15 October 2015
Monarch butterflies are notoriously choosy when picking out a spot to lay their eggs. Of all the plants in the world, only milkweeds pass muster. But monarchs sick with a debilitating protozoan parasite are pickier still. Butterflies infected with Ophryocystis elektroscirrha (OE) flit from plant to plant, searching for those with enough cardenolides—a toxic steroid—to reduce infection in their offspring. Now, researchers have found that how much cardenolide a plant stores may depend on an unlikely ally: soil fungi.