Blistering brigands

By Athayde Tonhasca

Flying over coastal dunes in the American state of Oregon, a male silver digger bee (Habropoda miserabilis) picks up the scent of a female. The smell takes him to a dark shape hanging from a twig: he zeroes in for a romantic encounter before she has time to flee. But as he touches the female-to-be, the object breaks up into a mass of louse-like creatures who in no time crawl onto him, clinging on for dear life. These hitchhikers are planidia (singular planidium, from the Greek planis, meaning ‘wanderer’), which are a type of larva that don’t look at all like larvae. They have well developed legs, are quite nimble, and they’re phoretic, that is, they use another organism to be transported to a new location. 

The puzzled male bee moves on carrying his passengers, and eventually finds a real female. During their encounter, the planidia jump ship again and attach to the female bee. She will then carry the uninvited guests to her nest, where their joyride ends and their parasitic nature is revealed. They start eating the bees’ pollen stores, eggs and larvae before moulting into ordinary, grub-like larvae, which carry on depleting the bee’s provisions and offspring. The larvae go through several stages, and in the following spring emerge from the host’s nest as adult blister beetles of the species Meloe franciscanus.

Aggregations of Meloe franciscanus planidia on grass stems and on the dorsal surface of a male silver digger bee © Saul-Gershenz et al. 2018. Proceedings of the National Academy of Sciences 115(39) 9756-9760; DOI: 10.1073/pnas.1718682115.

The tale started with a sophisticated deception: the planidia lured the male bee with chemical signals that mimic the host’s sex pheromones. But the trickery goes further: another population from the Mojave Desert (California) produces different chemicals to attract another species of bee. Not only that, the planidia match their perching heights with the height at which local male bees normally patrol for females.

The cunning behaviour of M. franciscanus is just one of the distinctive facts about blister beetles (family Meloidae). They undergo hypermetamorphosis, by which one of their larval stages is radically different from the others: planidia don’t look or behave at all like other immature phases. Females are prodigiously fecund, sometimes laying several thousand eggs. This reproductive potential possibly evolved to counterbalance high mortality: planidia will climb onto any insect they can reach, so the risk of picking the wrong host is considerable.

Blister beetles are also known as oil beetles, as many species exude drops of oily haemolymph from their joints when alarmed or disturbed. This liquid contains cantharidin, a toxic and irritating chemical. Apothecaries of ancient times used dried and ground up oil beetles as a blistering agent to remove warts and moles. One species, Lytta vesicatoria, is the source of a concoction named Spanish fly, the usage of which is not appropriate for discussion in a family-orientated blog. Medicinal and aphrodisiacal uses of blister beetles are unsubstantiated and risky to say the least, as cantharidin is about as toxic to humans as cyanide and strychnine. And there’s no known antidote. Remarkably, dozens of insect species, including some biting midges, feed on this oil beetle exudate, presumably to acquire its defensive properties.  

There are five species Meloe oil beetle in Britain, but only two are common: the black (M. proscarabaeus) and violet (M. violaceus) oil beetles. Adults feed mostly on flower parts, pollen and nectar. They are most often seen between March and July on open soil, as they search for a mate or a site for oviposition. Our oil beetles are all parasites of solitary bees, but we don’t know whether they use similar ruses to their American cousin: they probably do.  

A violet oil beetle. Note the drop of haemolymph on its thorax.  ©Darkone, Wikipedia Creative Commons

Many people can’t warm to parasitic insects, especially when their victims are lovable bees. This attitude is prejudiced and ill-informed: parasitic insects are important components of ecological communities by regulating population sizes, with profound effects on food webs and biodiversity. Oil beetles’ life histories are magnificent examples of adaptation and survival skills. And they need our care as well: some of our species are declining. 

Solitary bees have many natural enemies, and oil beetles are one of the main factors of brood mortality for those species that nest on the ground such as Andrena spp. mining bees. Despite this, it is estimated that less than 20% of their brood is lost to natural enemies. Bees may be facing several environmental challenges, but oil beetles are not one of them. Bees and beetles can be both valued, admired and protected. 

Violet oil beetle planidia. ©Natural History Museum,