An enemy’s enemy is a friend

By Athayde Tonhasca

A creature named ‘bee wolf’, ‘bee killer’ or ‘bee hunter’ cannot bode well for a bee. Indeed, the solitary wasp Philanthus triangulum, the European bee wolf, can be a serious headache for the honey bee (Apis mellifera).

From early July to mid-September, a female bee wolf can often be found busy digging a long tunnel (up to a metre deep) on the ground, usually on a sunny, sandy bank. This main tunnel will branch into several shorter burrows, each to become a brood cell. When she’s finished with house building, it’s time to go hunting to provide for her brood. Solitary and cuckoo bees would do as prey, but honey bees are the main draw. The bee wolf snatches a honey bee and stings it. The honey bee may try to defend herself with her own sting, but she’s no match for the stronger enemy. Attacker and attacked fall to the ground, and the honey bee quickly becomes paralyzed by the wasp’s powerful neuromuscular venom. Sometimes the bee wolf seems to be doing a mouth-to-mouth resuscitation attempt on her victim, but in fact she is lapping up nectar collected by the honey bee. The bee wolf grabs her prey firmly and brings it back to her nest. Watch the whole drama.

A bee wolf returning home with a paralyzed honey bee © Stevelaycock21, Wikipedia Creative Commons

The bee wolf continues hunting, bringing enough booty to stock each brood chamber with up to six honey bees. When the nest is full, she lays one egg on a honey bee in each chamber and seals the tunnel entrance. The larvae hatch in 2-3 days and feed on the living but incapacitated honey bees. When a larva is finished eating, it spins a protective cocoon to hibernate through winter, and a new adult emerges in spring.

Bee wolves are solitary but tend to nest close to each other, possibly because good nesting sites are hard to find. And these wasps can make excellent use of good spots: in continental Europe, nest aggregations may be 15,000 strong. With each wasp capturing up to 100 bees over the season, local honey bees can be depleted quickly. 

Honey bees are widespread, and with 20 to 50 thousand residents per hive during summer, it would seem that bee wolves have it easy. Enter the cuckoo wasp Hedychrum rutilans.

Hedychrum rutilans © Pudding4brains, Wikipedia Creative Commons

This colourful, iridescent creature is one of the 3,000 or so species of chrysidid wasps (family Chrysididae). Chrysidids are parasitoids, that is, they lay their eggs on the host or inside its nest, and their larvae eat the host’s offspring. Unfortunately for the European and other bee wolf species, this cuckoo wasp targets them. 

Most chrysidids wait for the host’s nest to be sealed up to lay their eggs inside the brood cells. H. rutilans has different ideas. It often hangs around the host’s nest entrance, waiting for the bee wolf to leave on another hunting expedition. It then enters the nest and lays an egg on a paralyzed honey bee already there, waiting to be entombed. Or it may take a blitzkrieg approach, laying an egg on a honey bee as it is dragged by the bee wolf into her nest. If the cuckoo wasp is successful either way, the bee wolf is in trouble. The emerging cuckoo wasp larva kills and eats the bee wolf larva, and the honey bees as well.

The cuckoo wasp seems to avoid detection inside the bee wolf nest thanks to chemical mimicry, that is, by releasing the same substances that bee wolves use to recognize each other.  Chemical mimicry protects the cuckoo offspring too; if the bee wolf picks up the smell of an intruder inside a brood chamber, it throws away the stored honey bees. The cuckoo wasp is not so safe in the open: if spotted, it will be attacked by the bee wolf. But like most chrysidids, H. rutilans is thick-skinned and rolls up in a defensive position so that sensitive parts are protected from stings and bites. Watch a cuckoo wasp escaping an attack.

A female chrysidid in a defensive position ©

The European bee wolf was first recorded in southern Britain in the 1990s, and since then it has moved north (with no notable consequences for honey bees so far). This wasp responds well to warm summers, so its expansion is likely to extend into Scotland. Predictably and unavoidably, H. rutilans followed its host into Great Britain, although records are still scarce and confined to the south. Time will tell whether cuckoo wasp distribution will expand as well. In continental Europe, this parasitoid can wipe out local bee wolf populations, but nobody can predict how the honey bee/ bee wolf/cuckoo wasp triangle will shape up in Britain. Curious naturalists will follow the plot closely.

Does H. rutilans have its own parasitoids? We don’t know, but that’s possible. No species is free from parasites, predators or pathogens. So Jonathan Swift’s witticism about an infinite chain of fleas has some biological truth:

So, Nat’ralists observe, a Flea

Hath smaller Fleas that on him prey,

And these have smaller yet to bite ’em,

And so proceed ad infinitum:

Thus ev’ry Poet, in his Kind

Is bit by him that comes behind.

On Poetry: A Rhapsody (1733).