The praying mantis, that gigantic predatory insect of Vineyard yards and gardens, is suffering from a barrage of bad press. Known officially as the Chinese mantis, Tenodera sinensis, this non-native species, the only mantis that occurs on Martha’s Vineyard, has acquired a reputation in the popular mind as a dangerous invasive.
A post of a mantis photo in the New England insects group I participate in on Facebook invariably prompts comments insisting that these insects, or their egg cases, must be destroyed on sight. Charges levelled against the species include that they kill hummingbirds and deplete populations of native pollinators, which are already beleaguered by a range of hazards. Similar sentiments can be found on some popular ecology and yard-and-garden websites.
Yet, on the other hand, the species continues to be commercially available as a pest control agent, to be released in settings ranging from home tomato patches to organic farms.
I grew up fascinated by praying mantises, considering them an exciting treat to watch. At the same time, as an ardent conservationist, I fully appreciate the risks non-native wildlife can pose to ecosystems. Given the sudden wave of conflicted messaging, I figured it was time to take another look at Tenodera.
A friend helped out with a quick search of the scientific literature on mantis ecology, which presents a nuanced picture. Yes, Chinese mantises prey on hummingbirds — but rarely, not routinely. There seems to be scant or no evidence that these insects actually harm hummingbird species at the population level. And to the extent that predation of hummingbirds occurs at hummingbird feeders, much of the blame reverts to well-intentioned humans who create unnaturally dense concentrations of hummingbird activity, which mantises, reasonably enough, exploit.
Some studies, likewise, show reduced insect biomass or diversity in sites with Chinese mantises as compared to sites without them. But again, it is not clear whether or how often the differences are great enough to matter. Moreover, many of the arthropods eaten by mantises are themselves non-native species, or ones deemed undesirable by humans. I suspect that a good portion of the prey consumed during the course of a mantis’s life may be other mantises: Immatures are cannibalistic, a strategy that ensures that aggressive, fast-growing individuals live to reproduce and that the species generally doesn’t become so abundant that it depletes its own prey resources.
My own experience, if just anecdotal, supports a moderate stance. I find a couple of adult mantises in my yard per year, on average. The species I’ve seen them preying on include insects I consider desirable — for example, red-legged grasshoppers, native bees, and, once, a mourning cloak butterfly — as well ones I’d like to see fewer of, such as the non-native cabbage white butterfly, the blowfly Lucilia sericata, and the western honeybee, which, I’m sorry to say, I view as a non-native competitor against a suite of native bee species I’m interested in encouraging. On balance, it’s hard for me to say whether Tenodera is a plus or a minus in my yard.
In native habitats on the Vineyard, I encounter Chinese mantises regularly but infrequently. I don’t know what they’re eating out there, but the low density of these predators suggests that they’re not eating enough of it to matter much, one way or the other. Based on my own observations, then, it’s hard to justify the term “invasive” to describe Chinese mantises. I’d describe them as a naturalized, non-native, generalist predator; how one views the impact of the species, in my view, depends largely on context and on the values of the observer.
None of this in any way invalidates the concept of invasiveness. Some non-native species — indeed, it’s not a short list — truly are destructive and worth trying to eliminate at every opportunity. But such determinations should come from a scientific consensus based on careful study or experimentation, not on extrapolation from casual observations or rare, dramatic instances.
The exercise of thinking about mantises reminded me of two important points. The first is the importance of critical thinking. Content producers for websites and social media accounts are motivated to produce simple, sensational messages, the kind of information that sticks in your mind and cries out to be shared or forwarded. But virality often promulgates flawed information: simplistic, distorted, or just plain wrong. (This rule, clearly, does not apply only to natural history.)
The second lesson is that value judgments about wildlife — whether a species is “beneficial” or “harmful” — tend to be human-centered, subjective, and context-dependent. Nature is incomprehensibly complex; we understand general principles and some specific pieces of the big picture, but our knowledge of how the whole shebang actually works is dwarfed by our ignorance. Simple interpretations are usually reductive or misleading.
Tenodera sinensis, then, reminds me that two crucial qualities for a good naturalist are skepticism and a double shot of humility.