Pollen Microscopy for Beekeepers

I keep a few honey hives in my backyard, in North Seattle area, Washington, USA. While I've observed honey bees on a variety of flowers positioned at eye level or lower, spotting them on large trees has always proven challenging. To learn more about the flowers they like, I started looking at the pollen they bring back through 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 gathered three pollen chunks from the hive. To capture any pollen accidentally dropped by honey bees, I had inserted a removable plastic board on the hive bottom (this arrangement is primarily employed 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 have collected bee pollen from hind legs of dead bees and from varroa mite monitoring boards. I have never used pollen traps but they should work too. I usually analyze pollen within a few days of collection. Wet samples tend to give poor results, but overly-dry samples may still be used by adding some water.

Record pollen color before microscopic analysis. Place pollen chunks on a white paper and take a photo under bright light. It probably looks darker than the same pollen on a honey bee, but may still help you to identify the source plant.

Mounting pollen samples and shooting photos under microscope

Sample preparation and mounting methods greatly affect how pollen grains look under microscope. To use my flower pollen library as reference, please prepare your bee pollen as follows.

Place a chunk of bee pollen on a slide glass.
Put a small drop of glycerin (aka glycerol, available at drugstores) beside the pollen.
Place a coverglass and smear pollen lightly.
Immediately take pictures, within a half minutes (some pollen grains cannot retain their shapes in glycerin).
Reuse slide glasses and coverglasses after washing/drying, if you wish.

Microscopes

You don’t need to spend thousands of dollars on a microscope. I use a Plugable 800x Inverted Digital Optical USB Microscope, which I bought from Amazon for $80 in 2017 (unfortunately discontinued), connected to a computer. I believe inverted microscopes that can connect to smartphones are available for around $100–$500. I also purchased other accessories from Amazon, such as slides, cover glasses, and a calibration slide.

Cross-referencing your bee pollen to my flower pollen library

My flower pollen library includes the majority of honey bee-friendly plants that grow in Pacific Northwest of America, except uncommon garden plants. I think you can also find many plants that grow in northern states of central and eastern US, Canada, UK, western-central-eastern Europe. However, plants that only grow in USDA Zone 9 -10 areas (central and southern Florida, southern Louisiana, southern Texas, southwestern coast of California, Hawaii, southern Europe and etc), are not included.

Identification to species is usually not possible, except when the species of interest does not have close relatives in the area (e.g., tulip poplar, Douglas fir, English ivy, and etc) or the species is far more abundant than other similar looking species (e.g., dandelion in winter, cherry plum in early spring, big-leaf maple, red-osier dogwood, English holly, Pacific willow in May, Himalayan blackberry, Japanese knotweed, jewelweed, and etc).

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

Pollen wiki : Contains over 2000 species of pollen images. Google translation to English is available.

Pollen pictures (Discoverlife.org) : Not as organized as Pollen Wiki, but still helpful.

Australian pollen and spore atlas: Includes tropical species.

Pollen grains of Canadian honey plants