THE BARKBEETLE DESTROYER
In an effort to continue the theme of the last “Meet the beetles,” let’s journey back in time once more, to 1899, when Dr. A. D. Hopkins wrote a short account describing the “barkbeetle destroyer.” He wrote that these “clerids” lay their eggs at the entrances to beetle galleries, and how these eggs hatch to form “minute-active worms” (larvae) that explore the galleries beneath the bark, feasting upon bark beetle eggs and larvae. There they eat until they are fully-grown, at which point they carve out a chamber for pupation in the outer bark. All stages spend the winter beneath the bark, and in the spring the adults emerge, following their prey to pine trees nearby, where they sit beneath flakes of outer bark, “awaiting an opportunity to pounce on any barkbeetle that comes near.”
In a 1920 paper, Adam G. Boving and A. B. Champlain describe the attack of a clerid (here “clerid”is a term that refers collectively to the family cleridae):
“The Clerid remains motionless until a wandering Scolytoid or some other insect approaches close enough. Then running with a rapidity that resembles a leap, it seizes the prey. Grasping it with the front and middle pair of legs and holding on to the bark by the hind pair, sometimes balanced by the tip of the abdomen against the bark, it proceeds to feed. With its strong jaws it breaks the chitin or separates the segments and feeds upon the soft tissue and viscera within.”
(Yes, this is something I desperately hope to capture in video later this summer).
LOOKING (SMELLING) FOR A GOOD RESTAURANT
In 1899, Hopkins also noted that “the American barkbeetle destroyer is often quite common on the bark of spruce trees infested with barkbeetles.” And indeed, even today, the presence of T. dubius on pine trees is a sign that the trees have been recently attacked. We now know that this strong association between the presence of beetles and clerids is mediated by plumes of semiochemicals (think pheromones) floating through the forest. For the clerid, the chemicals emitted by bark beetles smell, well… delicious. Wonderful. Enticing and powerful. And though most bark beetle chemicals attract T. dubius, it seems that one is a more powerful attractant than others – frontalin, a pheromone released by the Southern Pine Beetle.
What’s more, clerids may change their preference for one pheromone over another based on the density of their alternate prey. And a set of experiments by Arnaud Costa and John Reeve (which took place in little wind tunnels) have shown that T. dubius may change its preference or response to certain chemicals based on reward conditioning, which fits nicely with the density-dependent result. This idea of prey-switching may have important consequences for the population dynamics of SPB, T. dubius, and the other bark beetles (Ips species).
The “barkbeetle destroyer” is the most studied enemy of the Southern Pine Beetle, and it appears to have a significant effect on SPB populations. Exactly how important it is in keeping populations from reaching outbreak stage or in ending large outbreaks is still unclear, but it certainly seems that this little black, white, and red beetle may be a crucial driving force against the Southern Pine Beetle and the death of pine trees across the South.