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Peek into the strange genetics of sex determination in bees and learn how this unique system impacts bee conservation. This module's activity must be completed between September and March.






The process of biological sex determination in humans may be familiar to most of us.

People with ovaries generally have two X chromosomes. In contrast, people with testes typically have two different sex chromosomes, one X and one Y. However, sex determination in humans, just like we will see in bees, is not as simple as males are XY and females are XX. In humans, we see these variations with intersex individuals who have genitalia that does not match their chromosomes, have testicular and ovarian tissue, or have a combination of X or Y chromosomes other than XX or XY.

Figure 1.Sex determination by sl-CSD in haplodiploid bees, different colored chromosomes carry different alleles at the sex determining locus (Created with

In other organisms, sex determination is even more variable. Let us consider the diverse class Reptilia. Snakes and lizards determine sex based on their sex chromosomes at fertilization, but for many turtles and all Crocodilians, environmental conditions determine sex. You may be familiar with temperature-based sex determination in sea turtles and conservationists' work to ensure that populations maintain a sustainable sex ratio under the increased temperatures resulting from climate change. Much like the class Reptilia, insects have a diverse suite of sex determination pathways. In this module, we will explore the mechanism of sex determination in bees, the importance of sex ratio, and how these processes impact bee conservation.

Sex Determination

All hymenopterans, including ants, wasps, and bees, use a system called haplodiploidy, where male bees are (typically) haploid and female bees are diploid. Female bees can choose the sex of their offspring since male offspring arise from unfertilized eggs and female bees from fertilized eggs.

The term “haplodiploidy” is a bit of a misnomer for the best described mechanism of sex determination in Hymenoptera, single-locus complementary sex deter- mination (sl-CSD). Although the specifics are nuanced, the difference between the simplified description of haplodiploidy (males are haploid and females are diploid) and sl-CSD may be important for under- standing how population genetics impact bee conservation.

In the sl-CSD system, sex is determined by the zygosity of a single sex determining locus. In most large, outbred populations there are many alleles at the sex determining locus – sometimes as many as 20. Thus, if an individual is diploid (having two sets of genes, one inherited from their mother and the other inherited from their father), that individual will most likely have two different alleles, meaning they are heterozygous. These diploid, heterozygous individuals are females. In Hymenoptera, most males are haploid and thus hemizygous at the sex determining locus. However, when popula- tions become inbred such that there is low genetic variability at the sex determining locus, you can find individuals that are diploid but homozygous (Figure 1). One might think that these individuals would be female but in fact they end up developing into males who are reproductively sterile (see Sex Ratios, Haplodiploidy, and Bee Conservation below).

Figure 2. The proportion of offspring which are daughters is significantly lower when floral resource levels are low (p = 0.020) (Figure adapted from Kim 1999).

Mason Bee Sex Ratio and Sex Allocation

Haplodiploid sex determination provides bees with a unique ability to choose the sex of their offspring in response to external (environmental) conditions and as well as intrinsic conditions, such as their age and overall health. We briefly discuss several of these topics in the context of mason bee behavior in the Foraging Behavior and Nesting and Mating Behavior modules. One of the most notable and easily observed manifestations of this is the non-random distribution of offspring within the nests of stem-nesting bees, such as mason bees. In nearly all stem- and cavity-nesting bees, females preferentially place their female off- spring at the back of the nest and their male offspring at the front of the nest. Because males (typically) emerge before females, this arrangement allows the males to exit the nest before their sisters.

The females of many bee species, including mason bees, can also adjust the sex ratio of their brood in response to environmental conditions. Female bees will tend to lay more male eggs when conditions are bad and more female eggs when conditions are good. Floral resource scarcity, inadequate nesting materials, poor weather, and the presence of brood parasites can push female bees to lay more unfertilized, haploid (male) eggs (Figure 2). Female offspring require more food (floral resources) to reach a healthy adult size than male offspring, which are small compared to adult females. When floral resources are deficient within the foraging area, females tend to create smaller provisions and lay more haploid male eggs. (The alternative, diploid female eggs laid on undersized pollen provisions, can reduce female body size and increase female mortality.)

Intrinsic conditions, such as age and overall health, may also alter how females allocate resources. As actively nesting female bees near the end of their lives, they tend to exhibit reduced foraging efficiency and vigor. In response, female bees create smaller pollen provisions with insufficient nutrition for healthy female bee development. As a result, across the female bee’s life, there is a shift from laying female eggs towards laying more male eggs. Additionally, unmated females will exclusively lay haploid male eggs.

Figure 3. A diploid male vortex is a positive feedback loop which can drive a population towards extinction (Figure adapted from Zayed and Packer 2005).

Sex Ratios, Haplodiploidy, and Bee Conservation

The unique genetics of sex determination in bee populations create interesting conservation genetics dynamics. Perhaps most simply, is the manipulation of sex ratios within the population. In general, a greater ratio of males to females has little impact on population growth, and conversely, this makes female mortality significantly more impactful on population dynamics. An ideal ratio in mason bees is between 1.5 to 2 males per female. Having approximately two males per female helps limit the number of unmated females, which could result in a more male biased population, limit population growth, and reduce pollination, which is almost exclusively performed by females.

Several other hypotheses have been proposed regarding the impact of haplodiploidy on bee conservation, some suggest a benefit to bee populations others do not. One possible benefit is the purging of deleterious, or harmful, mutations from the population by haploid males. In recessive traits, diploid individuals may not experience the effects of a deleterious mutation by expressing the functional gene on the homologous chromo- some, haploid individuals are unable to compensate for the harmful mutation since they only have one copy of the gene. However, this also may reduce their ability to respond to environmental or biological pressures since they only have one allele for each gene, whereas the allelic variation within diploid females may make them more resistant to similar pressures.


