Biologists despise alien species. I don’t mean species coming from different planets, but species that colonize areas where they don’t belong. Rats in New Zealand, earthworms in New England, wild boars on New Britain, Wasmannia ants on New Caledonia (can you see a pattern?), they all have been introduced by people to new places, sometimes accidentally, sometimes on purpose. But introduction of such species nearly always has devastating consequences to the local ecosystems. Alien species, if introduced into an area that matches the physical aspects of their home turf, instantly gain the upper hand over the natives by the simple fact that they left behind an army of predators, parasites, and diseases that over thousands or millions of years had evolved along their side, and kept their populations in check. In the new place all these restraints are suddenly gone. The locals, however, still need to deal with their nemeses, but now these limiting factors are combined with the competition from the new arrivals. Not surprisingly, it often takes only one or two generations before local species are completely overwhelmed by the aliens, and either undergo a dramatic decline, or disappear completely.

As a biologist I also hate alien species. Nothing bothers me more than the sight of African grasses in Costa Rica or Japanese crabs in Rhode Island. But at the same time, when I walk in meadows around Boston, a part of me is thrilled to see Roeseli’s katydids (Metrioptera roeselii), a species I grew up with in Europe, its native habitat. It really is a pretty katydid, and also the first one to appear in the late spring. Now, in early July, adults of this species are the dominant orthopteran inhabitants of many New England grasslands. They are common across the street from my house, they sing in open places where I walk my dogs. But their presence here is bad news for native insects. Roeseli’s katydids are predators, and because they mature faster than native species, they can easily overpower local katydids and grasshoppers. They were first noticed in the eastern North America in early 1950’s, and have since spread all along the Canadian and US East Coast. Their impact on the local fauna still needs to be fully assessed, but the very fact that this is the only katydid species that you often find in New England meadows speaks for itself.

Roeseli’s katydids have two forms that differ in the development of their wings. In Europe the dominant form is brachypterous, which means that its wings are short, and it is incapable of flying. The second, macropterous form is long-winged and flies quite well, but is rare in its native Europe. For a while it was even considered to be a separate subspecies, but later it was discovered that long-winged forms appear when the population of these katydids becomes too dense, and dispersal into new habitats is a better survival strategy. Here in New England the long-winged form is quite common. This means that this katydid’s population densities are high, and that it will continue to spread into all available habitats.

As photographic subjects Roeseli’s katydids are truly gorgeous. I took these photos of the two forms using a white background technique popularized by the photographic project Meet Your Neighbours. But I wish that these little beauties weren’t my neighbors in Boston.