Wings, Sounds and Buzzy Bees
The buzzing sound of bees is generated by their wings and flight muscles. This movement of air generates an audible buzz as the wings vibrate rapidly following contraction by the thorax’s indirect flight muscles. Buzzing serves a variety of purposes including pollination, communication and defence. Elaborating on the function of the wings covered in the bee thorax, this article exams the mechanisms behind a common trait to flying insects.
How do Bees Create the Buzzing Sound?
A recap: bees have four pairs of wings: the forewings and hindwings. They are powered by indirect and direct flight muscles, which control the fine and fast movements during flight. The indirect muscles alter the shape of the thorax, causing the entire wing to move, while the direct flight muscles control the wing angle, enabling them to maintain the desired coordination (Iwamoto, 2011).
The high-pitched buzzing sound is generated by “thoracic oscillations” during flight, which involve alterations in the rate of dorsoventral muscle action potentials, thorax deformation, and wingbeat generation (reviewed in Vallejo-Marin, 2022). The author suggests that there are three primary types of bee buzzes: thermogenic activity, thoracic oscillations (standalone), and non-flight vibrations (elaborated below).
Flight Muscles and Wing Vibrations
In species that conduct buzz pollination such as bumble bee, buzzing enables them to release pollen from flowers (Thorp, 2000). Bees have specialised mechanisms for adjusting the intensity and frequency of floral vibrations. This involves varying the level of wing deployment, altering thoracic stiffness with accessory muscles, and changing the rate of action potentials in indirect flight muscles.
A strong correlation exists between wingbeat frequency and body/wing size, which can be affected by tiny holes or imperfections in the wing structure (As shown in stingless bees [Meliponini] by Duell & colleagues, 2022). The dorsoventral muscles, responsible for the upstroke of the wings during flight, assist in generating the buzzing sound. As the muscle contracts, the air passes through the wings, creating a buzz.
The frequency of wingbeats is regulated by accessory muscles in bees’ thorax. Bees also produce non-flight vibrations from their thorax for the purposes of communication, defense, or buzz pollination, each having a distinct buzzing tone (Hrncir et al., 2008; Pritchard & Vallejo-Marin, 2020).
The Purpose of the Bee Buzz
Bees buzz for a a variety of functions such as:
- Pollination: buzzing assists in the pollination of plants by dispersing pollen grains in the air.
- The efficiency of pollination differs between bee species, and honey bees (Apis mellifera) are most often used for commercial pollination operations due to their generalist foraging behaviour and large numbers.
- Evolutionary, a variety of flowering plants have developed structures to keep pollen grains locked inside (perhaps to prevent a pollen-feeding species from consuming it without benefiting the plant). When the buzzing bee’s body makes contact with the plant’s anthers, it results in shaking of the structure, releasing the pollen grains onto the bee (Vallejo-Marin, 2019).
- Communication: Buzzing enables bees to convey distinct messages like warnings or locations of food sources.
- Examples of communicatory buzzes include the high-pitched piping by queen bees and the anti-predator hissing by worker bees.
- Defense: Bees employ buzzing as a means of deterring predators, guarding the hive by creating an intimidating sound.
- For example, the defence against Asian Giant Hornet attacks by Apis cerana. Or the increasing volume of buzzing by Apis mellifera during your next (haphazard) hive inspection.
- Thermogenesis: In cold conditions, bees can buzz to generate heat, as in a winter cluster.
- A review by Stabentheiner and colleagues showed that buzz-generated heat increases comb temperature, not air temperature, enabling brood rearing in winter climates (Stabentheiner et al., 2010).
Communication Among Bees
In honey bees, communication via buzzing is referred to as “hissing” and “piping”. Workers communicate to one another using these vibrations in situations like swarming (Seely & Tautz, 2001). When hissing, the indirect flight muscles deform the thorax with a low rate of wing movement, causing vibration with a broad sound frequency, and a pretty (threatening) sight.
Piping is used as a defensive communication, as in when the colony is disturbed by a predator or a rough beekeeper, it triggers other bees to hiss and coordinate their response against the threat. Piping is also heard when a colony is busy raising bee queens for swarming or supersedure. This tooting by the newly-emerged queen is an announcement of herself to the hive (and also a a way to identify her sister queens so that she can kill them first – go read that article). Unlike hissing, piping can be done with the wings folded back or stretched outwards (Seely & Tautz, 2001).
Vibrations and Mating in Solitary Bee Species
Male mason bees have been shown to buzz during mating, using thoracic vibrations for mate recognition (Conrad & Ayasse, 2015). Other solitary bee species such as male digger bees from the species Centris pallida use vibrations during copulation to prevent the same female from mating with another drone (Alcock & Buchmann, 2010).
Defence Mechanisms
The aforementioned hissing and piping are one way bees utilize buzzing as a defence mechanism. We hear this as a high pitched buzzing sound, and it serves to deter predators and safeguard the bee colony from potential dangers. Don’t believe me? At your next hive inspection, consider oversmoking bees, handling frames aggressively and behaving rowdily… you will certainly hear a ferocious buzz, as well as attempts to sting your (well-protected) body.
Technically, the defensive buzz is distinguished from a flight or communicative buzz by the increase in thoracic contractions. In bumble bees, The dorsolateral muscles are more active than the dorsoventricular muscles, resulting in a greater stretch of the thorax (and perhaps a more intimidating look; Josephson & Ellington, 1997).
