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The Good News about DDT

Illustrations by RACHEL TAYLOR

Mother Nature is cleaning it up, but we need to do our part.

Twenty-five years ago the United States banned DDT. By then, populations of peregrine falcons, eagles, songbirds, brown pelicans, California sea lions and many fishes had dwindled to dangerously low numbers because of DDT poisoning. Once use of DDT stopped, Nature began to recover. While the DDT crisis of the 1970s may be over, its story is a lesson of the danger of chemical pesticides and a model of the resilience of Nature.

In 1948, when Paul Müller received the Nobel prize in medicine for his discovery of the insecticidal property of DDT, no one could have predicted how the story of DDT was to play out. The World Health Organization used DDT for the noble goal of eradicate malaria from the face of the earth. By 1958, massive spraying of DDT was underway in Africa, Asia and South America to control the malaria transmitting mosquito, Anopheles. Their goal was approached, but never realized and finally abandoned. DDT was highly effective against the mosquito for a while, until the insects became resistant to its toxic effects. As soon as DDT spraying ended, the number of malaria cases each year quickly returned to pre-DDT levels in most tropical countries. However, disease control was only one use for DDT. As the Green Revolution reached full swing, increasingly larger farms throughout the world relied on DDT to ensure generous harvests.

DDT and the Green Revolution

Farmers used more and more DDT throughout the 1950s and 60s, as mono-culture farming became the norm. The World Bank encouraged third-world countries to replace their local subsistence farming with large-scale growing of a cash crop, such as rice, coffee or corn. The old farming system grew many different kinds of plants in small plots, supporting an array of insects that fed on each one. Under that system, the small plots, variety of plants and genetic diversity within the plants kept insect populations in check. The new farming method encouraged farmers to plant vast areas with a single crop, often a hybrid bred for a specific trait. A field of hybrid plants lack genetic diversity, so if a bug finds one plant tasty, it finds them all tasty. On larger farms, the bugs have a nearly endless dinner table and no other guests to compete with for space or food. To keep insects in check in large-scale farming, where an entire crop could be wiped out by hungry insects, farmers needed DDT.

DDT controlled agricultural pests, but it wiped out beneficial insects too. It also killed fishes, birds and mammals whose prey was contaminated with the pesticide. Fish ate tainted worms, birds and sea lions ate tainted fish. Through each step up the food chain, DDT concentrations increased. The levels became high enough to cause severe eggshell thinning in birds and premature birthing in mammals. Thirty-three years after Müller's discovery, the United States officially recognized DDT as more an enemy than a friend and banned its use.

DDT is no longer used in the U.S., but it degrades slowly and thus remains in river and ocean sediments. South American, Asian and African countries continue to use DDT, contributing to global contamination. Scientists find DDT in soil, water, and even in ice from the arctic to Antarctica. A blanket of DDT covers the earth, punctuated here and there with highly tainted areas where it was, or is still, in heavy use.

DDT and California Sea Lions

California's waters between Los Angeles and the Channel Islands are one of the world's most DDT-tainted areas. Between 1949 and 1970, Montrose Chemical Corporation, at the time the world's largest manufacturer of DDT, dumped thousands of tons of DDT into the ocean where California sea lions mate and birth their pups.

Burney Le Boeuf, a marine biologist at the University of California at Santa Cruz, witnessed the effects of DDT on sea lions. He regularly toured the sea lion breeding grounds and in the late 1960s started seeing a strange and very disturbing trend. "We had noticed when going to sea lion rookeries in Mexico and California that there were scores, in some places hundreds of aborted fetuses that were strewn on the beaches, half-finished and dead," Le Boeuf recalled.

Le Boeuf and his colleagues counted all the pups born that year and fully half of them were dead. He suspected pesticides might be involved, partly because he knew Montrose dumped DDT right where the sea lions hunted for food. There was already correlation between DDT and harm to wildlife. The metabolized form of DDT, called DDE, was known to have killed house-cats in Bolivia's campaign against malaria. Mink that were fed fish from DDE-tainted Lake Michigan were unable to reproduce. So Le Boeuf took some sea lion blubber samples back to the lab for analysis.

What he found amazes him to this day. "The values of DDE were extraordinarily high, higher than anything we were familiar with in other species," said Le Boeuf. Each animal collected an incredible quarter-pound of DDE in their fatty tissues during the short time they spent in the Channel Islands each year. Le Boeuf only took samples from male animals, who mate at the Channel Islands then head north again to cleaner waters at the end of summer.

