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Animal Behavior, Reproduction

Where do bees live, and why do they collect pollen? Learn about how mason bees prepare for the next generation and why it is such a monumental task. This module's activity must be completed in mid to late May.






In stark contrast to the large intricate nests constructed by honey bees and their characteristic repeating hexagonal wax comb, or the papery and sometimes grandiose nests of some social wasps, mason bee nests are understated and easy to overlook.

The elegance of the mason bee nest is in its simplicity. They built their nests in elongated cavities, often in wood, which the bee divides into individual chambers, or brood cells, with mud partitions (or in some species leaf or plant fiber partitions). These partitions give mason bees their names; just as a brick mason constructs brick and mortar walls, female mason bees construct mud walls within their nests.

Figure 1. A cross section of an Osmia nest reed showing two complete brood cells each with a pollen provision and an egg, the empty vestibule space, and the nest plug or cap. (Image: © entomart)

Figure 1. A cross section of an Osmia nest reed showing two complete brood cells each with a pollen provision and an egg, the empty vestibule space, and the nest plug or cap. (Image: © entomart)

Adult female bees work with vigor over their short 20 days of activity. Between emerging from their cocoons and death, each bee constructs two to four nests (some long-lived female bees can build up to seven). Each of these nests contains an average of five brood cells. Each brood cell requires 25 loads of pollen to provision, and 75 floral visits per load, meaning each bee has to make over 28,000 floral visits in their lifetime— no wonder they are such prolific pollinators.

To add perspective, in 2015, Americans made an average of 1.5 trips to the grocery store per week (source: FMI U.S. Grocery Shopper Trends). If you consider each pollen-collecting trip analogous to a grocery store trip, and each bee makes 375 trips in 20 days, it would take the average American nearly five years (4.8 years) to make an equivalent number of trips. And keep in mind that the bees aren't just stopping at one "store" or flower; they stop at 75 on each trip. If that doesn't seem like enough of a challenge, mason bees don't only travel to the grocery store. They collect mud, lay eggs, protect their nests against parasites, and find and inspect new nest locations. It starts to make the phrase "busy as a bee" seem like a gross understatement.

In this module, we will explore this hectic period of the blue orchard mason bee life. We will focus on mating behavior, the prenesting period, and nest construction and oviposition. More detail on solitary bee foraging is available in the Foraging Behavior in Mason Bees module.

Mating Behavior

Most mason bees have one generation, or brood, per year. Nesting and mating activity begins in spring, with males emerging from their natal nests two to four days before the first female. Since mating occurs almost immediately after females emerge, it is not uncommon to see male bees waiting at the nests' exit or patrolling the surrounding area. A short-lived pheromone released by female bees as they emerge likely helps facilitate this behavior. Females often mate multiple times on their first day post-emergence, storing sperm in a spermatheca (for more information on bee reproduction see Life Cycle, Reproduction, and Development).

The Prenesting Period and Nest Selection

Shortly after mating, females leave the nest site for two to five days. During this absence, the prenesting period, the female bees complete their development, and their ovaries mature. While away from the nest site, the female bees will rest in crevasses of bark or shelter amongst clusters of males.

Most females will return to the nest site from which they emerged after several days. Although this isn't always the case, as some females will disperse from their natal nests during the prenesting period. A multitude of factors can influence the dispersal rate. One significant environmental factor is the weather during emergence, with rain and wind encouraging female dispersal.

Figure 2. A pair of Osmia cornuta mating. (Image: © Gilles San Martin, Flickr).

Figure 2. A pair of Osmia cornuta mating. (Image: © Gilles San Martin, Flickr).

Researchers are still working on identifying all of the causes of prenesting dispersal. However, in managed populations, they report higher dispersal rates when floral resource availability is low relative to pop- ulation size. How the bees are released, as loose cocoons or in nest tubes, also influences the dispersal rate, with greater rates of dispersal observed in populations released as loose cocoons.

Figure 3. An Osmia bicolor female near the entrance of her snail shell nest (Image: © Frank007, iNaturalist).

