Stylopized, emasculated and zombified: the risks of visiting a flower

By Athayde Tonhasca

‘Bizarre’ and ‘weird’ are overused adjectives for describing many characters and events of the natural world. Life is way too complex and varied to conform to familiar patterns, so the out-of-the ordinary is all around us, even though we not always see it. But when the discussion turns to stylopids, it’s difficult to avoid talking about the bizarre and the weird.

Stylopids (or stylops) are small, seldom seen and poorly known insects with about 700 described species, 10 to 16 of them in Britain; the true figures are likely to be much higher. They are parasites of other insects such as bees, wasps, plant hoppers and leaf hoppers. From a distance, male stylopids can be mistaken for flies, but their ruffled wings give them away and explain the name of this group of insects: the order Strepsiptera, from the Greek strephein (to twist) and pteron (wing). Twisted-wing insects is another common name for them. 

Males have branched antennae, and their eyes are berry-like structures comprising dozens of image-forming eyelets. This unusual array inspired the development of new cameras of reduced size and sharp images, which are handy for smartphones. Males cannot feed because their mouthparts are not developed. But never mind going hungry; they don’t live for more than a few hours. Their only objective in life is to use their fancy eyes to find a female and mate.

A male stylopid © Mike Quinn, TexasEnto.net, Mckenna & Farrell, 2010, PLoS ONE 5(7):e11887; and detail of a male head © CSIRO, Australian Insect Families

Females look nothing like the males. In fact, they don’t look like your ordinary insect at all because they don’t have wings, antennae, legs, mouthparts or eyes: they are neotenic, i.e., they retain their larval features. An adult female does not need a fully-formed body since she never leaves her host: she will develop and die semi-buried in another insect. ‘Semi-buried’ because the tip of her cephalothorax (the head and the thorax fused together) protrudes from the host’s abdomen. 

A bee carrying a female stylopid. Scale bars = 1 mm © Soon, V. et al. 2012. Entomologica Fennica 22: 213-218.

Through this exposed area, the female releases a pheromone to attract males. Once suitors finds her, they face an anatomical challenge. Only parts of her head/thorax are exposed, which doesn’t bode well for conventional insect romance. But this setback is nothing compared to the facts that she doesn’t have genitalia, and her eggs float in the haemolymph (‘blood’). So a male has only one course of action: the disturbingly sounding traumatic or hypodermic insemination. He pierces the female’s cuticle with his penis and injects his sperm into her haemolymph.

Two male stylopids going after a female tucked in a bee © W. Rutkies at Peinert et al. 2016. Scientific Reports 6: 25052

The deed done, males soon die. The fertilised eggs hatch inside the female, giving birth to thousands of tiny six-legged, very active and agile larvae called planidia (just like blister or oil beetles). The planidia feed on mum’s innards and eventually crawl out of her body to disperse and start looking for a host of their own. 

A stylopid planidium, and planidia emerging from a female stylopid. Scale bars = 0.1 mm © Kathirithamby, J. 2018. Biodiversity of Strepsiptera

A wandering planidium climbs a flower to wait for an unsuspecting visitor. When a bee or wasp lands, the planidium somehow hitchhikes a ride to their nest. We are not sure how it does this: it could hide in the pollen, or possibly be swallowed with the nectar sucked up by the flower visitor, then released when the host regurgitates nectar inside the nest. Chances are it will end up in the wrong nest, so most planidia are done for. But the staggering fecundity of female stylopids compensates bad odds: they can dish out 750,000 planidia, so a few are likely to find a suitable host.

Once inside the right nest, the planidium burrows into a host’s egg or larva and transforms into a traditional legless, grub-like larva. It is followed by other larval stages, pupation, and finally adulthood – by then the host has also become an adult. If it’s a male stylopid, it squeezes out of the host and flies away, usually leaving a big gap behind and killing the host. If it’s a female, it will park itself in the host’s abdomen. 

A male stylopid emerging from a wasp © Kathirithamby, J. 2009. Annual Review of Entomology 54: 227–49

The story above is a peep at stylopids’ life histories, as there is considerable variation depending on the species and type of host. And if all this sounds outlandish, there is more to come.

Stylopization (parasitism by stylopids) causes all sorts of physical and behavioural eccentricities in the host, all for the parasite’s benefit. Such as infertility. Reproduction involves mating, nest building, nest provision, etc., which are risky and energy-consuming, therefore not beneficial for a parasite. Stylopids solve this problem by disabling the host’s reproductive organs, functionally castrating them. Some stylopized bees have reduced scopae (pollen-carrying structures) and seldom if ever carry pollen: there’s no point, as they don’t have a brood to provide for. Contrary to what happens to most parasitized insects, stylopization often lengthens hosts development; they live longer so that stylopids have more time to mature. Some stylopized bees are led to stand still on a grass or flower stems with their head downwards. Such a zombie state greatly facilitates stylopids’ mating business. 

