more natural habitats, although the reasons for these
 correlations remain unclear. It is possible that forest fragments and
 edges provide resources for pollinators in the important early weeks of
 spring, after which time those insects can then “spill over” into crops
 such as apple orchards during bloom. To explore how forest edges may feed
 and therefore promote flower visitor communities in adjacent agricultural
 habitats, we sampled springtime pollinators in nine orchards and their
 adjacent forest edge canopies and understories. We identified pollen
 consumed by pan-trapped bees and flower flies to assess if bees ate pollen
 where they were caught and if their diets similarly “spill over” from
 forest to orchard. We further explored sex differences in habitat usage.
 Our spatially replicated sampling found that bee and flower fly abundance
 peaks first in the forest understory, then in the forest canopy, and
 finally in the orchard. Analysis of digestive tracts showed significant
 usage of forest canopy pollen throughout the spring, especially before
 apple bloom. Pollinators had often eaten pollen from a different habitat
 than the one in which they were caught, suggesting frequent movement
 between habitats. Digestive tract pollen is an underused but powerful
 avenue for ecological insight. In Andrena, which are important orchard
 pollinators and one of the most abundant wild bee taxa in this study, male
 bees were primarily found in the woods but not the orchards where
 conspecific females were later active. Synthesis and policy implications:
 Forested areas, especially forest canopy trees, provide large amounts of
 early spring resources that facilitate the build-up and spillover of wild
 pollinator populations into apple orchards during bloom. Forests also
 provide critical habitats for male bees, which were rarely found in
 orchards. Despite their importance for bee reproduction, the needs of male
 bees are usually not considered in conservation planning. Overall, our
 data indicate that ensuring there is adequate forest habitat adjacent to
 orchards can improve the long-term sustainability of pollinator
 populations that provide essential crop pollination services.
description: <strong>Bee and flower fly collection</strong>: This
 study was completed in the Finger Lakes region of New York, in the
 northeastern United States. Bees and flower flies (Syrphidae), two major
 groups of agricultural pollinators (Rader et al., 2016) were collected in
 nine second-growth deciduous forest patches that were selected for their
 adjacency to orchards. The most abundant tree species across sites were
 <em>Acer saccharum</em>, <em>Acer rubrum</em>,
 <em>Quercus rubra</em>, <em>Fagus sylvatica</em>,
 and <em>Betula</em> spp. Several sites also had
 <em>Tilia americana</em>, <em>Populus
 deltoides</em>, <em>Juglans nigra</em>, <em>Carya
 ovata</em>, or <em>Carya cordiformis</em> (see percent
 basal area at each site in Supplemental Figure A1). For simplicity, we
 will refer to bees and flower flies as “pollinators” hereafter. For full
 forest sampling methods, see Urban-Mead et al., (2021). Briefly, five
 paired canopy and understory pan traps in sets of vertically randomized
 blue, yellow, and white were deployed in canopy-dominant trees in
 second-growth forests and woodlots beginning in mid-March 2018-2019. A
 BigShot® slingshot was used to set a line of paracord with which to rig up
 the canopy traps at 25-30 meters above ground; canopy traps were not
 visible from the forest floor. Understory traps were ~1-meter
 aboveground. In addition to our canopy and understory
 sampling, we placed identical sets of traps in the branches of five
 randomly selected apple trees haphazardly spaced across an orchard block
 &lt;300 meters from the forest edge. Management varied between
 orchards, whose selection was constrained by forest adjacency. These pan
 traps were emptied and reset once every 7-10 days, on the same schedule as
 parallel forest sampling, until after apple orchards stopped blooming
 (corresponding roughly to the first week of June in both years).
 Pollinators were transferred directly into Whirlpaks (Nasco Whirlpak, Fort
 Atkinson, WI) with 95% ethanol in the field. Specimens were identified to
 species with a variety of taxonomic keys (Bouseman & LaBerge,
 1978; Gibbs, 2011; Gibbs et al., 2013; Laberge, 1971; LaBerge, 1973, 1980,
 1985). Difficult taxa were supported by expert knowledge (Jason Gibbs
 assisted with <em>Lasioglossum</em>
 (<em>Dialictus</em>), and Kaitlin Deutsch and Andrew D. Young
 with family Syrphidae), and DNA barcoding (for full methods, see
 (Urban-Mead et al., 2021). All specimens have been accessioned to the
 Cornell University Insect Collection. For clarity and
 consistency, we will use the term “farm” or “site” to refer to a given
 orchard and its adjacent forest location, while “habitat” refers to the
 unique sampling location within each replicate site: the canopy,
 understory, and orchard. The five trees per habitat (canopy, understory,
 or orchard) were 100-200 feet apart, so are summed or averaged for most
 analyses due to their small spatial scale relative to pollinator flight
 distances (Greenleaf et al., 2007).
