A Maple Leaf Mystery

By Kent Goodwin

Douglas maple (also known as rocky mountain maple or mi₵kik in Ktunaxa) is a tall deciduous shrub or small tree found in many parts of the the Nature Park. For years, I have been noticing that many of the bright green leaves of the plant would acquire patches of red in the late spring and early summer. I never looked too closely and assumed that the reddish patches were some kind of rust fungus.

Red stain on Douglas maple along Duck Pond Trail

This year, I decided to have a closer look and things became very interesting. The reddish patches were not just a stain, but seemed to be a three-dimensional velvety coating of some kind. On some leaves, the patches were tiny, and on others, the majority of the leaf was covered. Leaves with a lot of red would often curl up a bit at the edges and look a bit desiccated.

Multiple patches of red coalesce and cover most of this leaf.

I decided to collect some leaves and take them home for a closer look with a microscope. It turned out that each stain was made up of hundreds of tiny round balls of what looked like jelly. Their colour, size and shape was somewhat variable and their arrangement across the surface of the leaf was often sinuous.

Under a microscope, the tiny balls that make up the stain become apparent.

So I zoomed in a little more and found out, to my astonished delight, that each ball was perched on a little red stalk. What the heck were these things?

A close-up of the stain showing the stalks and globes.

Now it was time for some research, and I turned, as I always do, to Google Scholar. Here is what I learned. The reddish patches on the leaves are called erinea (singular: erinium). They are produced by the plant itself in response to the feeding activity of a tiny eryophyoid mite, in this case likely Eriophyes calaceris. This tube-shaped, four-legged mite overwinters in crevices in the twigs and bark of the plant and crawls out on the leaves in the summer to feed, mate and reproduce in the erinea, which I presume offer it some protection from predators.

The Eriphyes calaceris mite between two patches of erinia

Researchers believe that there are chemicals (and possibly bacteria and viruses) in the saliva of the mites that enter the leaf tissues and somehow cause them to start producing these little ball-topped papillae. While scientists have been able to isolate a number of chemicals from the saliva of the mites (an achievement in itself), no one yet understands exactly how that process could work, what the specific chemicals and other agents do, and how tissues that normally make green leaf material could suddenly start producing little balls of red jelly. In many ways, the red stain on maple leaves is still a mystery.

All photos by Kent Goodwin