In Britain, furrow bees (genera Halictus and Lasioglossum), yellow-face bees (genus Hylaeus) and especially mining bees (genus Andrena) are victims of stylopids, but we have little information about their interactions and no idea about consequences of parasitism. 

Stylopids are odd and enigmatic, but they are also one of the most complex and intriguing groups of animals. They are evolutionary marvels that have puzzled and awed generations of entomologists and naturalists, and more surprises should be revealed from future research. It seems quite fitting then for the august Royal Entomological Society to have adopted a stylopid (Stylops kirbii) as a representative of the organisation.

Royal Entomological Society badge © Wikipedia Creative Commons

Smorgasbord or Spartan: the consequences of pollen diets

By Athayde Tonhasca

There is nothing visibly remarkable about the mining bee Andrena florea. This bee, one of the 67 Andrena species in Britain, is found in open scrubby areas, grassland and woodland edges of south-east England. But one thing makes this bee unusual; it only takes pollen from white bryony (Bryonia dioica).

Andrena florea, which is commonly and unsurprisingly called the white bryony mining bee, is a rare British example of a bee that forages on a single plant species. This dietary restriction is circumstantial, because white bryony is the only plant of this group occurring in Britain. In continental Europe, A. florea has other Bryonia species available. So in a wider geographical context, this bee is oligolectic (or an oligolege) that is, it collects pollen from a few related plant species (from the Greek oligo: few, scant; and lect: chosen, picked).

A white bryony mining bee and its pollen source, white bryony © Aiwok (L) and H. Zell (R), Wikipedia Creative Commons

Pollen specialisation can be a considerable drawback for a bee because food may be scarce even in a landscape full of flowers, and this may limit populations of some species. For example, until recently the white bryony mining bee was rare and threatened in Poland. This has changed with the spread of white bryony into the country’s urban areas. And yet, a considerable number of species are pollen specialists; in some habitats, they make up the majority of the bee fauna. So pollen specialisation must have its advantages, for example by allowing more efficient flower visitation and pollination rates, which benefits bees and plants.

Polylectic bees are at the other end of the spectrum: they collect pollen from various unrelated kinds of flowers. The advantages of being a pollen generalist seem evident: there is more food to choose from and it’s available for longer, as flowers blossom at different times. But these bees must also have an array of physiological adaptations to overcome a variety of chemical and physical barriers to different types of pollen. This could be too costly for a bee’s metabolism.

Pollen is a rich source of protein, lipids, vitamins and minerals. But it also contains secondary compounds that may be noxious to some bees, and pollen grains are often protected by indigestible coating. These barriers explain why few insect taxa rely on pollen alone for food, and could also explain why most polyleges (polylectic bees) exhibit a degree of pollen specialisation: for example, heather (family Ericaceae) and legumes (family Fabaceae) make up over 70% of the pollen collected by British bumble bees, despite local abundance of other pollen sources.

Experiments with the closely related red mason bee (Osmia bicornis) and horned mason bee (Osmia cornuta) show the effects of different types of pollen. Red mason bee larvae develop well on buttercup pollen (genus Ranunculus), but fail to do so on pollen from viper’s bugloss and related plants (genus Echium); the reverse happens for the horned mason bee. Both bees do well on field mustard pollen (genus Sinapis), while neither develop on pollen from tansies and related species (genus Tanacetum). But the story is a bit more complex: neither bee shows any negative effect as long as they are not restricted to ‘bad’ pollen. In fact, unsuitable pollen is part of the bees’ natural diet. Other bee species show similar patterns.

Viper’s bugloss (1), creeping buttercup (2), field mustard (3) and tansy (4): nutritious/poisonous food for the right/wrong bee. © Wikipedia Creative Commons

So what can we conclude from all this?

Oligolecty and polylecty are both successful evolutionary strategies. Some bees depend on a few plants, others have diversified pollen diets. The range of hosts can be narrow or wide, depending on the species, but setting aside a handful of exceptions, bees need pollen from different plants to complement nutritional imbalances or to mitigate the effects of harmful secondary metabolites. But even pollen of low nutritional quality or digestibility is taken, as long as it’s a portion of a balanced diet.

These aspects have important consequences for the conservation of bees. They need a diversity of flowers, and plenty of them. Habitats such as semi-natural grassland, hedgerows, field borders, cover crops, brown sites, road verges, wild gardens and weedy parks are all suitable. Planting is helpful, but except for the honey bee and some bumble bees, we know little about what plant species to use. The safest action is to let our wild plants go wild, so that we have bigger, and more diverse flower-rich habitats. That’s not much or too difficult a task to assure the future of our most important pollinators.