Heat Generation by Buzzing
Shivering is how we generate heat. In bees, this thermogenesis is generated by the thoracic muscles as the bee is walking or stationary. It does not produce a sound, and there is no movement of the thorax despite contraction of the indirect flight muscles (recall that their purpose in flight is to change the throax’s shape and thus the wing angle). This process is controlled by the bee, who can increase their temperature from ~6°C (on a cold morning with the sunrise) to 37°C in less than 20 minutes (reviewed in Vallejo-Marin, 2022).
Does Fanning make a Sound?
In warmer weather, beekeepers see honey bees rapidly flutter their wings while remaining stationary at hive entrance (Terenzi et al., 2020). It produces a soft buzzing that’s less intense than the flight buzz, as their wing strokes are less forceful. When many bees fan simultaneously, the collective sound can be more noticeable. Bees fan for three main reasons:
To circulating air within the hive and cool it during hot conditions,
- This also reduces the built-up carbon dioxide generated by the respiring bees
The speed up the evaporation of the water in nectar in the process of making honey (after all, honey is ~20% water),
Dispersal of pheromones, especially after swarming, to help the disoriented bees find the hive.
What Makes Buzz Pollination Unique?
Buzz pollination, or “sonication” by bee species like solitary and bumble bees, extracts pollen from flowers. This technique involves the use of vibrations to dislodge the pollen, thereby allowing them to collect it more effectively (De Luca & Vallejo-Marin 2013). Notably, this specialized pollination is particularly effective for certain types of flowers that have evolved to require buzz pollination for successful reproduction.
Buzz pollination equips bees with the means to access a wide range of pollen-rich plants, offering them an edge over other pollinators. For many native bees and solitary bees, buzz pollination is how they gather pollen for their larvae.
Scrabbling, the Other Major Type of Pollination
A tangent: Buzz pollination is one form of gathering pollen from plants who conceal pollen in deep, tube-like anthers (“poricidal morphology” in the scientific literature). What about in plants where the pollen is exposed, as in dandelions and roses? Here, the bees use their legs and mandibles to knock the pollen free, whereby it is dispersed in the air and gathers on their hairy bodies. The bee then uses her legs to comb herself, and pack it into the specialised pollen baskets found on their hind legs(“scrabbling”).
Scrabbling is only observed in non-poricidal flower species, being inefficient for removing pollen grains from partially-concealed floral tubules (Thorp, 2000)
Different Types of Insect buzzing
For the science-minded readers, consider finding a multimeter, sticking both ends onto a bee and measuring the frequency of its wingbeats. For those who enjoy the chances of a prolonged lifespan, you can use your eyes and ears to distinguish between buzzing sounds.
Bumble bees vs. Honey bees
Bumble bees are larger than other types of bees and possess a fuzzy exterior. When in flight, they make a loud, low-pitched buzzing sound. In contrast, honey bees will not buzz without moving, and in the field their buzzing is during flight. The male drone bees buzz louder than the female worker bees.
When distinguishing between the two, look for behavioral cues: if a bee is seen vibrating on a flower without flying, it’s likely a bumble bee as it conducts buzz pollination (Miller-Struttmann et al., 2017). Also, bumble bees generally have a deeper, slower buzz compared to the higher-pitched, more rapid buzzing of honey bees. If all else fails, use your eyes to observe the hairy fuzzball that is a bumble bee.
Bees and Wasps
To the lay person, distinguishing bees and wasps is simple: one flies at you (wasp), another flies around you (bee). Some people perceive the wasp buzz as having a more aggressive, urgent tone compared to that of bees. Many bee and wasp species live and work independently, rather than in a colony. The flight of wasps results in a slightly higher pitched buzzing sound, generated by the vibration of their wings and flight muscles.
If you are close enough to observe their hind legs, wasps do not carry pollen as they are carnivorous (but many species do eat nectar; Phillips et al., 2021). Their bodies are less hairy than bees and generally sleeker, more streamlined. For a general overview of wasps, consider the article on the Asian Giant Hornet.
Hornets, Flies, Termites….
Getting more niche here. Hornets have a buzzing sound similar to wasps, which might be perceived as more aggressive or urgent than honey bees. Flies often have a higher-pitched and a more erratic buzz compared to the more consistent hum of honey bees. Termites don’t produce a typical buzzing sound, so they’re easily distinguishable (reviewed in Low et al., 2021). Hornets are aggressive. Flies often hover and are a bother (that rhymes!), termites are a rarity and do not often encountered in flight by the backyard beekeeper. If a termite colony swarms, you’ll know it.
Visually, flies have larger eyes relative to their head size and lack the pollen-carrying structures of bees. Termites, in their winged form (alates), are easily distinguishable from bees by their straight antennae, thick waist, and two pairs of similarly sized wings. Hornets have a sleeker body compared to honey bees and often have bright black, white, or yellow patterns.
Ecological Importance of Buzzing Bees
Bees buzzing has a beneficial effect on our world, as they aid in food production and pollination, maintaining a healthy ecosystem. Without bees, a considerable amount of the foods we consume would not be available. As bees travel from flower to flower, they spread pollen, which allows plants to reproduce and promote the diversity of plant species. This, consequently, fosters the general health and equilibrium of ecosystems.
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