Deadly Larva, Deadly Snails

Four and twenty tailors went to kill a snail . . .

Run, tailors, run, or she'll kill you all e'n now.

--Old Nursery Rhyme

The sluggish snail has turned out to be far more deadly than the anonymous poet knew. For years, scientists have been busier than the old song's tailors, trying to kill certain species that carry human and animal diseases, notably the microscopic parasite that causes schistosomiasis, an ancient and virtually incurable ailment common in many warm countries. Though a selective chemical capable of destroying the guilty snails is under development and shows high promise (TIME, July 5), Cornell Entomologist Clifford O. Berg thinks that a more practical approach would be to encourage the snail's natural enemies.

Most promising of the snail hunters is one of the world's messiest killers, a blind, translucent larva that is the aquatic young of a sciomyzid marsh fly. No species is more than three-quarters of an inch long, but they tear into the snails that are their natural prey with fierce abandon, ripping their flesh to ribbons with sharp mouth hooks.

Tundra Killer. Berg discovered the sciomyzid's taste for snails quite by accident. While doing research in Alaska on mosquito control, he occasionally dipped sciomyzid larvae from tundra pools. One afternoon he happened to put a single larva into a dish along with five snails. Half an hour later, he had a chilling surprise: "I saw the larva with its head thrust into the opening of a snail shell, its mouth hooks working. When I came back the next morning, the larva had pulled out, but half the soft parts of the snail were gone."

Berg searched scientific literature for earlier reports of such attacks, but apparently no other entomologist had recognized sciomyzid larvae as snail killers. Working with small grants from the National Institutes of Health and later the National Science Foundation, Berg and a handful of graduate students set out to make their own confirmation.

"So far," he says, "we have reared 105 species, and every one we have found is either a snail killer or a slug killer." Some of the larvae will kill only one particular species of snail; others eagerly attack almost any snail up to 19 times their own weight. Many scio-myzids are death for two of the three types of snails that carry schistosomiasis. The third snail host, prevalent in Formosa and the Philippines, has a limestone trap door inside its shell that mangles the attacking larva.

No Taste for Babies. Though there are no immediate plans to use the scio-myzids in large-scale attacks on snail populations, Berg did send five dozen larvae by air mail to Hawaii. There they are being bred to combat a snail-borne liver-fluke disease that has been plaguing the Hawaiian cattle industry.

But what will happen once the job is done? There is always some danger that an insect introduced to kill a pest may attack friendly insects or even humans. Berg does not believe that the marsh fly--either in its hungry larval stage or as a weak-winged grey or brown adult --poses any threat at all. Unlike the disease-spreading housefly, the sciomyzid avoids human company; its larva is hooked on snails to the exclusion of other food supplies. Says Berg: "Anything which is so highly specialized is not going to change its eating habits and start attacking babies."

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