Each module includes background information for educators, vocabulary words, activities, and discussion questions. All content, images, and videos are downloadable at no cost.
Without pollinators, our diet would be very bleak. Insects pollinate about 85% of major food crops, but some bees are better pollinators than others.
Humans rely on pollinators. The ecosystem services they provide are essential for agriculture and global food security, but the extent of this contribution can be difficult to quantify.
In 2014, the White House estimated that insect pollination services in the United States are valued at $24 billion per year for all insect pollinators, $9 billion of which is from native pollinators. Of course, these exact numbers are difficult to estimate, but with over 90% of angiosperms utilizing insect pollination for reproduction the overwhelming importance of insect pollination is clear.
Up to 85% of major commercial crops depend, at least partially, on animal pollination for seed and fruit production and for achieving maximum yields. The demand for fruits and vegetables to feed our growing population has only continued to increase. However, we are facing a decline in wild pollinators under threats from climate change, habitat loss, pesticides, and disease (see Drivers of Bee Decline and What You Can Do). Historically, we have utilized managed populations of the European honey bee, Apis mellifera, to supplement our pollination needs, but honey bees are not always the best pollinators for the job.
Mason Bee Pollination
Bees can be classified as specialists or generalists depending on the diversity of plants they forage from. There are two categories of specialist bees, oligolectic bees, which forage on plants within a single family or genera and monolectic bees which are only known to collect pollen from a single species or handful of closelyrelated species of plants. Generalists bees, or polylectic bees, forage on a wide variety of flowering plants in different taxonomic groups. Honey bees are the most well known generalist bees, but many mason bees, including Osmia lignaria are also generalists. It is important to note that generalist species don’t forage indiscriminately, in fact, many generalists focus on a diverse set of preferred taxonomic groups. O. lignaria for example, forages primarily on plants which flower in early spring, which coincides with nest provisioning (see Foraging Behavior in Mason Bees and Nesting and Mating Behavior).
Figure 1. Osmia are extremely efficient pollinators due to both their anatomy and behavior. This pollen covered Osmia georgica female shows us why (Image: Missouri Department of Conservation).
This may beg the question, why should we care about specialist bees if generalist bees pollinate a broader array of plants? Not only does biodiversity increase an ecosystem's resilience to change, potentially reducing the impacts of climate change, urbanization, and environmental contamination, but specialist bees and narrow generalist bees may be the only suitable pollinators for certain plants or just more efficient pollinators. As an example, the focal bees of this program, Osmia, are incredibly efficient pollinators of spring blooming flowers in the family Rosaceae, which includes grocery store staples like apples, pears, and cherries (Figure 2). Honey bees, which are broad generalists, are inefficient in comparison (we will discuss why in the next section).
Some farmers noticed how efficient mason bees are as pollinators and now manage populations on their farms. Farmers can either set out artificial nests and collect bees from local wild populations or nowadays bees can even be purchased online from a supplier. The process of managing mason bees can vary from a relatively hands off approach to a much more complex process where farmers will use refrigerators to manipulate the timing of bee emergence, matching it with the peak of their crop’s bloom, and ensuring maximum pollination. However the basic process is about the same, a population of bees is placed in the field and allowed to pollinate crops and build nests in artificial structures. At the end of the bee’s active period and following the bloom the nests are collected, screened for parasites, and then stored for the winter. In early spring the nests are returned to the field to pollinate the next year’s crop, allowing the process to restart.
Mason bees vs honey bees
Honey bees are finicky and it may come as a surprise, but they are not particularly good pollinators either. For the early spring flowers, like those favored by some bees, they are downright inefficient. Honey bees will not forage in poor weather, which is risky for apple and cherry farmers — and anyone that enjoys the fruit they produce — who depend on crop pollination during a short window in the spring when weather can be variable. Mason bees are less picky and will forage even under poor weather conditions, such as on cool and cloudy days with light wind and rain (see Foraging Behavior in Mason Bees).
Figure 2. An Apple orchard in full bloom. Apple trees bloom in early spring when the weather can be variable (Image: Bonnie Moreland, Flickr)
First, let us start with anatomy. Mason bees store and transport dry pollen in their abdominal scopa (see Bee Anatomy). When the bee visits each flower there is contact between the bee’s scopa and the flower’s female reproductive structure, the stigma. The pollen easily falls off of the bee’s scopa and on to the stigma during almost every visit, which facilitates efficient pollination. In contrast, when a honey bee visits a flower, it stores pollen as a sticky mixture with nectar attached to the sides of their hind legs which limits pollen transfer to the plant’s stigma.