Figure 3. An Osmia bicolor female near the entrance of her snail shell nest (Image: © Frank007, iNaturalist).

Whether the females dispersed or returned to their natal nest site, they must select a suitable location to build their own nests. In the wild, mason bees usually nest in beetle holes in logs and stumps or hollow stems, but one European species, Osmia bicolor, even nests in empty snail shells (Figure 3). In managed systems, artificial nests consist of hollow reeds and straws inserted into a box, stacked grooved boards, or holes drilled into lumber. In any case, the female bees will inspect each potential nest before starting construction. For O. lignaria, the ideal cavity for a nest is about 8mm in diameter and 15 cm (6in) deep but other mason bee species prefer a wide range of cavity sizes depending on the size of the bee.

To limit parasitism, the cavity should be free of cracks, spaced away from the outer edge of the log or nest box to prevent parasite intrusion, and oriented to increase sun exposure and, in turn, foraging time. Females will inspect multiple cavities, multiple times each before settling on one to nest in. Once they decide where to build their first nests, they mark the entrance with a glandular odor and fly an orientation flight to memorize its location.

The Nest Construction and Oviposition

After selecting a nest, females will visit a nearby clayey mud source and collect mud balls that they carry back to the nest between their mandibles and forelegs. The mud from approximately ten trips is needed to build the nest's innermost wall.

After completing this initial partition, the females shift their focus to foraging for floral resources. When they first return, the bees will enter their nest headfirst and regurgitate the nectar they collected onto the mud partition. They will then back out of the nest, turn around, and back into the nest to deposit pollen by scraping it off of their abdominal scopa with their hind legs (see  Bee Anatomy). Each bee repeats this process an average of 25 times to build up a sufficiently large pollen provision for the developing larvae. The number of trips varies depending on the sex of the egg they will lay in the brood cell, with male offspring requiring smaller provisions than female (see  Sex Determination and Allocation). The female bees do not collect pollen on the final trip, only nectar which they regurgitate on the pollen provision’s surface. The bee then turns around and lays an egg (oviposits) on the provision's surface. The female bee will then collect mud to create another partition and seal the brood cell.

Figure 4. Different species of cavity nesting bees incorporate different soil types, leaves, or pollen into their nest plug. (Image:

Figure 4. Different species of cavity nesting bees incorporate different soil types, leaves, or pollen into their nest plug. (Image:

Each female bee repeats this process and progressively fills the cavity with individual cells. As the nest nears completion, the bee will seal the final brood cell before building a mud cap or plug the nest entrance (Figure 4). The plug is usually a thick mud wall spaced away from the last brood cell partition creating a vestibule space (Figure 1).

Each female bee will build several nests, usually 2-4, over their lifetime, with males making up a greater proportion of the offspring in the later nests as female foraging efficiency declines. Within each nest, male offspring are typically found in the outermost cells.

Common Misconceptions 

Not all bees live in hives; of the over 4000 bee species in the United States, only a fraction are social. Only one, the European honey bee (Apis mellifera), lives in the characteristic box hives we often associate with bees. Another major difference between our common conception of bee nesting and the reality for most bees is division of labor. In honey bees, workers are responsible for nest construction and foraging, leaving the queen free to focus solely on reproduction. As their name may suggest, solitary bees work independently to build nests. Each female is responsible for mating, nest construction, foraging, provisioning, and laying eggs.


Brood cell - A subcomponent or structure in a bee's nest which houses offspring during development. In solitary bees, each cell contains the pollen provision and an individual egg. In honeybees, each brood cell is left open, and the bees use other cell types for food storage.

Pollen Provision - A mass of pollen and nectar which serves as the food source for developing larvae.

Scopa - A dense patch of long pollen-collecting hairs, usually on the legs or underside of the abdomen

Spermatheca - A sperm storage organ associated with the female reproductive system in many bees and ants.

