Epizootics to Order

Killing pests with poison can be expensive and dangerous. A better way to get rid of them is to let some other pest do the job. Last week Drs. Edward A. Steinhaus and Clarence G. Thompson of the University of California told how they have waged successful biological warfare against green caterpillars (Colias philodice eurytheme) that chew up alfalfa.

Wilting Caterpillar. After a few weeks of chewing alfalfa, green caterpillars usually get a virus disease (wilt) that turns their insides to mush. The trouble with this epizootic is that, before the virus gets them, the caterpillars have time to eat up a lot of alfalfa.

Steinhaus & Thompson reasoned that they could speed up the disease. From wilted caterpillars they made an infectious brew and sprayed it on infested plots of alfalfa. For a few days the caterpillars increased as rapidly as they did on untreated plots. But suddenly the epizootic started. Caterpillars stopped eating and wilted. After a few more days the caterpillar population in the treated plots fell sharply, while it went right on rising in the untreated plots.

Dr. Steinhaus says that the virus spray still needs more testing. But it is much cheaper than chemical sprays, and is not poisonous to anything but the alfalfa caterpillar.

Ravenous Snail. A much more serious pest, the giant snail Achatina fulica, which is chewing up many Pacific islands and threatening California, may also soon be attacked by biological methods. These mollusks have shells five inches long, and slimy bodies that are as long as nine inches. Native to East Africa, they have worked their way half around the world in the last 100 years. Before World War II, the Japanese, who consider achatinas a delicacy, took them to many Pacific islands and raised them like chickens. When the Japanese were driven out, their snails multiplied undisturbed.

Like many snails, achatinas have a biologically efficient system of reproduction. There are no single-purpose sexes. While mating, each snail acts as a male and fertilizes his partner. Then both become mothers, laying up to 300 eggs that look like golden peas and bounce like ping-pong balls. The baby snails grow rapidly, eating almost everything in sight: rubber seedlings, coconut fronds, truck gardens, sugar cane. Half a dozen of them can strip a tree. On Saipan they got so thick that their slimy, crushed bodies made autos skid off the roads. Poison is little help: if only a few survive, their double-acting multiplication soon makes them a pest again.

With great effort, Hawaii has managed to fight off the snails. Their latest appearance was in California, where they probably arrived in military vehicles. So far, they are not established securely in the U.S., but agricultural experts fear they may dig in unless some natural enemy is found to keep them in check.

At the request of the Navy, which is responsible for many Pacific islands, the National Research Council sent Mollusk Expert Dr. Francis X. Williams to Africa to look for the big snail's enemies. In Kenya he found small, fierce, carnivorous snails boring into big achatinas with sharp, file-like teeth. He also found snail-eating beetles, and took both finds back to Hawaii, where they are still penned up carefully for observation. Some biologists fear that if the beetles and small snails exterminate the giant snails, they might look around for other food and become pests themselves.

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