Exoneura robusta facts for kids
Quick facts for kids Exoneura robusta |
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Exoneura robusta is a type of bee often called a "reed bee." These bees are special because they can change how social they are depending on where they live. They get their name "reed bees" because they often build their nests inside the soft, spongy parts of dead fern stems.
You can find E. robusta bees in southeastern Australia, living in both cool, mountainous areas and heathland (open, shrubby) places. Usually, they have one group of babies (brood) per year. But in warmer places, like some heathlands, they can have two broods. When they have two broods, the older siblings can help raise the younger ones, which is a type of social behavior. These bees don't have different body types for different jobs (like a queen bee looks different from a worker bee). This allows them to be very flexible in their social lives.
Contents
About Exoneura robusta
What are Reed Bees?
Exoneura robusta is a species of Australian bee. An American scientist named Theodore Dru Alison Cockerell first described this bee in 1922. It belongs to the Apidae family, which is part of the larger group called Hymenoptera. This group includes ants, wasps, sawflies, and other bees.
Bees in the Exoneura group are commonly known as "reed bees." This name comes from where they build their nests: often inside the soft stems of plants, like reeds or ferns.
How to Spot Them
E. robusta bees have a black head and a black middle body part (thorax). Their back part (abdomen) is orange-brown. Their legs are yellow-orange, and they have hind wings. They can give a small sting, but it's not usually a big problem.
You can often identify these bees by their nests. They almost always build their nests inside the dead stems of a tree fern called Cyathea australis. New nests look very clean, with only a little bit of pollen inside. Older nests, however, look darker. This is because pollen and nectar from the previous year's baby-rearing activities have aged and stained the nest.
Where They Live and What They Do
Homes and Habitats
E. robusta bees live in southeastern Australia. They can be found in two main types of places: mountainous areas and heathlands. In the mountains, they build their nests inside the dead fronds (leaves) of C. australis tree ferns. In heathlands, they often choose branches from eucalyptus trees.
Their nests are simple: a single, undivided tunnel dug into the soft center of the plant stem. E. robusta bees are very important for the forests of southeastern Australia because they help pollinate plants. The way these bees live together (their social behavior) changes depending on their habitat.
Life Cycle of a Colony
Starting a New Nest
New E. robusta nests are started only during a short two-week period in early spring. These bees usually don't travel far from where they were born when they start a new colony.
It's unusual for most bees, but E. robusta colonies are often started by two to eight female bees working together. These co-founding females are usually related, like sisters. But if no relatives are around, they will start a colony with unrelated females.
Growing Up and Overwintering
Female bees lay eggs from late winter through spring. The new adult bees come out of their pupae (a resting stage) during the summer. Adult bees then overwinter (spend the winter) inside the nest.
Nests can be used again for six to ten years by the babies of the first founders. E. robusta usually produces only one group of babies (brood) per season. However, this can change based on their habitat, as mentioned before. Colonies are usually small, with about 6 or 7 babies per nest. The females who lay the most eggs produce about 4 or 5 babies in a season.
Bee Behavior
Flexible Social Lives
E. robusta bees have very flexible behavior compared to other bee species. They don't have different body types for different jobs, so all females can potentially do any role. They can live alone or in groups.
In larger groups, however, bees will take on different jobs. Some become guards, protecting the nest. Others become nurses, caring for the young. And some become foragers, finding food. These are behavioral differences, not physical ones. This means any bee could do any job if needed.
The social structure of E. robusta is polyphenic. This means that different behaviors can come from the same bee genetics, depending on the environment. Depending on where they live, these bees can be solitary (living alone), semisocial (living in groups where all can reproduce), quasisocial (living in groups where all can reproduce and help care for young), or eusocial (living in groups with a queen and non-reproducing workers).
The type of social life often depends on how many broods they have per season. If they only have one brood, they are usually quasisocial. But some heathland groups have two broods. This allows older siblings to help raise the younger ones, leading to eusocial behavior. This flexibility helps E. robusta adapt to different environments.
Who's in Charge?
In new nests, all the founding females can reproduce, and there isn't a strict leader. It's a more cooperative, equal setup.
However, in nests that have been used for a second year or longer, a social leader usually appears. The first female bee to emerge from her pupa (the first one to hatch as an adult) becomes the main reproducer. This can happen even if she hatches just a day before the others!
Females who hatch later often become foragers, meaning they go out to find food. They also tend to have shorter lives than the main reproducing female. The first-hatched female often takes on a guarding role. This job is risky because it puts her in danger from predators and competitors at the nest entrance. But this risk pays off because it allows her to control who reproduces in the nest.