 <strong>Digestive tract dissection and pollen slide
 preparation</strong>: Each insect was dissected directly upon
 removal from the Whirlpak bags while still pliable and soft from ethanol
 storage. Under a dissecting scope, each bee’s sternal segments were peeled
 backward towards the thorax with fine forceps, allowing for a second pair
 of forceps to extract the full digestive tract (crop, ileum, midgut, and
 rectum) without damage to external characters, as these were required for
 later species identification (Urban-Mead et al., 2021). The gut contents
 were placed on a microscope slide and macerated with forceps to release
 internal pollen and homogenized until evenly distributed across the slide.
 We removed large tissue fragments and broke up pollen clumps using
 forceps. Several drops of Calberla’s solution were added until pollen
 grains turned distinctly pink; a constant volume was not possible due to
 different pollen volumes. Each slide was covered with a 22x50mm
 coverslip. Pollen identified from an individual adult
 bee is usually collected from its scopal loads following net collection,
 and represents the current foraging bout’s collection for brood
 provisioning. However, for bees caught in pan traps or other liquid traps,
 this scopal pollen can wash off or become contaminated, making it
 difficult to assign external pollen confidently to a given specimen. In
 these cases, identification and characterization of digestive tract pollen
 can provide insight into adult consumption (e.g. Cane et al., 2017; Dobson
 & Peng, 1997; Käpylä, 1978; Taniguchi, 1956; Urban-Mead et al.,
 2022). Pollen eaten by an adult bee may differ from that collected for
 brood provisioning, so gut pollen should only be interpreted in the
 context of an adult bee diet unless both are simultaneously recorded. We
 suggest that this method has the benefit of likely representing multiple
 visited flowers over several hours rather than simply the most recent
 foraging bout; many bees are constant within a single foraging bout but
 not between bouts or days (Brosi, 2016). Although poorly studied, healthy
 bumble bees can take over 13 hours to defecate after a meal (Giacomini et
 al., 2022), so digestive tract pollen likely represents foraging bouts of
 at least a day. <strong>Pollen
 identification</strong>: In both years, we collected pollen from
 blooming plants to create a reference library and maintained detailed
 phenological records at each site. Only the site name and date were
 revealed to allow for phenologically informed identification; all other
 metadata was hidden to avoid bias. We used a compound Olympus BX41
 microscope at 400x magnification (40x with 10x ocular). We began each
 sample at a randomly generated point along the slide’s short edge and
 moved along the length of the slide categorizing all pollen in the field
 of view. We categorized up to 300 grains or up to ten unique transects per
 slide; slides were marked as “no pollen” if there were fewer than 30
 grains of pollen (Harmon‐Threatt & Kremen, 2015).
 Each grain was identified and then sorted into one of three
 pollen categories: canopy, Rosaceae, and others. Pollen types in the
 category “canopy” included: <em>Acer</em>,
 <em>Populus</em>, <em>Betulaceae</em>,
 <em>Carya</em>, <em>Fagus</em>,
 <em>Juglans</em>, Pine-type, <em>Quercus</em>,
 <em>Fraxinus</em>. The “Rosaceous” category, due to similar
 morphology, broadly included all apple, peach, apricot, and other related
 orchard trees. It may also include Rubus and wild strawberry, known to be
 favored hosts of many Syrphid species, and
 <em>Amelanchier</em>, all of which were often found within the
 orchard or along edges. Black Cherry (<em>Prunus
 serotina</em>) is also rosaceous but was not a dominant tree in our
 woods, so we do not believe meaningfully impacts our inferences (mean
 percent of total basal area 3.3% ± 1.9SD). Not all understory species
 could be identified with confidence; these were combined into a macro
 category “Understory, Other, and Unknown Pollen.” We took multiple voucher
 images for each slide; see Supplemental Table A1 for detailed pollen
 categories and vouchers.