The Beech Aphid Poop-Eater

Interactions between organisms are what got me into ecology. After all, no living thing operates in a vacuum. I recently had an experience that reminded me of this on a recent hike with some friends. We stumbled across a strange black growth on the limb of a beech tree. It was like a crusty black stalagmite on the branch. Luckily, my friend Kristen happened to be there. Her expertise in plant/fungal interactions was exactly what this moment called for. What we had found was one of the most unique fungi I have seen in a long time. What's more, it only lives on American beech trees (Fagus grandifolia).  

The fungus in question is a type of sooty mold known scientifically as Scorias spongiosa. I'm not sure if it has a widely used common name but at least one source I referenced lovingly referred to it as "the beech aphid poop-eater." Though that name doesn't come close to rolling off the tongue, it nonetheless describes the life cycle of this bizarre fungus quite accurately. 

A Scorias spongiosa colony changing from its asexual phase (beige) to its sexual phase (black)

A Scorias spongiosa colony changing from its asexual phase (beige) to its sexual phase (black)

We begin with an aphid called the beech blight aphid (Grylloprociphilus imbricator). It is not the insect responsible for beech bark disease but it is specific to American beech. These aphids are gregarious little creatures and large populations can quickly accumulate on beech trees. They secrete a white woolly substance, making them quite easy to spot. Like all aphids, they feed on sap and poop out mass quantities of honeydew (sugary aphid poop) in the process. 

A colony of beech blight aphids

A colony of beech blight aphids

It is this honeydew that the sooty mold feeds on. Spores that manage to land on the accumulation of aphid poop begin to grow fungal hyphae. At first, the fungus takes on a beige coloration. Gradually it forms a tangled mass of fungal tissues. It is important to note that it is not a parasite on the beech tree whatsoever.  S. spongiosa simply feeds on the sugary beech blight aphid poop. The reason S. spongiosa can grow into such a large formation has to do with the colonial nature of the beech blight aphid. The more aphid poop that accumulates, the lager S. spongiosa will grow.

By mid summer, some S. spongiosa resemble large sponges on the branches and trunks of beech trees. At this point they are producing asexual spores. Later in the year, the fungus switches over to producing sexual spores. This shift also causes the fungal mass to produce more melanin, which gives it its black coloration. It also becomes much more durable, meaning there is a good chance of finding this fungus outside of the growing season. 

Patches of Scorias spongiosa covering the bark of a beech tree

Patches of Scorias spongiosa covering the bark of a beech tree

What amazes me the most is that this fungus will only ever be found on American beech trees. The reason for this is because it feeds only on the poop of the beech blight aphid, which, as its common name suggests, feeds only on beech trees. This is but one example of the myriad ecological interactions that makeup life on this planet. It is also a reminder that single species conservation efforts forget most of what makes a species special. 

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2]