People passionate about nature

The Strange Case of the Eastern Red-Winged Blister Beetle

by: Robert E. Wrigley and Deanna Dodgson

Blister beetles are a fascinating family of colourful, world-wide ranging beetles, so-named for the presence of the blistering agent cantharidin in their bodies. Cantharidin is a powerful defensive chemical which is ‘bled’ readily when the insect is disturbed, as Wrigley found out when he handled specimens of the genus Epicauta, which he was collecting for museums.

A cantharidin extract of one European green species (Lytta vesicatoria) was ingested in small doses in historic times as an aphrodisiac, under the name of ‘Spanish Fly.’ Irritating (sexually stimulating?) the membranes of the urogenital tract, cantharidin is highly toxic, and no doubt poisoned many individuals who ingested excessive amounts of the concoction (knowingly or surreptitiously administered by others). It is also responsible for the deaths of livestock when accidentally ingesting a few live or dead beetles in their feed.

Manitoba is host to 20 species of blister beetles, and likely others will be added to the list with further research. A new species for Manitoba (which we named the Eastern Red-winged Blister Beetle) was discovered in 2021 by Deanna Dodgson and Garry Budyk in several sandhill habitats in southern Manitoba. The species is known by the imposing but apt scientific name Tricrania sanguinipennis, which refers to the triangular head, and red wing covers (elytra). The rest of the body is black and ‘hairy.’

Above: The Eastern Red-winged Blister Beetle (Deanna Dodgson).

Surprisingly, the species had emerged in late March (an unusually early spring) while the weather was still freezing at night, and there were still patches of snow present. In fact, while we focused on finding additional locations throughout April, our research was interrupted for days at a time on several occasions by significant snow storms. It was then not surprising that many of the beetles were found dead (from being frozen) on the sand surface.

Severe weather, plus the fact that adults are not known to feed significantly or at all, lead to an adult life stage lasting only around two to three weeks. With its six short legs, the adult beetle has a very limited crawling ability, and if overturned by falling into a rut in the sand, or being blown over by the wind, must spend many minutes and considerable energy attempting to right itself.

Figure 2. Sandhill-trail habitat of the Eastern Red-winged Blister Beetle (Robert Wrigley).

Figure 3. Beetle habitat at the sparsely vegetated basin of a blow-out (Robert Wrigley).

On checking the species’ distribution, we learned that there are actually two species of Tricrania, the eastern North American species, which occurs in Manitoba, and a western relative (Tricrania stansbury). Although appearing identical, the eastern species cannot fly, while the western species is fully flighted.

The nearest records for the eastern species are in the Minneapolis area, 650 kilometres to the southeast, so the question arises, how did a non-flying, slow-crawling beetle make it to the southern Manitoba sandhills, with few suitable habitats in between.

Our conclusion was that it arrived here in larval form while attached to successive generations of dispersing ground-nesting bees in the genus Colletes. This dispersal could have been achieved over several millennia following retreat of glacial conditions 9,000 years ago.

And here, the strange case of the Eastern Red-winged Blister Beetle becomes even-more fascinating.

The adult beetle emerges in early spring and crawls slowly over the sand, attempting to find a mate by following the scent trails of other individuals. During brief copulation, the male transfers his cantharidin gift in his sperm packet.

The female lays around 1000 eggs under debris over a period of one to two weeks and then dies. The eggs soon hatch in the form of highly active, long-legged larvae called triungulins, which search on the ground for a male bee, which in turn is attempting to locate a female with which to mate. It is possible that the triungulin releases a chemical cue which mimics the sex pheromone of the female bee, in order to attract a male bee.

A successful larva attaches with its strong mandibles to the underside hairs of the male bee, and then later transfers to the female bee during copulation. When the female bee descends into her burrow to deposit nectar and pollen for her egg, the beetle larva uncouples and proceeds to feed on the bee egg or larva and food provisions.

The beetle larva grows larger via up to 12 molting stages (instars), and then pupates over the winter in the bee burrow, protected from the elements and predators. The following spring, the adult beetle emerges to renew the species’ life cycle over again.

It seems that the chances of a beetle triungulin finding a host bee would be infinitely small, and yet we found over 250 adult beetles along a few sandy trails. Consequently, the beetle occurs only in the vicinity of a colony of specific ground-nesting bee species (Colletes inequalis bees in Manitoba). The emergence of the beetle is closely timed to the early spring emergence and nesting of this bee, colonies of which were observed along the bank of the sand trails.

Figure 4. When placed in close proximity, specimens instantly commence mating. Notice the similar-looking Aspen winter bud scale to the right of the lower couple (Robert Wrigley).

When searching the ground for beetle specimens, we frequently misidentified the cast winter bud scales of Aspen Poplar (which are shed in abundance in early spring) for the beetles. We could only determine which was which by bending down for a closer look. The similar size, shape, and reddish-brown colour of both would likely help camouflage the beetles from predators such as birds.

Since we did not get blisters on our fingers from handling numerous beetles, we suspect that they produce only a low concentration of cantharidin compared with some other species of blister beetles. Effective camouflage would then be of great survival value in avoiding predators. Early spring emergence of the beetle also means that its reproductive stage is completed prior to the appearance of many potential insect predators such as tiger beetles, most species of which appear above ground from late April to May.

This little insect provides amazing examples of natural selection acting on disuse of former essential adaptations – a former flighted species whose wings have degenerated, an independently reproducing species that is now completely dependent on specific host bees for provision of food, burrow and dispersal for its young, and perhaps decreased concentrations of its defensive chemicals due to camouflage and early emergence. Some of these adaptions may be related to the isolated occurrence of small pockets of partially vegetated sites in sandhill habitats.

The unanticipated finding of this unusual beetle, with its complex life cycle, is suggestive that many more fascinating natural-history stories remain to be discovered in our province.