Spring is famously elusive here on the Vineyard, but it finally gets here, and as you set about the yard and garden cleanup that inevitably follows, keep an eye out for the egg cases, or oothecae, of praying mantises.
Papery masses about the size of a lemon wedge, these odd, foamy structures are found stuck to stems, usually close to but not on the ground. They were deposited last fall by females of what is arguably the most fearsome predatory insect to be found in our region.
What you do with your ootheca once you find it is up to you, but these days, it’s a fraught question. Among gardeners, mantises are held in high regard as all-purpose insect pest control units. Mantis egg cases are widely available commercially for placement in gardens, on the assumption that a mantis population will help control smaller sucking and chewing insects that damage crops and ornamentals.
There is surely something to this. Growing to as much as four and a half inches in length, and equipped with powerful, spiny front legs modified for grasping, a praying mantis can overpower and eat virtually any insect it encounters. A typical garden pest — say, a squash bug or bean beetle — is barely a snack for one of these predators, and over its season-long life span, a mantis surely eats scores or hundreds of prey items. Especially if you’re trying to minimize pesticide use, the notion of hosting these eating machines is an enticing one.
But among naturalists, the reputation of praying mantises is much less rosy. For one thing, a mantis found anywhere in New England is almost certainly a non-native species. The range of one native species, the Carolina mantis, approaches southern New England, and like many insects, this one may be expanding its distribution northward as the climate warms. But I’ve never heard of this species being found in Massachusetts, let alone on the Vineyard.
Of the 20 or so mantis species found in the U.S., most are southern or, especially, southwestern species. Two widely established non-native species, the European mantis and Chinese mantis, were introduced to the Eastern U.S. a little over a century ago — the former reportedly by accident, the latter deliberately as an insect control tool. On the Vineyard, it is the Chinese mantis that is the most common and possibly the only species present (anyway, it’s the only one I’ve seen).
The problem with non-native species, of course, is that they introduce an artificial dynamic onto the landscape, a new ecological force that can knock things out of kilter for lack of anything to control it. From the naturalist’s perspective, then, the mantis’ dining habits are an issue.
Mantises don’t confine their endeavors to agricultural pests, but rather take anything they can catch, including insects usually deemed desirable by humans. In my experience, honeybees are a frequent victim of mantises; so are butterflies. Indeed, mantises often hunt by staking out flowers and then grabbing whatever lands on them. This habit virtually ensures that pollinators, generally considered desirable insects, figure prominently on the mantis menu.
The size and strength of an adult Chinese mantis allows it to take prey of astonishing size. While I can’t imagine it happens often, there are documented instances of this species catching and eating hummingbirds. So while there’s no question that mantises eat agricultural pests, they also eat unknown numbers of species that you probably like having around.
However, another behavioral trait of this species limits its impacts, whether they are unwanted or desirable.
Each foamy ootheca contains several dozen eggs, which hatch at about the same time. The tiny mantises prey freely on each other. Between this and other sources of mortality, mantis populations never seem to get very large. And on the Vineyard at least, they rarely turn up in natural habitat.
As foreign as cannibalism may be to human ethical standards, it’s probably more the rule than the exception among predatory insects that lay their eggs in concentrations. Food is food, after all, and from an evolutionary perspective, sibling cannibalism might make it more likely that the few offspring reaching reproductive age are the healthiest and most vigorous ones.
If you leave your mantis egg cases alone, they’ll hatch once spring has settled in, and after the resulting free-for-all, a few will mature by fall, mate, and produce another set of oothecae. Oddly, I rarely encounter developing mantises: Young ones apparently stay well hidden, and it is only when full-size that they hunt from exposed positions.
My own view, then, is that mantises, while non-native, are here to stay, and have taken up a position as reasonably well-behaved predators in an ecosystem that is already full of predators. In fact, I think it’s kind of cool to have such large and fearsome insects stalking our yard. Unlike some of my friends, I don’t destroy mantis egg cases or execute adults. But I do see this as an insect we should be very cautious about encouraging.