Second, let us consider the differences in bee behavior. Honey bees cheat. Mutualistic biological relationships rely on both parties doing their part and plant-pollinator relationships are no different. However, honey bees don’t always do their part. Specifically, they will often collect nectar from off to the side of the flowers they visit, never coming in contact with the plant’s reproductive structures (Figure 3). This behavior is called nectar robbing and allows honey bees to “steal” floral resources without fulfilling their end of the deal, pollinating the plant (youtu.be/PICUGaDY5FM). Mason bees' more direct, “headfirst” approach results in contact between the bee’s abdomen and the flower’s reproductive structures, facilitating pollination at nearly every visit (youtu.be/xyd9DgvjhF4). More discussion about the foraging behavior of mason bees and how this impacts pollination can be found in the Foraging Behavior in Mason Bees module.
Concerns with mason bees as managed pollinators
There are downsides to all managed pollinators and mason bees are no exception. The introduction of non-native mason bee species to the United states for use as managed pollinators has facilitated the spread of disease, led to the introduction of invasive bee species, and resulted in increased competition for native bees. Unfortunately, even the process of shipping pollinators within the United States is not without risk and can facilitate disease spread to new areas placing managed and wild bee species at risk.
Figure 3. A honey bee “sideworking” an apple blossom. Notice how there is no contact between the bee and the reproductive structures of the flower (Image: born1945, Flickr, cropped)
Figure 4. Many native Osmia species in the eastern US are in decline, however, two introduced species are increasing in abundance (Figure from LeCroy et al. 2020).
Recent research has documented the impacts of introduced mason bees in the eastern United States, where many native species are in decline. Here, populations of introduced species, such as Osmia cornifrons and Osmia taurus, are stable or even increasing (Figure 4). Of course, these concerns aren’t unique to mason bees. In fact, we can hypothesize that many of the same threats to native bees followed from the introduction of honey bees to North America in 1622 and the introduction of managed populations of nonnative bumble bees. Careful practices, such as prioritizing the use of native bee species, using locally reared managed bee populations, and parasite/disease screening, can help limit, although likely not eliminate, the impact of managed pollinators on wild species (See Drivers of Bee Decline and What You Can Do).
Honey bees are not the only bees used to pollinate crops. There are a number of alternative managed crop pollinators which are used and can be even more effective for certain crops. Native bees also play an important role in crop production and pollination of wild plants.
Ecosystem services - The benefits to humans provided by natural ecosystems.
Monolecty - Refers to highly specialized bees which forage for pollen from a single plant species. Occasionally the term will also be used to refer to bees which specialize on a few closely related plant species within the same genus.
Nectar Robbing - A foraging behavior in which a bee (or other pollinator) collects nectar from a flower without facilitating pollination by avoiding contact with the reproductive structures.
Oligolecty - Refers to bees which specialize on plants from a single family or genus.
Polylecty - Refers to generalist bees which collect pollen from a diverse group of plants.
Scopa - Specialized hairs (setae) used to collect and transport pollen.
Stigma - A flower structure on which pollen germinates during plant reproduction, which is located at the end of the pistil, the female reproductive organs.
Pollinator Limitation— Data Analysis
We tend to think of pollination of flowers as either “yes - the flower is pollinated,” or “no - the flower is not pollinated,” but the process isn’t quite so clear-cut. Pollinator limitation is where a plant may not be able to maximize fruit size or seed number if too little pollen is transferred to it by pollinators. Pollinator limitation may reduce agricultural productivity and also plant reproductive success.
In this activity, we will explore the importance of bee diversity in watermelon pollination by analyzing the results of a 2018 publication by Campbell et al. and then explore pollination limitation in store bought apples with help from Garratt et al.’s 2014 study on the topic.
Figure 1. Pollinator limitation results in lower seed set. Plants which are fully pollinated such as hand pollinated plants have the highest seed set, followed by flowers which are able to be pollinated by many different bees species. Finally, flowers that are only pollinated by one species (in this case bumble bees) have the lowest seed set.