    Haplodiploidy - A system of sex determination in which females are diploid and develop from fertilized eggs and males are haploid and develop from unfertilized eggs.

    Hemizygous - Having a single allele at a given locus

    Heterozygous - Having a different allele on each chromosome at a given locus

    Homozygous - Having the same allele on each chromosome at a given locus

    Single-locus Complementary Sex Determination - A mechanism for haplodiploidy in which the sex of offspring is determined based on the zygosity of a single sex-determining locus. Females are heterozygous and diploid, and males are haploid or homozygous and diploid.

    Zygosity - The degree of similarity of each copy of a gene within an individual

    Overwintering Adult Nest Dissection— Hands on

    The location of male and female bees in mason bee nest and the ratio between the sexes can teach us alot about sex allocation in mason bees. In this activity, we will examine the pattern of male and female adult mason bees within a nest.


    1. Mason Bee nest reeds, one per group, containing overwintering adults


    Note: Nest reeds can be collected from your mason bee hotel or ordered online. If you use nests collected from your bee hotel you may encounter a number of different species. However, you are likely to encounter Osmia lignaria in western states or an introduced species, Osmia cornifrons, in eastern states.


    2. Knife


    3. Small pair of scissors


    4. Forceps (optional)


    5. Magnifying Glass, dissecting microscope, or other magnification device (optional)


    Nest Structure:

    We cover the fundamentals of mason bee nest structure in the Nesting and Mating Behavior module.

    Osmia cocoons are separated in the nesting reeds by mud walls. You will also find many small black pellets (feces) that have been excluded from the cocoons by the bees (Image: © entomart)

    Identifying the sex of Osmia lignaria adults:

    Osmia lignaria males can be identified by the white patch of hairs on their face just above the mandibles. Females also have white hairs on their face, but closer to the top of their heads, near their antennae. Male bees are also generally smaller in size than female bees. Female bees also have scopa, a dense mat of pollen collecting hairs, on the bottom of their abdomen.

    Female Osmia lignaria:


    To identify female Osmia lignaria take note of the white facial setae (hairs) which are primarily the top of the head not near the mandibles (photos: Chelsey Ritner, USDA).


    Male Osmia lignaria:


    Male Osmia lignaria can be identified by the white setae (hairs) covering their face and the distinct patch near the mandibles (photos: Chelsey Ritner, USDA).

    Identifying the sex of Osmia cornifrons adults:

    Adult Osmia cornifrons males have a white patch of hairs on the lower part of their face just above the mandibles. Female bees lack the white tuft of hairs on their face and have coppery orange or tan hairs on the bottom of their abdomen, called scopa. O. cornifrons were introduced to the United States from Japan as a managed pollinator and are now common in the Northeast.

    Female Osmia cornifrons:


    Female Osmia cornifrons can be identified based on the presence of coppery orange or tan colored scopa (pollen collecting hairs) on the bottom of the abdomen. They are generally larger than male O. cornifrons (photos: Chelsey Ritner, USDA).


    Male Osmia cornifrons:


    Male Osmia cornifrons have a distinct patch of white setae (hairs) on their face just above the mandibles, they lack abdominal scopa, and are generally smaller than female O. cornifrons (photos: Chelsey Ritner, USDA).


    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.


    3. Record the total number of brood cells and cocoons within the nest.


    Opening the Cocoons and Collecting Data:

    1. Starting at the innermost cell, the cell furthest from the sealed entrance, gently remove the cocoon.


    2. Using a small pair of scissors, carefully cut around the edge of the cocoon and gently pull it open, being careful to avoid damaging the adult bee inside.


    3. Examine the adult bee, identify and note the sex, the cell's position in the nest, and any other observations you make (e.g., variation in sizes of the bee, presence of mites or other parasites, etc.)


    4. Repeat with each of the cocoons, working your way from the inside out.


    5. Combine each group's data into a class dataset.

    Discussion Questions:

    1. What is the ratio between male and female bees in the sampled nests?


    2. Is there a relationship between the location of a cocoon in each nest and the sex of the bee? Why might we see this pattern?


    3. Other than the identifying features listed for male and female bees, what other differences between male and female bees did you observe?


    Sex Determination Module PDF



    Sex Determination Activity PDF 

    Sex Determiantion Activity Answers PDF

    Figure 1. Osmia cocoons are separated in the nesting reeds by mud walls.

    Figure Front: Female Osmia lignaria

    Figure Side: Female Osmia lignaria

    Figure Front: Male Osmia lignaria

    Figure Side: Male Osmia lignaria 


    Gilbert, S. F. (2000). Environmental Sex Determination. In Developmental Biology (6th edition). Retrieved from

    Bosch, J., & Kemp, W. P. (2002). How to manage the blue orchard bee. Sustainable Agricultural Network. Kim, J. Y. (1999). Influence of resource level on maternal investment in a leaf-cutter bee (Hymenoptera:

    Megachilidae). Behavioral Ecology, 10(5), 552–556. Kopec, K., & Burd, L. A. (2017). Pollinators in peril: a systematic status review of North American and Hawaiian native bees. Center For Biological Diversity.

    Tepedino, V. J., & Torchio, P. F. (1989). Influence of Nest Hole Selection on Sex Ratio and Progeny Size in Osmia lignaria propinqua (Hymenoptera: Megachilidae). Annals of the Entomological Society of America, 82(3), 355–360.

    Zayed, A., & Packer, L. (2005). Complementary sex determination substantially increases extinction proneness of haplodiploid populations. Proceedings of the National Academy of Sciences of the United States of America, 102(30), 10742–10746.