Alan Dove writes about his grandfather's involvement in the history of DDT (my emphasis):
DDT owes its notoriety to American applied research during World War II. At the start of the war, chemists had known how to synthesize the compound for decades, and a few knew of its insecticidal properties, but nobody had tested it rigorously or turned it into a practical product. It seems unlikely that anybody would have, if it hadn't fallen into the hands of an obscure group of entomologists at the US Department of Agriculture in 1942.
The USDA scientists had recently been drafted into a critical public health project: preventing louse-borne typhus in troops. They worked methodically, testing every chemical they could find to see what would kill lice. Among thousands of other samples, they received a waxy, granular substance from the Geigy corporation in Switzerland. The manufacturer called it Gesarol.
Gesarol did kill lice, and every other insect in the lab, but a crumbly wax doesn't work well as a delousing treatment, so the USDA crew did the unglamorous but essential job of reformulating it. By 1943, they were producing large quantities of several formulations, including powders and sprays, and they were referring to Gesarol by its generic name, abbreviated DDT.
The USDA scientists promoted DDT only for a few circumscribed uses, including delousing and malaria control. Does this sound familiar? Indeed, the entomologist leading the effort specifically cautioned against spraying the stuff willy-nilly outdoors, arguing as early as 1944 that DDT was "definitely poisonous," and that its environmental consequences might be bad. I happen to have an archive of his papers sitting in my closet, as he was my grandfather.
That message became unfashionable after the war, when the USDA reverted to its traditional mission of boosting American agricultural yields. Walter E. Dove left the agency, and the USDA started promoting widespread spraying of DDT on crops.
(Via Ed Darrell.)