For the first time in U.K. history, an alien species (meaning one that is not native to the area) will be let loose in the kingdom to combat the growth of another species—also introduced.

Millions of sap-sucking psyllids (Aphalara itadori) will be imported into the U.K. to fight the Japanese knotweed (Fallopia japonica), an invasive species first introduced during Victorian times that has since become an ecosystem-choking threat. Alien species are considered "invasive" once they become difficult to control and squeeze out native species.

According to a report in Britain's Daily Mail, knotweed "grows through concrete and asphalt, damages buildings and walls, weakens flood defenses, and crowds out other plants." The U.K. spends $2.4 billion annually fighting the plant.

The knotweed has no natural predator in the U.K. but the psyllid, also known as jumping plant lice, is the knotweed's natural enemy in Japan, where the insect helps keep the plant in check. In fact, the psyllid eats knotweed exclusively. And according to tests conducted by the nonprofit Center for Agriculture and Biosciences International (CABI), the psyllid will not feed on any native plants. (Although it will, itself, be eaten by existing local predators.)

Knotweed has also spread throughout North America, and the U.S. Department of Agriculture classifies it as an invasive species, as well. According to the Plant Conservation Alliance, which provides some resources on how to fight the weed, "Japanese knotweed spreads quickly to form dense thickets that exclude native vegetation and greatly alter natural ecosystems. It poses a significant threat to riparian areas, where it can survive severe floods and is able to rapidly colonize scoured shores and islands. Once established, populations are extremely persistent."

Obviously, introducing one alien species to fight another sometimes has  unintended consequences, as Australia's cane toad problem readily illustrates. But CABI points out that the cane toad's introduction in 1935 to control pest beetles was carried out against the advice of scientists. And writing for the BBC, CABI chief scientist Matthew Cock points out that out of 400 different "biological agents" released to combat "weed species" in the last 110 years, "only nine produced any collateral damage, such as feeding on native species."

Meanwhile, U.K. wildlife minister, Huw Irranca-Davies, says this is the best way to rid the country of Japanese knotweed: "This project is not only groundbreaking, it offers real hope that we can redress the balance."

A similar psyllid was released in Florida a few years ago to fight melaleuca (Melaleuca quinquenervia), an invasive tree from Australia, although results so far are inconclusive. (An introduced weevil was more successful at controlling the trees.)

Psyllids aren't always beneficial to their environments. One psyllid species, a more invasive insect that somehow snuck into Florida around 1998, has transmitted huanglongbing (HLB), or citrus greening disease, throughout Florida's citrus industry.

Image: Japanese knotweed, via Wikipedia