Nature knows no real balance, just moments of apparent equilibrium before some rise or fall.
We are studying scale insects—a kind of immobile (scientists say “sessile”) animal that lives on plants and sucks at them until, in some cases, they die (and by we, I mostly mean one of my students, Emily Meineke, and her other advisor, Steve Frank). Scale insects are everywhere once you learn to look for them, under nearly every leaf, sucking.
The sucking of scale insects is mostly harmless, an indiscretion, an innocent obsession, but scale insects can kill trees and maybe worse. A year or so ago, we began to wonder whether or not they might also affect humans. This is the kind of thing one wonders when contemplating whether the obscure topic one has chosen to spend one’s life on might have some meaning. It was lovely and unadulterated speculation, then we read a new paper about emerald Ash borers and everything changed. Well, not everything. Actually not so much at all, but it was a cool paper.
Image 1. Look closely. These are the scale insects who blend in with tree bark all over the city and world, convincing you and me to move along. There is nothing to see. Photo by Becky Kirkland/NC State University Communications
Ash trees are brush-topped, lovely and, in many cities in North America among the most common urban trees. Or they were. In 2002 emerald ash borers arrived from Asia in Michigan where they began to kill ash trees. At last count tens of millions of ash trees had been killed in Michigan and surrounding states.
If there is any good news about the emerald ash borer, it is that it set the stage for a clear study of what happens when you lose trees (I know, this is like saying that the good news about the titanic is that it taught us how giant ships sink). This beetle kills trees anywhere it invades and so, with each death, the consequences of their loss can be studied relative to what was observed where ash trees have not died. Trees offer--people who study, love and hug trees argue--many health benefits, including their effects on air quality and urban climate. The presence of trees, it has been suggested, might even lead to a local decrease in some pollutants and hence the incidence of those human health problems associated with pollution (heart disease, respiratory problems, low birth weight babies, etc...).
The anticipated effect of the loss of many trees, in this case many ash trees, would be that in areas where the trees died the health of humans should be more negatively affected. More beetles = fewer trees = more human health problems. More beetles might even = more human death. Such are the hypotheses one can come up with when sitting on the patio of a favorite bar, but typically elegant bar hypotheses get messy when they are brought out into the clear headed air of morning and confronted with data.
Not this time. The results held. The beetle and its effects on trees were statistically associated with more than 21000 extra deaths (yes, 21000!). In counties where the emerald ash borer killed more trees, more deaths occurred. Indirectly, the emerald ash borer may be killing people (it is also possible that something else has happened in concert with the tree deaths to affect human health, but it is unclear what this might be). Other studies have been found links between the number of trees around houses and the birth weight of babies (more trees = reduced risk of having a low birth rate baby. See similar results from a separate study from Spain). These new results do not prove the positive effects of trees on human health, but they certainly do not reject the possibility. All of this brings me back to the scale insects we are studying. It gets me wondering anew about the effects of scale on health and well-being.
Recently, Emily Meineke has shown that scale insects are becoming more abundant in the warmer parts of cities.Emily’s story goes something like this. Warm cities make scale insects do better (this we know). Because scale insects sometimes kill trees, this probably leads them to kill more trees. Because the death of trees can negatively affect human health, it may even be the case that warm cities lead to more scale insects, which lead to fewer trees, which leads to more negative human health outcomes. At least we can hypothesize that these are possibilities (And then Emily can go test them. Advising can be fun.).
Image 2. Emily Meineke, scale insect whisperer. Photo by Becky Kirkland, NCSU.
But there is one more fun piece in this living puzzle. There is the question of why the scale insects don’t always kill the trees. This is an old question, one might pose it more simply as, “why is the Earth green?,” green rather than the brown that would result if all the insects ate all of the plants.
The answer is not immediately obvious. One might argue that it is in the best interest of the insects to not eat all of the plants, but insects aren’t so smart. I’ve never seen a beetle or a scale insect turn down an edible tree. They are not in the habit of leaving one for the future.
Instead, the Earth is green in large part because animals eat and kill herbivores. It is always shark week up among the leaves of the trees. Bless the predators who keep things green. In the case of the beetles, it may really be the predators. But in the case of the scale insects, more than predators it appears to be another clan of organisms— the real monsters of the backyard—parasitoids.
Parasitoids are animals that lay their eggs inside the bodies of other animals. They are among the most common life forms on Earth, far more diverse and common than birds and mammals combined (and yet they have, as one measure of how little we take note of them, never been mentioned in the Atlantic Monthly or the New Yorker). Many parasitoids specialize on scale insects; in as much, they are defenders of trees. It appears that these parasitoids are, in part, what keeps them in check and so to the extent that scale insects threaten trees, whose absence threatens our health, these parasitoids might just be saving our lives.
Image 3. A tiny defender of trees (about 2 mm), a parasitoid wasp of the genus Encryrtus. This wasp is one of the most common parasitoid wasps found living inside the female scale insects on willow oak trees. We haven't been able to figure out the species, despite having contracted a parasitoid expert to identify the wasps of scale insects. Maybe it is a new species. Maybe it is just a species so hard to ID that there is just one old guy in the Czech republic who can do so. Photo by Andrew Ernst (the contracted parasitoidologist).
But life is never easy. There is another group of organisms, the hyperparasitoids. They lay their eggs inside the eggs or bodies of parasitoids that are, in turn, inside of other organisms. If the parasitoids are the friends of humanity (and trees), “hypers” are, once again, like the scales themselves, our enemy.
Image 4. A tiny hyperparsitoid wasp (about 1 mm) of the genus Pachyneuron, nemesis of Encryrtus wasps and, indirectly, trees and humanity. Photo by Andrew Ernst.
I’ve now walked ten steps past what we know is to what might be and so I should return to the ash trees, which in their absence appear to make us sick or even dead, and simply suggest that all around us the trees and our health depend on a diversity of forms that, without meaning to, keep the tree eaters in tenuous check, a check that constantly slipping this way and that as the trees, herbivores, parasitoids and hyperparasitoids win battles (but seldom wars). Who knows, your life may have already been saved by a tiny wasp. The world is green because of such organisms, but, as the case of the emerald ash borer (who, in leaving Japan for the U.S. has escaped both predators and parasitoids) makes clear, it could be, at any moment, otherwise. Some of the things we need to do to keep the world green are relatively beyond your control as an individual (now that the emerald ash borer has arrived, your ability to check its spread is limited, for example), but one thing that definitely tips the balance of power, one thing that keeps the green team winning is planting a tree. Planting a tree brings back all the benefits that turn into costs when trees are killed. A planted tree (and its menagerie of scale insects, parasitoids and hyperparasitoids) also helps to cool hot streets and so reduces the impact of scale insects. If you plant the right tree, it will grow fruit you can eat, big branches to hang a swing on, or broad leaves beneath which you can walk in admiration, keeping an eye on the scale insects and the deadly parasitoids flying leaf to leaf, searching for the one they love, and then shoving a needle into its back and implanting a near microscopic, alien egg.
For more on these animals, see Lynn Felman's story, a bug in a bug in a tree.