Myco-kleptomaniacs

This is the second annual Week of the Fungi on STC, a sporadic undertaking. It is our way to hail the start of the fall mushroom collecting season in parts of our home territory (the northern hemisphere).

SEM micrograph of adult southern pine beetle. The
arrow indicates the location of a mycangium. Source.

by Elio

Anyone who has gone out hunting for wild mushrooms and came back with a nearly empty basket has been tempted to "borrow" specimens from someone more lucky. This cardinal sin now has a name: mycoklepty.

Stealing fungi is not limited to humans. Beetles do it too.

A graduate student at Michigan State, Jiri Hulcr, observed that small bark beetles in the forests of Papua New Guinea were consistently found in proximity of larger ones. These beetles make galleries in the bark of trees and, in this case, those made by these two particular beetle species, the larger and the smaller, were always adjacent to each other. How come?

First, something more about these beetles. Like leaf-cutting ants and termites, these insects depend on fungi for sustenance. They sow spores of specific fungi in the tunnels they make into the sapwood of healthy trees. The spores germinate, the fungus grows into a glistening mat known as "ambrosia," Food of the Gods. This ambrosia is Food of the Beetles. A pioneer student of these insects, H. G. Hubbard, wrote in 1897: All the growing parts of the fungus are extremely succulent and tender. The young larvae nip off the tender tips as calves crop the heads of clover, but the older larvae and the adult beetles eat the whole structure down to the base, from which it soon springs up afresh, appearing in white tesselations upon the walls. The growth of ambrosia may be compared to asparagus, which remains succulent and edible only when continually cropped, but if allowed to go to seed is no longer useful as food. The fungi provide the beetle not only with all nutrients but also with the steroids needed to make the hormones that regulate the beetle's metamorphosis, a most unusual relationship between nutrition and development.

Observing the beetle habits in Papua New Guinea, Hulcr suspected that the smaller beetles were sponging off the larger species rather than collecting and hauling their own spores. To check this out, he looked closely at their anatomy. Sure enough, the smaller species lacks a specialized fungus spore-carrying structure called a mycangium. Furthermore, DNA analysis of the fungi found in the galleries of the two kinds of beetles matched, leading to the conclusion that the smaller beetles did indeed appropriate their fungi from the larger ones. Mycoklepty indeed!

Comments

[Nathan Myers]

So the small beetles are of a different species from the larger ones? Or are they immature, and will develop such a feature later? Or were they malnourished as grubs, missing out on the steroid needed to grow their own mycangium?

Merry replies:

Thanks for pointing out that we did not make that clear. When one gets too close to a subject, such matters seem obvious---until someone reads it with fresh eyes. They are indeed different species. I'll edit the text slightly to specify that for other readers.