Awesome bee flies

Bee flies, Bombylius major have emerged in the warm weather and I have come across them in several places. Bee flies are early spring fliers, the peak of adult activity is in April and May. They are found throughout the UK, where this species seems to be expanding north. Thee are some cool facts about them.

A male at rest.

Parasitism. Bee flies are parasitoids of several species of ground-nesting solitary bees, including Andrena mining bees. Their larvae crawl into the bees' nests and feed on larvae of the host bee once it is fully developed. Only large populations of mining bees are able to sustain a parasitoid population, so the presence of the bee fly is an indicator of healthy mining bee populations.

Female loading her sand-brush

Egg shooting. Bee flies collect sand or dust at the tip of their abdomens (which looks a bit like a brush) to coat their eggs before laying, as the one above is doing. Why? It is unclear if it is to camouflage them or to make them heavier. Females lay eggs in a curious way. Instead of just getting into the bee nest to lay, they fly low over bee aggregations, and throw their eggs against dark spots resembling nest entrances, including shadows swinging their bodies against it. Bee flies can afford some inaccuracy when egg laying though, as females can lay thousands of eggs per day. Their unusual egg laying behaviour might have something to do with the fact that they are soft-bodied insects, and could be too susceptible to the bee's sting to actually risk a one-to-one confrontation with the female bee. This is how they do it, note the sand-brush at the tip of the abdomen:


Mimicry. As their name strongly suggests, Bee flies are flies that look like bees, that is they are bee mimics like many hoverflies: they are similar in size to a honeybee, brown-tawny and as furry, with quite long hairs, as a bumblebee. It is unclear though if they mimic bees to get close to their nests unnoticed or to avoid predation from insect eating birds or other predators. Other mining bee parasites do not resemble bees at all (I'm thinking the wasp-like Nomada bees) and actually get inside nests to lay eggs.
Look into their eyes. You can tell male and female bee flies in the same way that hoverflies: males eyes are larger and actually meet a the top of the head, while in females they are smaller and separated by a hairy patch. This suggests that males use the sense of vision to look for females. Males actually hover in particular spots some times high up, near flowers, possibly to meet females.

 

male

female

Male feeding on Lungwort showing the fully extended tongue

Look at that tongue! One of the most impressive features of bee flies are their proboscis. These spear-like, non-retractable structure at the front of their head looks positively dangerous (like a giant mosquito!), however, it is harmless, its only purpose is to allow the fly to reach and suck nectar from flowers with deep, narrow corollas. Their mouth parts can be extended further as they feed to as long as their total body length.

A male feeding on grape hyacinth on Monday

Fantastic fliers. Bee flies are very fast and agile fliers, they hover a lot and can fly in any direction, including backwards. They don't settle to feed on flowers like hoverflies, instead, they behave more like a Hairy footed flower bee, or a Hummingbird Hawkmoth: their wings never stop moving when they feed from flowers, they are actually virtually invisible. They use their spindly legs to stabilise themselves in front of the flower. Males will hover at height, as many hoverflies do, and spin when they meet females.

Bee fly feeding on primroses on Monday

Pollinators. Bee flies feed on early spring flowers: Primroses, Grape-hyacinths, Forget-me-Nots, Violets, Lung-wort, Lesser Celandines and Wood Anemones. They will also feed on Blackthorn and sallows. Bee flies not only feed on nectar, they also consume pollen, with females feeding on pollen to a larger extent than males. They can be effective pollinators of these early plants.

A basking Bee fly yesterday.

Basking. Bee flies are active in warm, sunny days, and they like to bask to thermoregulate, sitting still, with their wings kept open at an angle. Then, you can try and identify the species as the wing patterns - hard to see in flight - can be diagnostic. The most common species, the large bee fly, Bombylius major, has a distinctive dark edge at the front of their wings.

More information
An identification sheet of British Species by the BRC. Click here.

Natural History Museum page. Click here.

Boesi, Roberto, Carlo Polidori, and Francesco Andrietti. "Searching for the right target: oviposition and feeding behavior in Bombylius bee flies (Diptera: Bombyliidae)." Zoological Studies 48.2 (2009): 141-150.

