Those vexing but intriguing cryptic bees

By Athayde Tonhasca

Imagine it is springtime, you are enjoying the sunshine in a beautiful garden, camera/phone in hand, ready to start your list of pollinating species (a New Year’s resolution). Lo and behold, a bumble bee is feeding on a flower very close to you. The lighting is good, there is no wind, and she waits politely for you to take several perfect shots. Back at home, you get your guide book to identify the bee. By cross-referencing colour and band patterns, you conclude it’s a buff-tailed bumble bee (Bombus terrestris). Before jotting it down on your field notebook (another New Year’s resolution), you check it again. Now you think it looks more like a white-tailed (B. lucorum). You dig in further to find two additional candidates: the cryptic (B. cryptarum) and the Northern white-tailed bumble bee (B. magnus): all looking very much the same. You begin to wonder if you should stick to a list of butterflies.

Your frustration would be understandable and explainable. The colour patterns of many bumble bees are variable within and between populations, so that some forms may look very much like some other species. To complicate things, many sympatric bumble bee species – i.e., occurring in the same area – are similar to each other. This feature is believed to be a manifestation of Mullerian (or Müllerian) mimicry, which happens when two or more species harmful to predators converge to a similar appearance. This reinforces the message to predators to avoid that colour pattern for their own good, which gives mimics an evolutionary advantage. 

Bombus lucorum/cryptarum/magnus? A vexing problem © Ivar Leidus, Wikipedia Creative Commons

Experienced bumble bee enthusiasts can tell the buff-tailed from the lookalikes, but the white-tailed, cryptic and Northern white-tailed bumble bees are morphologically indistinguishable from each other: we need genetic analyses to tell them apart. So it is not surprising that Bombus lucorum has been given more than 180 other scientific names over the years, all because of different taxonomic interpretations. To sort out this Gordian knot, taxonomists refer to the three species collectively as Bombus lucorum sensu lato (‘in the broad sense’), abbreviated as s.l. or sens. lat. As opposed to Bombus lucorum sensu stricto (‘in the strict sense’), abbreviated as s.s. or sens. str., when we want to refer to this species alone. Bombus lucorum s.l. is not an isolated case: of the 250 or so bumble bee species in the world, only a handful are easily and unquestionably identified.

These taxonomic spitting images are known as cryptic species, sibling species, or as a species complex. With the fast expanding availability of genetic techniques, we are finding out that cryptic species are much more common among insects than previously thought. For example, DNA barcode analyses have shown that the two-barred flasher butterfly (Astraptes fulgerator), which ranges from the United States to Argentina, comprises three to seven species (the vagueness on the number reminds us that genetic tools are not a straightforward alternative to traditional taxonomy).

Astraptes fulgerator s.l.: 3, 4, 5, 6 or 7 species. © Chinmayisk, Wikipedia Creative Commons

Cryptic species have profound consequences for conservation. A species cannot be properly protected if we can’t tell exactly what it is, what it does and where it is. Because of these uncertainties, the conservation status of B. cryptarum and B. magnus can’t be assessed: they linger in the Data Deficient limbo of the IUCN Red List of threatened species. Moreover, biodiversity estimates may be hugely underestimated, and conservation efforts may be inadequate because true species richness and ecological interactions could be hidden. There are implications for human health and agriculture as well. There are over 3,500 species of mosquitoes of the order Culicidae, of which an alarming number are cryptic: telling them apart is essential because a few are vectors of malaria and other diseases. The same applies to Ceratitis fruit flies, some of which are serious pest of fruit crops. 

It would be short-sighted to see cryptic species only as a taxonomic headache. In fact they tell us life on Earth is much more complex and diverse than we thought. DNA analyses have revealed new species of bats, chameleons, lemurs and even giraffes. So we can expect a deluge of new plants, fungi, insects and other less studied organisms when – and if – their genetic profiles are investigated.

Approximately 1.5 million species have been described, but this figure is thought to be a gross underestimate. With cryptic species at play, the true number is anybody’s guess. 

The complexities of species complexes notwithstanding, bumble bee observers have reasons to persevere. Much of species identifications involve ecological information such as location, time of the year, plants visited, and even behaviour. Naming species this way is trickier, but also more challenging and educational. And those who become stuck in their efforts to identify bees should be philosophical: by tangling with cryptic species, they are experiencing some of the mysteries and wonders of nature.