I remember being fascinated by the Dr. Seuss book Horton Hears a Who as a child. It made me wonder: could each of those specks of dust dancing in the light shafts from my bedroom window be its own planet? Could our own planet be a speck of dust in some other creature’s universe?
As it turns out, there is something close to Who-ville right here in Vermont and New Hampshire. It is, if not exactly a self-contained world, then a surprisingly busy and biologically diverse ecosystem contained on a single leaf, specifically, a black cherry tree leaf.
Black cherry trees are among the most common trees in the northeastern United States. They are our biggest native cherry trees and the source for the cherry wood used for furniture and cabinets.
On the leaves of some of these cherry trees are eriophyid mites. There are hundreds of different species of eriophyid mites. Each species is tiny – Who sized – just one-hundredth of an inch long. And while we would expect a mite to have eight legs, these mites have just four, all the front of their bodies, so they wind up dragging their long, legless behinds around behind them.
It’s really hard to tell these various species of eriophyid mites apart by looking at them, even with high magnification. But luckily for the farmers and gardeners who want to identify them, usually to find out the best way to kill them, these mites can be told apart by the various galls, blisters, and rust that they create on the plants they live on.
For the cherry gall mite, that would be a finger gall on the glossy top surface of a black cherry leaf. “Finger gall” is the technical term, indicating its general shape, although again, they are tiny, the length of an eyelash. The galls are red, and there can be dozens of them on a single cherry tree leaf. Inside the gall, the eriophyid mite nestles in an abnormally lush growth of leaf-hairs.
It seems like an easy life, protected from the world inside a gall. The mite probably doesn’t even notice when, in late May, a small, pale-blue butterfly with wings the size of thumbnails, alights on the gall. The mite probably has no idea that when the butterfly departs, it leaves behind a tiny egg.
That pale-blue visitor is a cherry gall azure, also known as late spring azure. The cherry gall azure is more widespread in New Hampshire than in Vermont, but it is found in both states. It is closely related to the spring azure and was only named a species, and differentiated from the spring azure, a few years ago.
While other lepidopterists had noticed that there was something different about some of the spring azures, two amateur butterfly scientists, Harry Pavulaan and David Wright, studied the differences and described the new species, which they called Celastrina serotina, in a scientific journal.
The cherry gall azure’s white egg hatches into a small, green caterpillar only a fraction of an inch long. As the caterpillar grows, it changes color, becoming either creamy white or reddish, like the finger galls on the cherry tree leaf. Soon, the caterpillar starts eating those galls. The biggest cherry gall azure caterpillars – still less than an inch long – gobble up the entire gall, mite and all. It’s one of only two caterpillars in our area known to eat meat on a regular basis. (The other is the caterpillar of the harvester butterfly.)
It’s also these mature cherry gall azure caterpillars that have a special relationship with ants. The caterpillars have two different organs that ooze sweet stuff, or honey-dew, for the ants to eat. In return, the ants presumably defend the caterpillars against predators, which are mostly wasps. It’s similar to the relationship that some ants have with some aphids. While it sounds weird, many caterpillars in the same family as the cherry gall azures, including other species of azure and gossamer-wings, do the same thing with ants.
Then, some time from the end of June to the middle of July, the fun is over. The leaf and the mites keep doing their thing, but the ants disappear when the caterpillars turn into light brown pupae that look a lot like dried feces. Come autumn, the pupae fall to the ground with the leaves and stay in pupal form until the next spring. When they transform into adult cherry gall azure butterflies the next May, they will go off in search of cherry blossoms to sip nectar from and other cherry gall azure butterflies to start the cycle over with.
Finally, the females will look for a finger gall on a cherry leaf, a leaf that is almost a little world of its own, worthy of Dr. Seuss.