Jacquemyn, Hans, et al. "Biological flora of the British isles: Primula vulgaris Huds.(P. acaulis (L.) Hill)." Journal of ecology 97.4 (2009): 812-833.

Watching bees on foxgloves

June is peak Foxglove, Digitalis purpurea, season. This year, we have quite a few in the garden, and luckily one of them faces the conservatory, which allows me to observe visiting bees quite closely. Today I watched the bees feeding on it in sunny spells between showers. Foxgloves are said to be adapted to be pollinated by bumblebees: they have large, bell shaped flowers with a large landing lip and hairs that might deter smaller bees or other insects from entering. Indeed, bumblebees, and particularly long-tongue species, can reach the nectar deep inside the flower and actually pollinated it are the main visitors. A study in N Yorkshire found that Bombus hortorum (top, individual leaving flower with pollen load, and another individual just visible inside a lower flower) was the main visitor and preferred this flowers when available, but another study in Belgium found that B. pascuorum was the main visitor. The tree bee Bombus hypnorum is also a regular. All three species are commonly found on floxgloves flowers in my garden, with B. hortorum being the most common visitor. But other bumblebees, including those with short tongues do often collect pollen by 'buzzing' inside the flower even if they cannot reach the nectar.
 A few solitary bees also visit foxgloves. In my garden the forked tailed flower bee Anthophora furcata regularly visits them, and more rarely the carder bee Anthidium manicatum.
As relatively few species feed on this species, it is a good starting plant to familiarise yourself with bumblebees.
Today, I watched four bee species visiting this particular foxglove stem.

A male A. furcata (above) jumped on each visiting bumblebee to check them out, but was obviously feeding also on nectar. I haven't seen females around yet. The male has similar size and colour than a honeybee, but it has a bright yellow face and a visibly long tongue.

Bombus hortorum is the bumblebee with the longest tongue in the UK (above, with tongue extended). It can also be recognised by its double yellow band formed by the rear of the thorax and the front of the abdomen, and its white tail.

B. pascuorum is the most common all-brown bumblebee in gardens. 

Bombus hypnorum is a very distinctive bumblebee, three coloured: brown/black/white.

The stout black abdomen with lateral yellow patches of the Wood Carden Bee Anthidium manicatum which visited on 1/07/2011.

 Foxglove flowers open from the bottom of the stem towards the top. The plant reproductive parts are located on the roof of the flower, four stamens and a pistil in the middle, so that the body of a bumblebee entering the flower rubs against them. To reduce self-pollination, the flower anthers open first, delivering a pollen load onto bumblebees feeding. Five days later, when the pollen is likely to be exhausted the pistil becomes receptive, increasing the chances that it will be pollinated by the pollen of a different plant. I cut three flowers of a white foxglove in the garden and removed the roof of the flower to expose the reproductive organs. The one on the left, from the top of the flower spike just opened. The lobed yellowish sacs are the closed anthers, and the spike in the middle is the pistil, also closed and attached to the flower roof. The middle flower has open anthers and the pistil, although closed, has started to detach. On the left is the oldest flower from the bottom of the flower spike. Its anthers are almost devoid of pollen and the pistil is now pointing down, so that is easily rubbed by the bumblebee's back and then ready to be fertilised.

A bumblebee view of a foxglove. Note the open pistils on the lower, older flowers.

More information

Arthur A. D. Broadbent and Andrew F. G. Bourke (2012). The bumblebee Bombus hortorum is the main pollinating visitor to Digitalis purpurea (Common Foxglove) in a UK population. Journal of Pollination Ecology, 8 (7), 48-51.

