Only three types of insects throughout the course of evolutionary time have developed agricultural systems with fungi. Leafcutter ants, fungus-growing termites, and a group of tiny beetles that exist hidden, behind the bark of trees. All three form dependent mutualisms with several species of fungus. These insects utilize fungi to break down nutrient poor substrate like wood, into easily digestible carbohydrates, and other macronutrients like sterols, nitrogen and phosphorus. In return, fungal spores hitch a ride to another area of suitable substrate, where they can grow again. This is a common reoccurring pattern in nature, as one partner offers efficient dispersal while the other provides food.
There are two types of beetle that cultivate fungi. Bark beetles, which make up 3700 of the 7500 species within the subfamily Scolytinae, and the 3400 ambrosia beetle species. The remaining 400 species within the Scolytinae are seed and pith feeders that we know little about. Bark beetles bore into the outer phloem tissue of trees where they feed on a combination of plant and fungal tissue while ambrosia beetles bore deep into sapwood, where they grow and feed exclusively on their fungal partner. The areas of excavated wood which become colonized by fungus are called galleries. Eggs that make it to the larval stage not only depend on these galleries for sustenance, but their mothers for the health of these fungal galleries.
Signs of bark beetles on a dead hemlock in WNY
If females tending the brood die, the fungal gardens become contaminated with other fungi and bacteria. Soon the entire gallery along with its larvae become compromised. Because this is a recent observation of ambrosia beetle parental care, (Link 1, Link 2) there is little information on how adult females control fungal and bacterial pests. After reading Cameron R. Currie’s standout publication in Nature, I hypothesize a similar antimicrobial interaction.
In Currie’s 1999 paper, he isolated a bacterial strain of Streptomyces that lived on the leafcutter ants that directly tended fungal gardens. This bacteria that covered these ants produce antimicrobial compounds that target the fungal pests that parasitize their fungal gardens. Given enough time, similar microbes that help the growth of these galleries will occur and be passed vertically to the beetle’s offspring.
Before a mature beetle bores out of its tree to find another suitable one, it feeds directly on fungal spores that matured in the fungal galleries. Spores subsequently collect in specialized appendages called mycangium, where they can be safely transported to the next tree. These mycangia evolved separately, several times which show how important they are. We know the mycangium allows the fungi to colonize new substrate which ultimately provide nutrients to beetle offspring, but I propose another function-the collection of vertically transmitted antimicrobial producing bacteria.
Mycangium from ambrosia beetle
