Public-health specialists are debating whether or not to bring back DDT to help control mosquitoes. DDT advocates should think again. The environmental damage this pesticide can cause goes beyond the decimation of hawks, eagles, fish, and frogs documented during its previous use. (It was introduced early in World War II; a federal law banned its use in 1972.) Recent research shows that the class of pesticides to which DDT belongs stunts the growth of legumes such as alfalfa and soybeans, limiting their ability to fix nitrogen and so provide their own fertilizer and improve the soil. Also, a comprehensive survey has found that residues of discontinued pesticides such as DDT continue to contaminate streams, lakes, and groundwater throughout the continental United States.
Plants need nitrogen to grow. They can't take it directly out of the air. It has to be combined with hydrogen to form ammonia – a process called nitrogen fixation. Plants ingest the ammonia and recover the nitrogen they need. Nitrogen can be fixed in chemical factories and spread on farm land. Legumes can do the trick naturally. They send out chemical "signals" that recruit nitrogen-fixing soil bacteria. These bacteria form nodules on legume roots and fertilize the soil. Hence the age-old practice of maintaining soil fertility by rotating crops to include plantings of legumes.
For the past several years, Jennifer E. Fox at the University of Oregon in Eugene has used test tube experiments to study the subtle way pesticides impede this nitrogen-fixing process. Last June she joined several colleagues to report research with real plants. Their paper, published in the Proceedings of the National Academy of Sciences, shows that the pesticides block the bacteria recruitment "signal" that legumes emit. "In essence," Dr. Fox says, "the agrichemicals are cutting the lines of communication between the host plants and symbiotic bacteria."
This has serious implications for farmers. Heavy use of commercial nitrogen fertilizers is showing diminishing returns in terms of crop yields, while fertilizer runoff contaminates streams, lakes, and even coastal ocean areas. If legumes can't do their natural fertilizing job, even more artificial fertilizer will be required.
Some of the pesticides hang around for a long time. The US Geological Survey has made a comprehensive study of pesticide residues in surface and ground water across the continental United States. It used data collected nationwide between 1992 and 2001. A summary of that study, published in Environmental Science & Technology last May, states that the "assessment shows widespread occurrence of pesticides, with concentrations in many streams at levels that may have effects on aquatic life and fish-eating wildlife." Ground water is less contaminated. Nevertheless, one or more pesticides showed up in 33 percent of wells that tap major aquifers used for water supply.
The report notes that most of the DDT class of pesticides that were phased out decades ago "continue to persist in the environment." It adds, "The frequent occurrence of pesticide mixtures, particularly in streams, implies that the combined toxicity of pesticides in aquatic ecosystems may often be greater than that of any single pesticide that is present."
Research like that of Fox and the USGS raises a significant warning: We don't know the full extent of DDT's harmful influence in our environment. We do know that, once it gets into our environment, it stays there. We need to find better ways to control mosquitoes.