A recent communication from a friend in Vineyard Haven provided an excuse to write about a group of beetles that has long fascinated me: the so-called “checkered beetles” constituting the taxonomic family Cleridae.
My friend’s message, sent from Ground Zero of the ongoing Southern pine beetle outbreak along Lambert’s Cove Road, included nice photos of a beetle my friend correctly identified before I even had time to respond: a colorful Clerid called Thanasimus dubius, or the wavering checkered beetle.
This beetle turns out to be an interesting one, and possibly an important one from a Vineyard perspective, and I’ll return to it for a closer look. But first, it’s worth discussing the family Cleridae as a group, because these are really cool insects.
This is not a large family by beetle standards, with only about 300 representatives in North America. And while the family is well distributed around the globe, it seems oddly scarce in our region. As far as I can tell, only a half-dozen Clerid species have been documented on Martha’s Vineyard. And none of these appears to be at all common: Of about 3,300 Vineyard beetle observations in iNaturalist, only 19 are of Clerids. This pattern is roughly similar throughout the Southern New England coastal plain.
But what this family lacks in diversity and abundance, it more than makes up for in snappy coloration and ecological importance. Clerids are called “checkered beetles” because of the bold patterns of bands or spots that many species exhibit. While there is a lot of variation across the family, a typical Clerid is a medium-size beetle, with the head and thorax considerably narrower than the wings. Most Clerids are covered with short, dense hair, and the combination of structure, coloration, and hairiness makes this family fairly easy to recognize.
The ecological importance of the checkered beetles stems from their predatory habits. Again, there is a wide range of variation. But generally speaking, Clerids eat other insects, often other beetles. Many species target wood-boring beetles; others zero in on galls, feeding on the insect larvae inside; and still others associate with carrion, sometimes eating the dead animal but also often preying on other insects attracted to the decaying flesh. As is always the case with predators, these habits mean that Clerids can be influential in regulating the population size of the insects they prey on.
So, back to Thanasimus dubius, which my friend observed “all over a knocked-over pitch pine” along Chappaquonsett Road. That area, most readers will be aware, has seen its extensive stands of pitch pines devastated by Southern pine beetle infestations. The Sheriff’s Meadow Foundation, and possibly some private landowners as well, have embarked on large-scale, expensive, but in my view necessary responses, removing dead pines and thinning surviving ones to reduce their vulnerability to Southern pine beetle infestation.
My friend’s discovery of large numbers of T. dubius on a dead, presumably beetle-killed pitch pine is anything but an accident. The prey of choice for this particular Clerid beetle happens to bark-boring beetles, including the Southern pine beetle. Thanasimus lays its eggs in or near the entrance of boring beetle galleries on the trunk of an infested tree. The beetle larvae enter the galleries to feed on the eggs of the boring beetle. And adult Thanasimus bushwhack adult boring beetles as they emerge from tree trunks to start their reproductive cycle.
Thanasimus dubius is, in short, an ally in the fight to control Southern pine beetle, a native originally of the Southeastern U.S. that arrived here recently as its range expands northward, driven by milder winters that allow more of the cold-sensitive Southern pine beetle larvae to survive. (Yes, Southern pine beetle devastation on the Island is a very direct, very tangible effect of global climate change, though a variety of other factors have amplified the beetle’s impact.) What my astute friend observed was clear evidence of at least one natural predator of Southern pine beetle responding to the sudden abundance of a suitable prey species.
I would not hold my breath waiting for Thanasimus to beat the Southern pine beetle into submission, though. In order for the predator to become abundant, the prey needs to become abundant first. And so I would expect a predator like Thanasimus to respond to local beetle outbreaks once they’re underway, rather than preventing them in the first place.
Over the long term, once the initial outbreak of Southern pine beetle has run its course, natural predators like Thanasimus will likely play a role in regulating Southern pine beetle numbers, as this newly established pest species gradually settles in as an established member of our insect fauna. But trees grow slowly, and forest regeneration takes decades or centuries to play out fully. A new equilibrium will eventually emerge involving pitch pine, its associated, lethal boring beetle, and predators of that beetle. But it will take time.