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March/April 1999 issue	In this issue Table of Contents Editor’s Notebook Inside Story Feature: Hunting Poachers Whale-eating bears

Of all God’s creations, few are less appealing to humans than parasites. Like a doctor offering a second opinion to an overweight patient ("all right, you’re ugly too"), we pile on the invective: repulsive, revolting, harmful, useless. You know — parasitic. Yet get past the initial disgust and these lowly creatures begin to exert a perverse fascination. They even demand a grudging respect for the ingenuity with which they conspire against their hosts.

Take, for example, a humble worm known as the lancet fluke. Like many parasites, it passes through several stages in the course of its life, each of which requires a different host. At one juncture, the fluke must pass directly from the body of an ant into a sheep — a transition that would be a lot simpler if the two species ordinarily met. But ants generally stay near the ground, while sheep browse on the tips of vegetation. The fluke overcomes this difficulty by invading the ant in the form of larvae, which infest the nervous system. Under this foreign control, the ant suddenly and unaccountably decides to climb a tall blade of grass and lock its jaws shut on the tip. Unable to escape the lancet fluke’s command, the ant has no choice but to await its ovine nemesis.

This bizarre plot was unravelled almost 40 years ago by Wilhelm Hohorst and colleagues in Frankfurt, Germany. Parasites, it seems, were not just unwelcome passengers in their hosts’ bodies; they were a potent force with an unexpected impact on ecological networks. Who had ever heard of a sheep that ingested ants?

Since then, dozens of quirky variations on this alien-invasion theme have been documented by parasitologists. In the early 1970s, for example, William Bethel and John Holmes of the University of Alberta studied the behaviour of small aquatic crustaceans called amphipods. Healthy amphipods, they discovered, prefer to stay near the bottom of ponds, where they are relatively safe from predators. But if they become infected with Polymorphus paradoxus, a parasitic worm, they begin to seek the bright lights of the surface. This increases their chances of being eaten by surface-feeders like mallards, muskrats and beavers — the very species within which P. paradoxus must complete its life cycle. Amphipods infested with a different worm, P. marilis, are drawn only part way up, into the realm of that parasite’s ultimate hosts, the diving ducks. And so it goes.

Even plants can be subject to hostile takeovers. Blueberries (both wild and cultivated) are afflicted by a disease called "mummy berry," which is caused by a fungus. The first symptoms appear in spring, when tender young leaves are infected by wind-borne spores. The foliage droops and turns brown and the leaves become coated with spores.

As unappealing as they are to human eyes, the leaves now attract insects, which crawl over the discoloured surfaces and lick them intently. Under the influence of the disease, the foliage has suddenly begun to produce sugars. What’s more, the leaves have also developed ultraviolet markings — visible to insects — which mimic the "nectar-guides" on blueberry blossoms.

When the insects bumble off to visit the plant’s real flowers, they deposit mummy-berry spores (picked up from the leaves) on the stigmas. As a result of this infection, the blueberry plant produces shrivelled, infertile fruit, within which the fungus survives the winter, ready to resume its dirty tricks next spring.

Only occasionally are victims able to shift the balance of power and defend their own interests. For example, bumblebees parasitized by a conopid fly often choose to spend nights out in the cold, rather than seek shelter within the communal nest. By letting their bodies cool, they slow the parasite’s growth and may prevent it from maturing before their own, natural deaths. Recent research by Murdoch McAllister and colleagues at Simon Fraser University suggests that pea aphids infected by wasps commit suicide to protect close relatives from coming in contact with the parasite. Death by desiccation — dropping to the parched earth — seems to be their preferred means of self-sacrifice.

Parasites are opportunists, with a cool, criminal disregard for the integrity of their victims. "What’s mine is mine and what’s yours is mine" might be their motto. But far from being an ecological offence, this is the way of a world in which all lives interpenetrate each other. Working from the inside to complicate and subvert natural relationships, parasites are clever hackers in the worldwide web of life.

Candace Savage is a Saskatoon-based writer and author of 18 books on wildlife, environmental issues and other subjects. Her latest is a history of beauty pageants, entitled Beauty Queens.

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