Female sea lions winter further south, in areas more contaminated than northern waters, so Le Boeuf thought they might had been getting DDE-spiked fish the year around. Le Boeuf speculated that the amount of DDE the female sea lions were ingesting could be responsible for the huge number of aborted pups he saw. Biologist Robert Delong at the National Marine Mammal Laboratory in Seattle, Washington answered that question in 1973 when he compared DDT levels in female sea lions who aborted their pups with those that carried their pups to full term. He also took tissue samples from dead and living premature pups and normal, healthy pups. His findings confirmed what Le Boeuf and others suspected. The females who aborted their pups contained at least eight times more DDT than the ones who had full-term pups. Delong drew the first direct connection between levels of DDT and premature pupping in sea lions.

DDT was, and still is, an attractive insecticide because it's cheap and effective. It's also persistent. It kills bugs that land on sprayed walls for months or years after application, and there's no immediate toxic effects on humans. Its persistence is exactly what worries many scientists. Estimates of its half life--the time it takes half of it to degrade--vary from 5 to 20 years depending on the environmental conditions. It still shows up in California's oceans and streams whenever sediments are churned up. The 1995 floods in California washed DDT contaminated sediments down to Elkhorn Slough at Moss Landing. Several hundred Caspian tern chicks died as a result. Le Boeuf said the turbulent El Nino current churns up sediment in the Channel Islands, dosing everything from benthic worms to bald eagles. These sudden, large doses of DDT also make the fish inedible for humans.

LeBoeuf and other scientists knew of the occasional DDT spikes in coastal areas of California, but they had no measure of the normal levels of contamination in marine animals. A student of Le Boeuf's, Patty Lieberg-Clark, decided to find out. From 1988 to 1992, Lieberg-Clark was involved in the Marine Mammal Stranding Network, which deals with dead marine mammals that wash up on shore. Through the network, she had access to the tissues of seals and sea lions from all over the California coast. Working with biologist Wally Jarman at UC Santa Cruz, and supported by Le Boeuf, she began to measure pesticide levels in the animal tissues she collected.

Lieberg-Clark found a huge drop in DDT levels from those recorded by Le Boeuf in 1970. "I didn't think anything interesting was going to come out of her study," Jarman recalls. "But when I looked at it, I realized it was unbelievable. The levels had dropped so much I questioned whether the data was right." DDT levels had declined in many animals, but to only about 10 percent of previous levels. The amount of DDT in the sea lions Lieberg-Clark analyzed had fallen to less than 1 percent of that measured in 1970; from 760 parts per million to 5.2 parts per million. "I'd never seen anything like it," Jarman said.

Jarman cautions that the amount of DDT they found in sea lions is not low, just lower than what Le Boeuf measured years ago. "There's a global contamination of DDE. There was so much DDT produced and used, and still used in Asian, African and some Latin American countries that there's a background level," Jarman said, "I'm not saying it's high or harmful, it's just there, but we definitely do not see a population effect anymore for sea lions."

Lieberg-Clark describes DDT as the poster child of dangerous compounds. "There are certainly a lot of things more dangerous than DDT," she said. The incredible recovery of the animals that were most affected by DDT is a clear sign that what we do makes a difference. If we continue to use chemical pesticides, we could see another crisis in wildlife and even human health. The harm is not so clear as in the heyday of DDT when whole species were being decimated, though many of compounds we still use in agriculture are known to reduce fertility and impair the immune system. The DDT story is proof that we can reverse some of the damage if we choose.

Sea lions are just one example of recovery. Peregrine falcons returned from the brink of extinction. At one time, only two breeding pairs lived in California. Their populations today are growing, although they still suffer some reproductive problems due to eggshell thinning. Brown pelicans, which were also nearly extinct, returned in number and also expanded their range. Bald eagles, though, are still unable to reproduce in the Channel Islands because of DDE poisoning. These are just a few representative species, the recovery of others varies but is generally good.

Stories of recovery and success blend with tales of poisoning and death in the ongoing struggle with pesticide pollution. Farmers use new kinds of chemicals in greater amounts than ever before. The potential for disaster on a larger scale than the DDT crisis looms in our future, Lieberg-Clark warns. She, Jarman, Le Boeuf and other scientists who saw the effects of chemical pesticides hope that we aren't making the same mistake again. We have come a long way in gaining the wisdom and the power to live in accord with Nature. Perhaps the phenomenal success story of California's sea lions will remind us of the delicate balance between health and sickness and lend us pause and caution in our choices.