In the 1500s, when men on sailing ships were casually spreading crop plants from one continent to the next, maize came to Africa. Fortunately, many of the maize pests stayed behind, in the Americas. But slowly, trade and travel are re-uniting maize with its pests. A caterpillar called the fall armyworm is the latest American pest to reach Africa, and it has spread across the continent, threatening one of Africa’s staple food crops.