The Pollinator/Herbivore Waltz

A bumblee on a Brassica rapa planf

Florian Schiestl

Insects and plants have long interacted in an evolutionary dance that has driven diversity. Over time, plants have evolved flower traits—variations in color, shape, size, and chemistry—that allure pollinators. Plants have also evolved defense mechanisms—toxins, tastes, and odors—that repel herbivores. Plant adaptations for pollination and for herbivory have been studied in isolation, but a new study has investigated these selective pressures in concert.

Botanists Sergio Ramos and Florian Schiestl of the University of Zurich, Switzerland used a turnip relative, Brassica rapa, to investigate how plant traits diverged in response to pollination and herbivory. The plants were split into four groups and each was exposed to a different treatment over the course of six generations: bee pollination with or without caterpillar herbivory, or hand-pollination with or without herbivory.

The researchers studied the traits of the experimentally exposed plants, plus two subsequent generations of insect-free offspring. Petal width and concentrations of volatile scenting chemicals were taken as measures of attractiveness to pollinators. Defensive traits were measured by glucosinolate concentrations—the chemical compounds that give plants their pungency and deter herbivores. Finally, the researchers tracked changes in mating systems by measuring the degree to which plants could and did pollinate themselves.

The offspring of bee-pollinated plants without caterpillars had the largest and most fragrant flowers, and they were the most attractive to bumblebees. In contrast, the offspring of bee-pollinated plants with caterpillars showed no detectable increase in floral size or fragrance compared to the offspring of hand-pollinated plants. Herbivore-deterring glucosinates increased following caterpillar exposure “because the better defended the plant is chemically, the less the larvae will eat and destroy the plants,” said Schiestl. He and Ramos speculated that plants faced a trade-off between spending resources on attraction versus defense.

The interaction of insect pollination and herbivory was also associated with changes in the plant’s mating system. B. rapa is normally self-incompatible, so pollen landing on its own stigma will not lead to fertilization. The offspring of plants exposed to both bees and caterpillars showed the greatest increase in self-compatibility and produced the most seeds without pollinators, suggesting that such plants were compensating for reduced pollinator presence by evolving increased self-pollination. Pistil length also decreased over the generations of caterpillar-infested plants, making self-pollination easier.

This study highlights the complexity of the evolutionary dance among plants, pollinators, and herbivores. (Science)