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Plants Weaken Without Insects Bugging Them

During each growing season for five years, eight of the 16 experimental primrose plots (yellow flowers) were treated biweekly with an insecticide; the other eight were not. (Credit: Anurag Agrawal)
Farmers' efforts to keep their fields free of insect pests could actually be weakening their crops, according to a new study.

Plants and insects have always had a complex relationship, and over millions of years, they have changed each other in significant ways.

Even as insect pollinators encouraged plants to produce colorful flowers, many plants also developed traits, such as powerful toxins, to protect themselves from hungry bugs.

Evolutionary pressures

The new study examines how insects and plants are still co-evolving.

Anurag Agrawal studies evolution in real-time. The Cornell University professor of ecology and evolutionary biology set up experimental plots of evening primrose to observe the evolutionary pressures that insects continue to put on the wildflower.

“We took insects out of the picture for half of the plots, using an insecticide, and we then watched the evolutionary change that happened in those plant populations,” said Agrawal.
Seed predation on evening primrose fruit by Monpha brevivitella moth larva. (Credit: Marc Johnson)
Seed predation on evening primrose fruit by Monpha brevivitella moth larva. (Credit: Marc Johnson)

Agrawal and colleagues were surprised by what they saw. In their five-generation, five-year study reported in Science, evidence for evolutionary change began showing up quickly, after just two or three years.

“When we take insects out of the picture, essentially the plants were less resistant. In other words, they evolved in the direction we expected them to," Agrawal explained. "If they are not experiencing the pressure of insect pests, they were flowering earlier and producing less toxins so they become essentially less resistant.”

Losing their resistance

In other words, plants that enjoyed chemical pest protection lost some of their natural bug resistance, compared to plants that were not sprayed.

Agrawal says he and his team were also surprised to discover that the speedy adaptation of the primrose to a bug-free environment triggered other changes in the local ecology. “And that had to do with the other species that were competing with our evening primroses,” he said.

Such as dandelions, whose population doubled over the five years, reducing the number of primroses.

“The suppressing of the insects favored the dandelions,” Agrawal added.

Most plants naturally produce defensive toxins to ward off insect attacks. But in today’s high-yield farming systems, growers have strong incentives to use powerful chemical sprays to keep pest populations under control.

Natural balance

Agrawal's study suggests farmers should consider ways to strike a more natural balance between crops and bugs.

“I think that what it tells us is that insects are really important, that over the millennia plants have figured out ways to co-exist with their pests and that when we perturb that system in any way, there are likely to be a set of consequences, some of which we can predict and others we can’t,” he says.

The researchers plan to maintain their experiment as a living, learning laboratory to track evolutionary changes over the coming decades.