The crafty nest parasites of solitary bees.
Life is hard for a female solitary bee. From the day she leaves her mother’s nest, she has only a few frantic weeks to mate, construct her own nest cells, stock them with provisions of pollen and nectar gathered from a scattering of flowers and lay her eggs before she dies. Bees do more than most insects to ensure their young survive to fly the next season, but their stores of protein-rich pollen and high-energy nectar are a lure for others.
Just as the cuckoo lays eggs in other bird’s nests, a number of insects have evolved to leave their young to grow in the cradles of bees. These species have devised a myriad of fascinating methods to achieve their goal.
Pulling a fast one -Nomad bees (Nomada spp.)
Many of these parasites are bees themselves, termed cuckoo bees. Nomad bees or Wasp bees are tricksters, mimicking stinging wasps to protect them from predators whilst they hover around bare ground, scenting for active nest burrows of Mining bees (Andrena spp.). When she finds one, the female Nomad bee checks the coast is clear, nips in to lay her egg and makes a swift exit before the owner returns from foraging. On hatching, the larva despatches host bee’s egg before tucking into the food stores. This exploitation of another’s food resource is known as cleptoparasitism.
Breaking and Entering-Blood bees (Sphecodes spp.)
Blood bees are more brazen attackers. They are close relatives of some ground-nesting bees, and good diggers themselves. The female’s thick-set body means she is not such an agile flier, so she crawls around in search of nests before digging her way inside. She breaks open a nest cell, destroys and replaces the host egg, then reseals it. If the owner catches her red handed, the Blood bee will fight and even kill her. The effort she put in to re-sealing the cell means the host’s work is no longer needed for the cuckoo bee’s offspring to thrive.
Cutting in-Sharp-tailed bees (Coelioxys sp.)
Sharp-tailed bees target leafcutter bees (Megachile spp.) as their hosts, a choice with presents a difficulty. When a leafcutter bee has finished her nest, she plugs the entrance with a wall of leaf fragments precisely to thwart predators and parasites . Unfortunately for the host, the sharp-tailed bee has an abdomen specially equipped with a blade-like tip. She simply slices a gash in the leaf wall and lays her own egg inside. The larva hatches before the leafcutter grub and crushes the host egg with its powerful jaws before tucking into the pollen ball.
Lethal injection- Gasteruption jaculator
Mason bees (Osmia spp.) guard their nests even more thoroughly than leafcutters, sealing the entrances with a cement of mud or chewed leaves and saliva. Most of their nest parasites nip into unfinished cells, but not Gasteruption. Thanks to its incredibly long proboscis (egg-laying tube) this ichneumon wasp can drill through the hard wall and inject an egg into the nest cell, on or near the host’s own egg. The larva will feed upon the bee grub, an act which makes it a ‘parasitoid’ (host-eater).
Bombs away! -Bee flies (Bombylius spp.)
These hairy, hovering flies are named after their resemblance to bumblebees, though they are easily identified by their long, permanently-extended proboscis which they use to sup nectar.
The way females lay their eggs in mining bee burrows is truly unique. She gathers sand or soil on the hairy tip of her abdomen, hovers over the open, incomplete nest of a solitary bee, then bombs the nest with her eggs, flicking them down the hole. The soil particles stick to the eggs as she ejects them, adding extra weight which improves her aim. This behaviour allows the delicate beefly to keep safely out of the way of the host bee, and gave rise to their other common name ‘Bomber flies’
Upon hatching, the larvae are slender and active, wriggling deeper into the nest to munch the food stores. They leave the egg and happily share the provisions when the bee larva itself hatches…for a while.
After a time, both larvae have grown on their rich diet, and the Beefly larva changes form, metamorphosing into a large, fat grub. It’s new hulking body allows it to attack the now-fattened bee larva and feast upon its adopted sibling. The beefly has found a way to exploit its host as much as possible by crossing the line from food-stealing cleptoparasite to host-eating parasitoid.
Hitching a Lift-Oil beetles (Meloe spp.)
Another way to exploit your nest host is to let them drop off the kids for you. Of all the bee kleptoparasites, none go to greater lengths than the Oil beetles. They are also nest parasites of mining bees, but the bulky, flightless beetles do not lay their eggs directly in the holes.
Instead of spending precious time dragging her distended, egg-bound abdomen around to different bee nests, the female digs her own hole and lays her hundreds of eggs all in one batch. When they hatch, the tiny, louse-like larvae called triungulins burrow to the surface, seek the nearest flower stem, climb to the top, and wait…
Hopefully, a mother mining bee will alight on the flower to feed and gather pollen. While she is distracted the triungulins climb aboard and cling on ready for take-off. They hitch-hike to the bee’s nest, where they depart, settle down and change into a second, grub-like stage to gorge themselves on the food stores and bee larvae.
The extra ‘triungulin’ larval stage is rare amongst beetles and the feat is even more amazing considering the triungulins cannot distinguish between a mining bee and a fly or other insect useless to them. This complex lottery is why oil beetles produce such abdomen-bulging quantities of eggs to maximise the chances that some will survive.
Some oil beetles have improved their lot by evolving triungulins which act as a lure themselves instead of relying on a flower. For example, those of Meloe impressus climb up a plant stem and cluster into a clump. They then release an air-borne scent which mimics the pheromones of a female bee. A hot-blooded male bee which has picked up the perfume attempts to mate with the cluster and the larvae clamber aboard, hoping their carrier finds a real partner so they can depart onto a female and be carried to her nest.
It’s easy from our human perspective of right and wrong to think of these nest parasites as nasty, devious, even evil. Yet they are just as much a part of the ecosystem as their host bees which we so adore. The relationship between the parasites and their hosts has been playing out for millennia, and there’s no evidence that these parasites cause serious declines in their host bee’s numbers.
To the contrary, the presence of these parasites is an indicator that host populations are a healthy size. The nest parasites are always much rarer than the hosts, otherwise they would kill off their life support system. The fewer and farther between the bee nests are, the tougher it is for parasites to find them, and tougher still for the next generation to get together and breed once they emerge. Thus these cleptoparasites and parasitoids are canaries in the coal mine, their numbers signalling the fates of their host bees, even when the decline of the hosts themselves goes unnoticed.
Sadly, we are seeing this this happen. The Six-banded nomad bee (Nomada sexfasciata) is one of Britain’s rarest bees, formerly widespread across Southern England but now just clinging on at a small strip of South Devon as its Long-horned bee host has declined (but not disappeared) across its range. A further eight species of Nomad bee and three blood bees are of principal conservation importance in UK countries due to substantial decline. Three species of Oil beetle have already gone nationally extinct in the UK, with the remaining five in severe decline, despite some of their host bees being regarded as common.
This is just a window into the lives of bee nest parasites. There are many more fascinating species and much more to discover about their incredibly inventive lifestyles. In their own struggle for survival, these intriguing insects are deserving of our attention, respect and protection as much as their hosts.
With thanks to Ed Phillips for kind permission to use his images in the article. Ed has many fantastic photographs of bees and activity in his bee hotels on his blog http://www.edphillipswildlife.com/
and flickr page https://www.flickr.com/photos/50342657@N05/
Oil beetles- Buglife
Steven Falk’s flickr collection of bees-With excellent photographs and descriptions of all of Britain’s bee genera
Beeflies- Soldierflies and allies recording scheme
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