Readers’ wildlife photos

Hooray! Two more groups of photos came in this morning and so we’re good through Friday (I have one in reserve and can cobble together a few others).

The regulars are pulling their weight, and here we have an informative text-and-photo educational post by regular Athayde Tonhasca Júnior. The subject is one of his favorites: bee behavior. You can enlarge Athayde’s photos by clicking on them, and his text is indented.

Witty impostors

On its release in 1956, Invasion of the Body Snatchers did not impress the critics. A spiel about alien plant spores growing into sociopathic duplicates of human beings was considered too outlandish. While the intelligentsia trashed the film, the producers laughed all the way to the bank because it was a commercial hit: the public loved it. One of the reasons for the film’s success was its ‘aliens among us’ theme. The idea of ill-intentioned beings circulating freely and unsuspectedly in the mist of our society is disturbing and gripping – especially during the McCarthy era, when Americans were inspecting their closets for hidden communists. ‘Enemies within’ inspired and inspires countless tales about spies, infiltrated assassins, covert extra-terrestrials, psycho cyborgs and zombified humans.

Fig 1. Invasion of the Body Snatchers received numerous accolades and is today considered a science-fiction/horror classic © Allied Artists, Wikimedia Commons:

Despite what assorted internet sages tell us, tales of aliens’ secret forays into world domination are entertaining fibs. But the natural world provides many real body snatching thrills such as parasitic flies that zombify their victims or induce them to dig their own graves, or wasps that make their hosts work for them. These cases involve species we may already suspect to be mischievous. That some bumble bees could play similar tricks may surprise many.

Superficially, cuckoo bumble bees, Bombus species of the subgenus Psithyrus, look like any of their social (non-parasitic) relatives. But a close inspection of a female’s hind leg shows no pollen basket (corbicula), which is a shallow cavity surrounded by a fringe of long hairs, a structure used to store pollen to be carried away.

Fig 2. Hind legs of a vestal cuckoo bumble bee (B. vestalis) on left, and a buff-tailed bumble bee (B. terrestris) © Alvesgaspar, Wikimedia Commons. [JAC: the buff-tailed bee has a pollen basket.]

She has no corbiculae because she gathers no pollen; cuckoo bumble bees do not found their own nests nor produce a worker caste: there are only male and female reproductive forms. Instead, a female invades the nest of a social bumble bee, lays her own eggs, which are cared for by her unsuspecting hosts. Raising the young at another species’ expenses is known as brood parasitism, a behaviour displayed by some cuckoo birds (order Cuculiformes) – hence Psithyrus bees’ common name.

Fig 3. A  common reed warbler (Acrocephalus scirpaceus) feeding a European cuckoo (Cuculus canorus) who has dispatched rival offspring by pushing them out of the nest © Per Harald Olsen, Wikimedia Commons:

Cuckoo bumble bees go beyond brood parasitism; they don’t just lay their eggs in a host’s nest and leave them to themselves like cuckoo birds do. These bees take over the victim’s colony, a form of exploitation known as social parasitism. Among insects, this strategy is employed mostly by bees, wasps and ants – of which slave-making ants are notorious – but also by other taxa such as the large blue butterfly (Phengaris arion). Once inside the host’s nest, the female cuckoo and her young live off pollen and nectar pilfered from their hosts, so they are also kleptoparasites – animals that steal food or prey from another animal.

We should pause to appreciate the challenges a cuckoo bumble bee faces. First, she has to locate the nest of a suitable host. She must then get in through a narrow entrance protected by a mob armed with poisonous stings and sharp mandibles. Once these defences have been overwhelmed, she must be able to usurp the colony from the host queen, lay her own eggs and induce the host workers to feed her and her developing brood. A tall order for any brood, social and klepto- parasite.

Fig 4. Cuckoo bumble bees coveting this tree bumble bee (B. hypnorum) nest must pass its sentries © Orangeaurochs, Wikimedia Commons:

Finding a nest is reasonably straightforward: like most social insects, cuckoo bumble bees rely on chemical signals from cuticular hydrocarbons to recognise nestmates, co-specific competitors and potential hosts. But locating a nest is just the start. It must be of suitable size: if too big, the defenders are likely to overwhelm and kill the trespasser; if too small, there will not be enough workers to care for her larvae. As an example, there’s a 100% survival for vestal cuckoo bumble bees invading buff-tailed bumble bee nests with five workers; survival drops to nil for colonies with fifty workers (Sramkova & Ayasse, 2009). To avoid disaster, the female cuckoo bumble bee assesses the size of the host colony possibly by their chemical signals and workers’ traffic (Lhomme & Hines, 2018).