Dominant females are less likely to let another female back into the nest if she has mated with a male from outside the colony. In new nests, however, you don't see bees being kicked out because the co-founding females work together equally.
Scientists think that dominant females might use special scents (pheromones) to stop other females from developing their ovaries (the parts that make eggs). Since leadership isn't about physical differences, a dominant female can pass on her status to her daughters through her behavior. By stopping her nestmates from breeding (or kicking them out), she can make sure her daughters are the first to hatch and become the next leaders.
Other females in the nest might lay eggs later, meaning their daughters will also be secondary, or they might lay unfertilized eggs that become males (who don't compete for dominance). One reason why these "subordinate" females might accept this system is that living in a group is very important for E. robusta. It might be better for a subordinate to stay in the nest and produce sons than to try to start a new nest alone.
Sharing Reproduction
Because of the leadership structure in older nests, reproduction isn't shared equally. This is called "reproductive skew." Dominant females have more developed ovaries and can produce more babies than other females. Some "worker" females might have ovaries that aren't developed at all.
Reproductive skew is less common when the bees in a colony are closely related. This suggests that dominant bees can't completely control the reproduction of others. Subordinate bees might limit their own reproduction to avoid being kicked out of the nest. However, this doesn't explain why there's reproductive skew in new nests where no one gets kicked out.
Who Becomes Male or Female?
The number of males versus females produced by E. robusta is greatly affected by the benefits of living in a group. More adults in a colony mean more babies can be produced. Very few females (less than 4%) nest alone, which shows how important group living is for raising young successfully.
Nests usually have only a few babies at a time, and there are almost always more females than males, especially in smaller nests. This is because a working colony needs female bees (males don't work). So, having more daughters helps the colony be more productive. Larger broods might have more males because enough daughters can still be produced without needing a lot more females.
Having a high number of females helps the colony by allowing them to share jobs like foraging for food, nursing the young, and guarding the nest. More females can also increase the number of babies by having a guard present all the time. This is important because ants can wipe out an unguarded nest's entire brood.
Family Ties in the Colony
How Related Are They?
E. robusta colonies are often started by several females. This means the babies in a colony are not as closely related as in some other social bees. Studies have found that the young bees in a colony are related by about 0.439. This is lower than what you'd expect if there was only one queen who mated once. This supports the idea that multiple females start the nests.
The adult bees in a colony are slightly less related than the young bees. This suggests that some adult bees might have moved into the colony from another nest. However, the females who start a new nest together are often quite closely related, sometimes as high as 0.6.
Recognizing Family
Living in a group is very important for E. robusta. This helps explain why they have a certain level of relatedness within colonies, but also why they don't seem to tell family members apart from non-family members.
Female bees prefer to nest with relatives, but they will nest with unrelated bees if no family is available. They tend to start new nests with bees they grew up with, whether they are related or not.
E. robusta bees don't seem to have a way to recognize their relatives. One reason for this could be that trying to tell family apart would make the colony less efficient. It would waste time that could be spent on other tasks. So, if recognizing family lowered the fitness of all colony members, then treating everyone equally would be better. This seems to be the case for E. robusta: the benefits of group living are greater than the costs of helping non-related nestmates.
Parasites of Exoneura robusta
E. robusta bees are often attacked by a parasitic bee called Inquilina schwarzi. This parasite is an inquiline, meaning it lives in the nest of another species.
Because E. robusta doesn't have a strict caste system (like a queen and workers), it's easier for I. schwarzi to blend into the colony. The way E. robusta nests are built also helps the parasite. Since the young bees are raised in a shared tunnel, the female bees can't easily control who gets fed. This makes it easy for I. schwarzi to lay its eggs in the common tunnel.
Older, longer nests are more likely to be parasitized. This might be because older nests have had more time to be found by parasites. Or, larger nests might have a stronger scent, making them easier to locate. Another reason could be that I. schwarzi looks for older nests because they are likely to have more resources.
Parasitism by I. schwarzi changes the behavior of E. robusta. For example, parasitized nests tend to be larger after the young bees leave. This suggests that the parasite makes the host bees less likely to leave the nest. This helps the parasite because more hosts mean more resources, and if the colony stays together, the parasite doesn't have to move.
In the spring, I. schwarzi can even make the leadership structure of E. robusta less strict. I. schwarzi usually waits until the host bee eggs have been laid before laying its own. This way, the smell of their eggs is hidden and harder to detect. Generally, I. schwarzi tries to have as little impact as possible on E. robusta colonies. This is because the parasite's survival depends on the host colony's survival. However, the parasites do lay enough eggs to put extra pressure on the colony's food and resources.
Images for kids
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Distribution map of Exoneura robusta in Australia.
