Pollen Microscopy for Beekeepers

I keep a few honey bee hives in my backyard in the North Seattle area, Washington, USA. While I have observed honey bees foraging on a variety of flowers at eye level or lower, spotting them on large trees has always been challenging. To better understand which flowers they prefer, I began examining the pollen they bring back under a microscope.

In the sunny afternoon of late January, my honey bees brought back yellow pollen loads. Where did they come from?

Honey bees bringing back yellow pollen, January 28, 2022

The next morning, I collected three pollen chunks from the hive. To capture any pollen accidentally dropped by the bees, I had inserted a removable plastic board on the hive bottom—a setup primarily used for monitoring Varroa mites.

Observing bee pollen under microscope

Place a chunk of pollen on a slide glass.

Drop small amount of glycerin.

Place a coverglass, lightly smear the pollen chunk, and immediately* take a photo.

*Because I do not make permanent slides that require fixing agents, I take a photo within a half minute of smearing pollen.

I used an inverted microscope (800x), connected to a computer

Bee pollen under microscope (x800)

Bee pollen 1

Bee pollen 2

Bee pollen 3

Scale

The images above correspond to the pollen images obtained from hazelnut species.

Pollen from European hazenut

European hazenut pollen, under microscope

In my neighborhood, the two most common hazelnut species are Corylus avellana (European hazelnut) and Corylus cornuta (beaked hazelnut). Both species produce yellow pollen, and their microscopic shapes are indistinguishable. Corylus avellana blooms from December to January, while Corylus cornuta begins blooming in February. Since the bee pollen samples were collected in late January, it is likely that they originated from Corylus avellana.

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Collecting bee pollen

I collected bee pollen from the hind legs of dead bees and from Varroa mite monitoring boards. I have not used pollen traps, but they should work as well. I usually analyze pollen within a few days of collection. Wet samples tend to give poor results, although overly dry samples can still be used by adding a small amount of water.

Before microscopic analysis, record pollen color. Place pollen chunks on white paper and photograph them under bright light. The pollen may appear darker than when carried on a honey bee, but color can still be helpful for identifying the source plant.

Mounting pollen samples and shooting photos under microscope

Sample preparation and mounting methods greatly affect how pollen grains appear under the microscope. I tested several mounting methods, and the results are summarized in the table below. Based on these results, I decided to use glycerin mounting (with immediate photography) in most cases. I also used corn syrup mounting to approximate the pollen shape found in honey. See the Microscopy – Miscellaneous page for photos of pollen in various mounting media and the Pollen Analysis of Honey page for additional images of pollen in honey and corn syrup.

Microscopes

You don’t need to spend thousands of dollars on a microscope. I use a Plugable 800× inverted digital optical USB microscope, which I purchased from Amazon for about $80 in 2017 (now discontinued), connected to a computer. Comparable inverted microscopes that connect to smartphones are currently available for roughly $100–$500. I also purchased basic accessories from Amazon, including slides, cover glasses, and a calibration slide.

Cross-referencing bee pollen with my flower pollen library

My flower pollen library includes the majority of honey bee–friendly plants commonly found in the Pacific Northwest of the United States, excluding uncommon ornamental garden species. It also covers many plants native to or widely cultivated in the northern regions of the central and eastern United States, Canada, the United Kingdom, and western, central, and eastern Europe. Plants restricted to USDA Zones 9–10 (e.g., central and southern Florida, southern Louisiana and Texas, the southwestern coast of California, Hawaii, and parts of southern Europe) are generally not included.

Identification to species level is usually not possible, except in cases where the species has no close relatives in the region (e.g., tulip poplar, Douglas fir, English ivy) or where one species is overwhelmingly more abundant than similar taxa at the time of flowering (e.g., dandelion in winter; cherry plum in early spring; bigleaf maple; red-osier dogwood; English holly; Pacific willow in May; Himalayan blackberry; Japanese knotweed; jewelweed).

To use my flower pollen library as a reference, please prepare your bee pollen as follows:

Place a chunk of bee pollen on a microscope slide.
Put a small drop of glycerin (also called glycerol; available at drugstores) beside the pollen.
Place a coverslip over the sample and gently smear the pollen.
Take photographs immediately, within half a minute, as some pollen grains do not retain their shape in glycerin.
Slides and coverslips may be reused after washing and drying, if desired.

Other resouces

Other online flower pollen libraries (to effectively use them, your samples should be prepared according to their methods)

PalDat (Pollen and Spore Database): A very large global palynology database with detailed descriptions and images (light microscopy and often scanning electron microscopy) of pollen grains from many plant families.

Pollen wiki (PollenAtlas) : A searchable online atlas of light micrographs of pollen and spores, including many seed plants, useful for quick visual reference. Google translation to English is available.

Pollen pictures (Discoverlife.org) : Not as organized as the above two databases, but still helpful.

Pollen grains of Canadian honey plants : A reference book showing microscope images and descriptions of pollen from common Canadian honey plants, widely used for honey pollen analysis.

Australian pollen and spore atlas: A searchable online atlas covering over 15,000 pollen and spore species from the Australasian region.

Global Pollen Project: A crowdsourced pollen reference library where users can explore digitized pollen slides, submit grains for identification, and compare against an expanding global collection of pollen taxa.