Space Invaders Are Here

I wonder why people began imagining that space invaders were green. Perhaps it was because humans had already appropriated most of the other colors: black, white, khaki, red, yellow, and blue. The ancient Brits painted their naked bodies blue with woad, so they say, but that was before football fans started painting themselves in the colors of their teams. Now you can see green people rooting for Brazil, but they have a yellow stripe also.

Be that as it may, the color of the invaders I am talking about is due to chlorophyll; they are plants, not humanoids, and they do not come from outer space, but from one part of the globe to invade the space of another. Classic examples are the Nile cabbage (Pistia stratiotes), responsible for blocking navigation over long stretches of the White Nile in the southern Sudan, has turned up in the Amazon. In revenge, the South American water hyacinth (Eichornia crassipes) - with its lilac flowers and gas-filled bladders that give buoyancy to drift down rivers and foul the beaches of Rio de Janeiro after torrential rains - has invaded the Nile.

I want to report on two much more insidious green threats, with the appropriately alien names of kudzu (Pueraria montana var. lobata) and hydrilla (Hydrilla verticillata). Kudzu is a broad-leaved Asian vine, imported to the United States a century ago by immigrant farmers to grow as cheap fodder for their cattle. It has spread out of control across enormous areas of the country, overgrowing and strangling every tree, bush, plant, or even object in its path (see the images at www.jjanthony.com/kudzu ).

The damage does not stop there. Kudzu is host to a serious crop pest. The fungus that causes soybean rust (Phakopsora pachyrhizi) was first identified in Japan in 1902 and took a century to cross the Pacific, reaching South America in 2000. In 2004 Hurricane Ivan picked up spores from there and dropped them over nine southern states of the United States. Once there, they not only infested soybean fields, but also found a congenial alternate host: kudzu. As a result, no matter how much fungicide farmers spread on their soybean fields, the rust will be hiding in kudzu leaves ready to make a comeback. But in a nice twist, weed-control groups are growing the fungus that causes wheat rust, another crop pest; the fungus is rubbed into cuts in the kudzu stems to kill the plant.

The other alien threat was discovered when American bald eagles (Haliacetus leucocephalus) and coots (Fulica americana) began to flop around in circles and show brain damage - I have no idea why it wasn't called mad bird disease - and to die in numbers on some Arkansas lakes. Between 1994 and 2002, at least 100 eagles and thousands of coots died at 11 lakes between North Carolina and Texas. The cause was traced to a neurotoxin in the water. But what was the source of the poison, and why did it affect mostly those two species?

It turns out that the neurotoxin is connected to hydrilla, an invasive species of water plant. Hydrilla was imported into the United States in the 1960s from its native Africa, Australia, and parts of Asia by the aquarium industry to prettify fish tanks. Inevitably, it escaped when aquarium owners emptied their tanks into the environment, and it now totally covers the surface of many lakes in Arkansas, Florida, and other states ?ￄ￮ 10,000 acres in South Carolina alone.

The damage does not stop there. Hydrilla attracts a blue-green cyanobacterium, of a species previously unrecorded in the United States, which flourishes on its slender fronds. Cyanobacteria as a class produce deadly toxins - not, I hasten to add, out of pure malice, but as an unintended consequence of getting rid of unwanted products of their metabolism. Vegetarian water birds such as coots and many ducks feed on the plant, sicken from the toxin and die. Eagles find the sick birds easy prey, feed on them, and die in their turn - a classic food-chain disaster just like that which led to the banning of DDT.

So beware of little green alien invaders. Chop down the kudzu on your land and when you empty your fish tanks, bury or burn the plants.

Jack Woodall is director of the Nucleus for the Investigation of Emerging Infectious Diseases in the Institute of Medical Biochemistry at the Federal University of Rio de Janeiro, Brazil.