Part 1: Watermelon Pollination
Watermelons are pollinated by a number of bee species. Managed bumble bees species such as Bombus impatiens are often used to help pollinate commercially grown watermelons. In their study, Campbell et al. explored how melon weight and fruit set are affected by pollinator availability. They used three treatments:
A bumble bee cage where all pollinators except B. impatiens were excluded,
A pollinator excursion cage where all pollinators were excluded, and
An open area or field where bumble bees, honey bees, and wild bees can all pollinate the flowers.
Using the figures below study how each of the trials affects melon weight and percent fruit set per flower and answer the following discussion questions.
Figure 2. Top: The mean melon weight (g) for each treatment. Bottom: Percent fruit set per flower. The error bars represent the Standard error and different letters show a statistically significant difference (Figures from Campbell et al. 2018).
Describe the general relationship between each treatment. Which trial has the least pollinator limitation? Which has the greatest? Are any of the trials statistically similar?
- If you were a watermelon farmer, how might this information inform your farming practices?
What other information might you want to know before making a decision?
Part 2: A hands on look at pollinator limitation in apples
Fruit quality is measured using many variables including width, weight, sugar content, and number of seeds. Using these metrics, Garratt et al. evaluated how pollinator limitation may affect fruit quality in two varieties of apples. They summarized their results in the Figure 3.
Note: Hand pollination is when researchers or farmers act as the pollinator and transfer pollen from one flower to another themselves, often using a small brush.
Figure 3. The number of seeds per apple in each treatment group for two varieties of apples a) cox and b) gala (Figure: Garratt et al. 2014)
A) Discuss the results: Which treatment resulted in the most seeds/apple? The least? Why might we see this relationship? Are these trends consistent between both apple varieties?
B) Hands on: Now that we have a better understanding of how pollinator limitation can affect the number of seeds per apple let us look at some apples of our own.
Apples (One per student or small group)
Create a table with columns for apple width, weight, and number of seeds.
Apple size: Weigh each apple individually. Carefully cut the apple in half and measure
the width at the widest point. Record the values.
Count the number of fully developed seeds in each apple and record on your data table.
Share your data with the class.
Data Analysis and Interpretation:
Using the class dataset, create a plot of the mean apple width (or weight) vs. the number of seeds per apple. Describe the relationship.
Is there any evidence of pollinator limitation? If so, what other information might you need to determine this with certainty? (Note: a fully pollinated apple will contain 10 seeds)
Aizen, Marcelo A., Lucas A. Garibaldi, Saul A. Cunningham, and Alexandra M. Klein. 2009. “How Much Does Agriculture Depend on Pollinators? Lessons from Long-Term Trends in Crop Production.” Annals of Botany 103(9): 1579–88.
Bosch, Jordi, and William P Kemp. 2002. Sustainable Agricultural Network How to Manage the Blue Orchard Bee.
Bosch, Jordi, William P. Kemp, and Glen E. Trostle. 2006. “Bee Population Returns and Cherry Yields in an Orchard Pollinated with Osmia Lignaria (Hymenoptera: Megachilidae).” Journal of Economic Entomology 99(2): 408–13.
Carpenter, Madeline H., and Brock A. Harpur. 2021. “Genetic Past, Present, and Future of the Honey Bee (Apis Mellifera) in the United States of America.” Apidologie.
LeCroy, Kathryn A. et al. 2020. “Decline of Six Native Mason Bee Species Following the Arrival of an Exotic Congener.” Scientific Reports 10(1): 1–9. https://doi.org/10.1038/s41598-020-75566-9.
Russo, L., M. G. Park, E. J. Blitzer, and B. N. Danforth. 2017. “Flower Handling Behavior and Abundance Determine the Relative Contribution of Pollinators to Seed Set in Apple Orchards.” Agriculture, Ecosystems and Environment 246(May): 102–8. http://dx.doi.org/10.1016/j.agee.2017.05.033.
The White House Office of the Press Secretary. 2014. “Fact Sheet: The Economic Challenge Posed by Declining Pollinator Populations.”
Vicens, Narcís, and Jordi Bosch. 2000. “Weather-Dependent Pollinator Activity in an Apple Orchard, with Special Reference to Osmia Cornuta and Apis Mellifera (Hymenoptera: Megachilidae and Apidae).” Environmental Entomology 29(3): 413–20.