After picking an appropriate target, the female cuckoo bumble bee must confront the residents, who understandably are not obliging. But the nest defenders face a formidable enemy: a cuckoo bumble bee is sturdier and better armed than her social counterparts. She has larger and stronger mandibles, more powerful sting muscles, an enlarged venom gland, and her ventral underside, a particularly vulnerable spot, is protected by thicker exoskeleton plates (sternites) (Richards, 1928). So, some cuckoo species use brute force: they bite, push and sting their way in.

Fig 5. Armed for breaking and entering: the variable cuckoo bumble bee (B. variabilis), a critically endangered North American species © USGS Bee Inventory and Monitoring Lab:

But violence is not always necessary. Some species are let in because they mimic their host’s chemical signs. Others have no chemical signatures and display no aggressive behaviours; the host bees are not aware an enemy has sneaked in. The cuckoo will hide in a corner of the nest for a few days, long enough to acquire the scents of her host and blend in (Dronnet et al., 2005).

Once inside, our intruder has to deal with the queen, the only egg-laying member of the colony and thus the mother of all other bees, whose activities are controlled by their mum’s pheromones. Most cuckoo bumble bees don’t beat about the bush; they kill the queen and eat her eggs. Some species spare the deposed monarch, who loses control of her colony for reasons not completely understood: probably the usurper’s pheromones and physical aggression assure her dominance over the queen.

Fig 6. A brown-belted bumble bee queen (B. griseocollis) is strong, but no match for a cuckoo bumble bee © USGS Bee Inventory and Monitoring Lab:

After sorting out the queen problem, the cuckoo bumble bee is free to lay her own eggs and induce the host workers to feed her and her developing young, although how this is done is largely unknown. The resulting male and female cuckoo bees will leave the nest by late summer and look for mating partners. Like most other bees, the male dies soon after intercourse, while the female will search for a safe spot underground to overwinter, just like her hosts. She will emerge from her slumber late, giving sufficient time for her hosts to establish their nests. The female cuckoo bee spends some time hopping from flower to flower, sipping nectar while her ovaries mature, so that she will be ready to find and conquer a bumble bee nest.

Of the 250 or so Bombus species, roughly 30 have evolved into parasitism. We have a poor grasp of cuckoo species’ biology and ecology, partly because they fly about for a relatively short time and their numbers are naturally low, since they don’t have a worker caste. Thus they are difficult to find and study. But the lack of information comes largely from prejudice. Parasites in general are not viewed sympathetically, especially those that target ‘cute and lovable’ victims such as bumble bees. As a result, cuckoo bumble bees are often absent from local species lists and conservation plans. But that’s a misguided view. Parasites and predators are integral components of ecosystems, preventing over-dominance of some species in favour of rarer ones (Frainer et al., 2018). Cuckoo species should be admired and valued for their physiological, morphological and behavioural adaptations that allow them to overcome the defences of highly organised colonies. These bees of ill-repute are in fact evolutionary marvels.

Fig 7. A female red-tailed cuckoo bumble bee (B. rupestris), a widespread European species and a parasite of the equally abundant red-tailed bumble bee (B. lapidarius) © Ivar Leidus, Wikimedia Commons:

References

Dronnet, S. et al. 2005. Bumblebee inquilinism in Bombus (Fernaldaepsithyrus) sylvestris (Hymenoptera, Apidae): behavioural and chemical analyses of host-parasite interactions. Apidologie 36: 59–70.

Frainer, A. et al. 2018. Parasitism and the biodiversity-functioning relationship. Trends in Ecology and Evolution 33: 260–268.

Lhomme, P. & Hines, H. 2018. Ecology and evolution of cuckoo bumble bees. Annals of the Entomological Society of America 112: 122–140.

Richards, O.W. 1928. A revision of the European bees allied to Psithyrus quadricolor Lepeletier (Hymenoptera, Bombidae). Transactions of the Entomological Society of London 76: 345–365.

Sramkova, A. & Ayasse, M. 2009. Chemical ecology involved in invasion success of the cuckoo bumblebee Psithyrus vestalis and in survival of workers of its host Bombus terrestris. Chemoecology 19: 55–62.