I'm rounding out The Salt's impromptu Pest Resistance Week (which started with stories about weeds and corn rootworms) with a little-known tale that may scramble your mental categories. You've heard about herbicide-resistant weeds (which farmers hate) and herbicide-resistant crops like Roundup Ready soybeans or corn (which farmers like). But here's a case — the only one I know of — in which a weed helped create a herbicide-resistant crop. The story begins in 1996, in a soybean field in Kansas. The soybeans in this field were able to tolerate a class of weedkillers known as "ALS inhibitors." This line of soybeans had been created through "mutation breeding." This technique involves exposing thousands of seeds to chemicals that cause genetic mutations. One of those mutations allowed the resulting soybean plant to withstand the herbicides. (Similar kinds of herbicide-tolerant wheat, rice, and other crops have been created using the same method.) Among the soybeans in this Kansas field, however, a few weeds also survived after the farmers sprayed their herbicide. The weeds were native sunflowers, wild relatives of the sunflowers that farmers grow as a crop. (As I reported a few months ago, sunflowers are one of a very small handful of crops that originated in our part of the world.) The farmer contacted Kassim Al-Khatib, who was then a weed expert at Kansas State University. Al-Khatib collected some of the surviving weeds from this field, did some tests on them, and confirmed that these sunflowers were indeed resistant to ALS inhibitor herbicides. A few months later, through a chance encounter at a scientific meeting, word of this discovery reached Jerry Miller, a sunflower breeder at the U.S. Department of Agriculture's Sunflower Research Unit in Fargo, N.D. "I couldn't believe it. I called Kassim right away," recalls Miller. He saw the possibility of a herbicide-tolerant commercial sunflower created through traditional breeding, avoiding controversies over genetic engineering. Miller did manage to create such a sunflower — although it took some heroic efforts to get the wild and cultivated sunflowers to exchange pollen and produce viable offspring. When Miller finally had some herbicide-tolerant offspring in hand, he broke the news to a big meeting of sunflower growers. He told the farmers that, very soon, they might be able to spray ALS inhibiting herbicides right over their sunflowers, killing a host of problematic weeds without harming their crop. "The room got completely quiet," he recalls. Today, commercial sunflowers from North Dakota to Turkey contain this genetic trait, and many sunflower growers rely heavily on ALS inhibitors to control their weeds. What's the lesson from this tale? For one thing, that it doesn't take genetic engineering to create resistance to a herbicide — whether in a weed or a crop. Probably more important, it's a reminder that our food crops are descended from plants that once grew wild, and the line that separates a despised weed from a valuable crop is sometimes a very fuzzy one. It's a boundary porous enough for genes to find their way through.