Spring Nest Dissection— Hands on

This activity should be completed in mid to late May, shortly after the bees seal their nests. Opening older nests will allow for many of the same observations, but the larvae may have already begun feeding on the pollen provisions.


1. Mason Bee nest reeds, one per group

Note: Nest reeds can be collected from your mason bee hotel or ordered online.

2. Knife

3. Optional: Analytic Balance with a readability of <0.010g


Mason bees construct nests in long and narrow cavities. You can find wild populations nesting in old wood-boring beetle holes and hollow stems. However, many species will also nest in artificial "bee hotels" made of reeds, straws, or holes drilled into lumber. The group's common name comes from the mud partitions they construct to separate each chamber, or brood cell, of the nest and a final thick plug that seals the nest's entrance.

The number of brood cells in each nest varies depending on the length of the nest cavity. Female bees lay each egg on a pollen provision, a mixture of pollen and nectar, which serves as the food source for developing larva. The size of the pollen provision depends on the sex of the egg. Since female mason bees are larger than males, they require more food during development. Female bees account for this variation by provisioning brood cells for males with smaller volumes of pollen.

Figure 1. A cross section of a mason bee nest. Notice the eggs, pollen provisions, mud partitions separating each brood cell, empty vestibule space, and mud plug (Image: © entomart).

Figure 1. A cross section of a mason bee nest. Notice the eggs, pollen provisions, mud partitions separating each brood cell, empty vestibule space, and mud plug (Image: © entomart).


Opening the nest:

1. Select a plugged nest reed

2. Insert the knife's edge into the end of the reed above the center point and twist to break it open

Opening the Cocoons and Collecting Data:

1. Sketch the internal nest structure

2. Label the major components of the nest

  1. Initial partition
  2. Pollen provisions
  3. Eggs
  4. Nest plug

3. Number each brood cell starting with the cell furthest away from the nest entrance.

4. Optional - Carefully remove and weigh each pollen provision using an analytical balance and be sure to note the brood cell number.

5. Record any differences between the brood cell sizes, pollen provision sizes, color and texture differences in the pollen provisions, and the presence of any other insects, eggs, or parasites.

6. Take note of where the egg is placed in the brood cell.

Discussion Questions:

1. What resources do mason bees require for nest construction?

2. Are there any differences between the size of the brood cells and pollen provisions?

3. Optional - are there any differences in weight between the pollen provisions? Are there any patterns in the distribution of these weights throughout these nest

4. How might intense drought impact the nesting and reproduction of mason bees?

5. What nest structures might stay the same for other species of mason bees that nest in different structures (e.g., snail shells)? What structures might change?


Nesting and Mating Behavior Module PDF



Nesting and Mating Behavior Activity PDF 

Nesting and Mating Behavior Activity Answers PDF


Boyle, N. K., Machtley, S. A., Hagler, J. R., & Pitts-Singer, T. L. (2018). Evaluating the persistence of fluorescent and protein powders on adult blue orchard bees, Osmia lignaria (Hymenoptera: Megachilidae), for mark-capture studies. Apidologie, 49(3), 378–385. 018-0564-4

Bosch, J., & Kemp, W. P. (2002). How to manage the blue orchard bee. Sustainable Agricultural Network. Bosch, J., Sgolastra, F., & Kemp, W. P. (n.d.). Chp. 6 Life Cycle Ecophysiology of Osmia Mason Bees Used as Crop Pollinators. In Managing Solitary Bees (pp. 83–104).

Guédot, C., Pitts-Singer, T. L., Buckner, J. S., Bosch, J., & Kemp, W. P. (2006). Olfactory cues and nest recognition in the solitary bee Osmia lignaria. Physiological Entomology, 31(2), 110–119.

Ladurner, E., Bosch, J., Kemp, W. P., & Maini, S. (2008). Foraging and nesting behavior of Osmia lignaria (Hymenoptera: Megachilidae) in the presence of fungicides: Cage studies. Journal of Economic Entomology, 101(3), 647–653.[647:FANBOO]2.0.CO;2