This picture is of an Edith’s checkerspot butterfly laying her eggs on some blue-eyed Mary, the plant usually eaten by its caterpillars. This week’s Nature, however, describes the fate of a population of the insect in Nevada that evolved to prefer ribwort plantain, a weed introduced from Europe that is common in American cattle pastures. This particular group of checkerspots was studied for 25 years by Michael Singer and Camille Parmasan of the University of Texas at Austin. The pair watched the insects evolve gradually over the decades until they would lay their eggs only on the alien invader, which provided more abundant feeding for their larvae than blue-eyed Mary did. Dr Singer and Dr Parmasan then saw the whole population vanish within a year, when a change of land use caused ribwort to disappear, even though blue-eyed Mary was still available. The butterflies had been trapped in an evolutionary cul-de-sac.