Wild Bees in the Trees: Pollen Analyses to Determine Wild Bee Foraging in Early Spring Canopies

View the project final report

Commodities

• Fruits: apples

Practices

• Crop Production: forest/woodlot management, pollinator habitat, pollinator health
• Natural Resources/Environment: biodiversity, hedgerows
• Production Systems: agroecosystems

As honey bee winter losses remain high, orchard pollination increasingly relies on diverse wild bee populations. Not much is known about the importance of early-season forest canopies for supporting pollinators, likely because many forest trees are wind-pollinated and canopies are difficult to access. Last year, the student employed novel canopy-sampling methods and caught over half of known pollinator species over 20 meters high in wind-pollinated tree canopies during the critical resource-poor weeks before apple trees bloom. An individual forest tree can have hundreds of thousands of flowers, orders of magnitude more than understory ephemerals. There is a large knowledge gap as to how these trees support pollination services. This project will use molecular methods to investigate the diet of those canopy-caught bees and identify the forest and hedgerow trees whose pollen is most consumed by important orchard pollinators. Using pollen samples collected from each of the common tree species, each will be analyzed for protein to lipid ratios, a metric relevant for diet quality and bee preference. Pollen production per tree and tree abundance per woodlot will be calculated, to create a complete picture of the quality, quantity, and bee preferences of each species. Finally, the pesticide residues will be analyzed at the Cornell Chemical Ecology Core Facility, and compared with the spray records at each orchard, to determine the toxicity of tree pollen use. This work will directly inform orchardists in management of multifunctional woodlots and hedgerows to increase biodiversity in fruit tree pollination services and overall on-farm sustainability.

In spring 2017 and 2018, I employed novel canopy-sampling methods using elevated “bee bowl” traps at 11 sites in NY state. I caught wild bee pollinators in the canopy and understory of woodlots adjacent to pollination-dependent fruit trees. Woodlots were dominated by wind-pollinated species, and more than half of captured bees were known apple pollinators (Russo et al. 2015, Blitzer et al. 2016). Canopy bees were highly abundant during tree bloom, and rapidly declined after, supporting the idea that bees were foraging.

Here, I propose three research objectives investigating(1) bee foraging preferences on tree species, (2) the trees that provide optimal nutritional quantity and quality, and (3) bees’ exposure to pesticides when visiting forest and hedgerow trees near orchards.

(1) I will use DNA barcoding to identify the pollen from guts of trap-caught wild pollinators. Traps demonstrated immense diversity of canopy-foraging bees, but not which trees the bees preferred. Molecular pollen identification methods will provide detailed tree pollen use by the wild pollinating bees. I hypothesize that pollen preferences vary over space, time, and the gradient of forest cover in the surrounding landscape.

(2) I will quantify the pollen quality and quantity of important forest and hedgerow trees with metrics relevant to bee health. To estimate pollen quantity, I will improve estimates of flower abundance for trees and shrubs in an in-development floral resource database for the Finger Lakes region of NY (Iverson et al. unpublished data), then combine these data with per-flower estimates pollen abundance. The only available pollen quality data is missing many local species and provides only protein content (Roulston et al. 2000), while protein:lipid ratio of pollen is most important for pollinator choice (Vaudo et al. 2016a, 2016b). I thus propose to collect and calculate protein:lipid ratios for the common wind- and insect-pollinated forest trees and compare these with apple pollen in adjacent orchards. I will ask if the preferred trees identified in objective (1) also have the highest quality or quantity pollen.

(3) I will quantify pesticide residues in the pollen of forest and hedgerow trees. Using pollen from trees collected 100-500m from the orchard, I will analyze pollen samples for 41 compounds commonly applied in our landscape at the Cornell Chemical Ecology Core Facilities, through the McArt lab. I will calculate toxicity estimates based on the literature and compare wild tree pollen toxicity relative to apple pollen.