Verboven, H., Brys, R., and Hermy, M. (2012). Sex in the city: Reproductive success of Digitalis purpurea in a gradient from urban to rural sites Landscape and Urban Planning, 106 (2), 158-164 DOI: 10.1016/j.landurbplan.2012.02.015

Night pollinators

Although daytime pollinators get a lot of press, night time pollinators are important too. Many moths feed on flowers at night, and some species of flowers appear to have evolved to be pollinated mainly by moths. If you go out to the garden on balmy summer nights, such as the ones we are enjoying this early September, you are likely to see Silver Y, Autographa gamma, feeding. This lively moth looks like a blur as it flutters continuously while it feeds - the flash creates a false 'frozen' effect. Like another summer visitor the Hummingbird Hawkmoth, the Silver Y has a very long tongue which allows it to feed on Honeysuckle, Lavender, buddleia, peas and wallflowers (above). All photos taken yesterday and today in the garden.

on honeysuckle

on lavender

...and on wallflower Erysimum Bowles Mauve

 And they must be careful on their comings and goings, while flying above them, Pipistrelle bats are hunting...

Unexpected pollinators

One of my spurge plants is in full bloom now. Spurges have complex flowers with exposed nectaries, which are available to insects lacking a long tongue, although they appear not to be very appealing to bees and butterflies. They attract bluebottles, hoverflies, ladybirds and wasps. Later in the season they will be taken over by ants, which will defend them aggressively from competitors. In the last few days, dozens of 7 spot ladybirds have been enjoying this early nectar, in the company of bluebottles and hoverflies. They pay back the plant by acting as its pollinators. Some of the ladybirds were covered in pollen and the one on top seems to be pushing up the plant pistil, so the anatomy of the plant is right for ladybirds to act as pollinators

This 7 spot ladybird, from 2/04/11 was laden with spurge pollen

Bluebottle enjoying the nectar

More 7 spots on the flowers, and possibly a harlequin

A little hoverfly, Meliscaeva auricollis

The CSI Blowfly

At this time of the year, when there has been a few frosts and the sun is weak, the most likely insect you are likely to see on the wing are bluebottles. I found this one yesterday enjoying the sun in my conservatory. It is the urban blowfly, Calliphora vicina, a very cold tolerant species which is the most common buebottle in the UK.
  Calliphora vicina is one of the most important species in forensic entomology, especially in investigating human remains. Different fly species arrive at a body at different stages of decomposition and they will lay their eggs on it. Blowflies can smell rotting flesh from large distances and are one of the earliest finding a body. They have a very fast life cycle, and they can produce up to five generations a year, depending on the temperature.

(Figure from Amendt, Krettek & Zehner 2004)

The female lays batches of 150-200 eggs in open wounds, rotten meat, or bodies. These eggs can hatch almost immediately after being laid, but they can take up to 9 days to hatch at 5 oC. The maggost will start feeding immediately. As the duration of each of the three larval stages depend on the temperature and is known in great detail, the age of the oldest maggots together with the average arrival time of the species allows to estimate the post mortem interval and approximate time of death.

Bluebottles will enter houses and lay batches of eggs in exposed meat either cooked or raw, and their attraction to rubbish makes them very abundant in cities. Bluebottles have an important ecological role as carcass decomposers...

... but they are also pollinators of several plants with exposed nectaries, such as ivy, spurges (Euphorbia) and plants from the carrot family. Some companies have even used bluebottles for greenhouse pollination of various crops as they fly at lower temperatures than bees. They also act as dispersers of fungal spores, and some fungi, like Stinkhorns (Phallus impudicus) have specific adaptations to attract blowflies, releasing chemicals that smell like rotting meat. The flies feed on the surface on the fungus and the spores attach to the fly, which can disperse them.
 Next time you are annoyed when a bluebottle enters your house, you might want to give a thought to how useful these flies are.

More information
Amendt, J., Krettek, R., & Zehner, R. (2004). Forensic entomology. Naturwissenschaften, 91 (2), 51-65 DOI: 10.1007/s00114-003-0493-5

Donovan, S., Hall, M., Turner, B., & Moncrieff, C. (2006). Larval growth rates of the blowfly, Calliphora vicina, over a range of temperatures. Medical and Veterinary Entomology, 20 (1), 106-114 DOI: 10.1111/j.1365-2915.2006.00600.x