At the end of February, I finally found a beetle I had been looking for here, for years. Perhaps I had not been scouting for it early enough, but we barely had a winter this year, and I suspect that most years it would have been evident in March. The key was finding an aggregation of its host organism.
Tricrania sanguinipennis is a blister beetle in the family Meloidae. The adult insects measure 9-15 millimeters, but are bright enough, and active enough, that they are easily seen. Their wings, hidden under their red, leathery elytra, are almost vestigial, so they are flightless. Instead they crawl, but rather rapidly, over the surface of the soil, periodically digging where they detect the possible presence of a host.
Kansas is about as far west as this beetle is found, though there are records well north into Saskatchewan, Canada. It occurs over the eastern U.S. to extreme northern Florida, and up into adjacent southern Canada.
The limiting factor is that Tricrania sanguinipennis is a parasitoid of solitary bees that nest in dense aggregations, namely cellophane bees in the genus Colletes.
We have the Unequal Cellophane Bee, Colletes inaequalis, nesting in our yard and/or the adjacent neighbor’s lawn, depending on the year, but those locations are apparently blister beetle-free. It took finding a small aggregation of the bees in a forested park to locate the beetles.
The bees nest in burrows, ideally in sandy soil. The vertical, subterranean tunnels branch into several individual cells, each one an “apartment” for a single bee larva. The grub feeds on a nearly liquid loaf of pollen and nectar. The walls of its room are coated in a type of natural plastic manufactured in glands in its mother’s abdomen, which essentially waterproofs the chamber, and retards mold and fungus.
Back to the beetles. What the female beetles lack in mobility, they more than make up for in fecundity. Each lady can produce hundreds of eggs, over a thousand in some documented instances. How, then, do the beetles gain entry into these tunnels? The answer is that they do not. The larvae do.
The larval stage is unusually lengthy, progressing through six instars. An instar is the interval between molts. It is also strange in that it includes hypermetamorphosis. In this case that means the larval form changes radically in both appearance and behavior from one molt to the next.
The first instar larva that emerges from the egg is a sleek, streamlined, highly mobile bee-seeking missile called a triungulin. At the time they are active, it is almost exclusively male Colletes bees that are active, buzzing about and frequently landing to investigate a potential site where a female could emerge. It is at these brief moments when a blister beetle triungulin scampers aboard, affixing itself to the hairs on the underside of the bee’s abdomen.
When the male bee at last is able to mate, the triungulin transfers to the female bee. In at least a few instances, the triungulin may attach directly to a female bee that it encounters on the ground. She will eventually, and unwittingly, ferry that parasite, and probably several others, to her new nest burrow. Once inside, it disembarks and infiltrates one of the subterranean nest cells.
The beetle larva usually consumes the egg of the host immediately, but not always. The bulk of its diet will be the honey and pollen left by the bee for its offspring in the cell. It may be a frequent occurrence that more than one beetle larva invades a single bee cell. In that event, cannibalism of the competition resolves the conflict.
The meal of the host egg or larva is usually enough nutrition to trigger the beetle larva’s molt to the next instar. This results in a shocking change from that sleek, active larva into the insect equivalent of a couch potato. The second instar is, shall we say….rotund, and boat-shaped. It commences feeding on the pollen and nectar stores in the cell of the now missing bee offspring. The insect retains this form for the remainder of its larval life. The first three instars shuck their old exoskeleton completely, but the fourth and fifth instars retain each molt in its entirety. Think of it as an object inside a balloon (inside another balloon by the fifth instar). Those larvae actually shrink in size to fit inside the shed “skins.” When molting into the pupa stage, the sixth molt is again broken during shedding, and compacted at the rear of the pupa, which is still inside those other exoskeleton balloons.
It takes until late summer or early fall for the life cycle to complete, the adult beetle remaining encased in its final one or two larval exuviae, where it overwinters, still inside the cell in the host’s nest burrow.
You would think that the bee species hosting this diabolical beetle would be decimated by it, but such is not the case. Each spring there are plenty of the adult bees. What is more of a threat is the potential disconnect between the bees and their nectar plants. Colletes inaequalis visits flowering trees almost exclusively, especially Eastern Redbud, and maple trees. As the phenology of the blooming cycles becomes increasingly unpredictable thanks to climate change, the appearance of the flowers may cease to always coincide with the emergence of the bees that pollinate them.
Sources: Messinger Carril, Olivia, and Joseph S. Wilson. 2021. Common Bees of Eastern North America. Princeton, NJ: Princeton University Press. 286 pp.
Parker, J.B. and Adam G. Boving. 1925. “The Blister Beetle Tricrania sanguinipennis - Biology, Descriptions of Different Stages, and Systematic Relationship,” Proc U.S. Nat. Mus. 64(2491): 1-40. This is a wonderfully exhaustive article, with illustrations of all larval instars.
