Some plants in the cabbage and mustard family pay a dramatic price to ward off hungry caterpillars: they kill patches of their own leaves on which butterflies have laid eggs. Without a living anchor, the eggs will shrink and die. The ability of these plants to kill eggs has been documented since at least the 1980s, but a new study shows that they are only found in a few closely related plants in this family – and they are only triggered by certain species of butterflies.
Nina Fatouros from Wageningen University in the Netherlands and her colleagues examined 31 plant species in the target family. First, they dabbed the leaves of the plants with liquid exposed to egg material from a species of butterfly known to lay eggs on it. Four closely related plant species reliably killed the treated leaf spots. Further testing confirmed that the species with the strongest response only reacted strongly when the egg material came from a group of butterflies. Pieris, which lays eggs on these plants in the wild. This is “clear evidence” that certain butterfly species may have stimulated the development of the defense against necrosis, says Fatouros. The researchers also tracked eggs laid by wild butterflies to confirm that the defense mechanism is drying them out or shedding them. The work has been detailed in New phytologist.
“It is very unlikely that you would find this by accident,” says Jurriaan Ton, a molecular plant biologist at the University of Sheffield, who was not involved in the study. He adds that the relationship between the plants and their exaggerated response to these butterflies suggests that a heated evolutionary “arms race” has taken place between the plants and insects.
“To my knowledge, this is the first study that really examined the appearance of this trait within a particular plant family,” says ecologist Julia Koricheva of Royal Holloway, University of London, who was also not involved in the work.
Future research could examine how the trait has evolved recently, says Fatouros. She notes that arms races rarely end – and there is evidence that the butterflies may fight back. Some prefer to lay their eggs in tightly clustered groups, which makes them less susceptible to the plants’ strategy.