Without any pests, plants relax quickly

CORNELL (US) — When researchers removed bugs, a species of native wildflower let its guard down after just three to four generations.

The new five-year study provides rare real-time data that demonstrates key predictions by Charles Darwin on the importance of ecology and natural selection in shaping a species’ evolution.

When insect pests were removed from experimental fields of evening primrose, a native wildflower, the plants evolved—in just three to four generations—to relax their defenses against pests and to better compete for space and other resources against dandelions, even when the weeds unexpectedly thrived without insects.

Without insects, the number of dandelions doubled, but the evening primrose plants were able to compete. (Credit: David Edwards/Flickr)


“What was most surprising to us was how rapidly the plants evolved, and we did not expect the evolution of competitive ability in the plants,” says Anurag Agrawal, a Cornell University professor of ecology and evolutionary biology and lead author of the study published Friday in the journal Science.

In the study, which began in 2007, Agrawal, Amy Hastings, a research support specialist working in Agrawal’s lab, and colleagues set up 16 identical plots that contained the same relative numbers of 18 unique genotypes of native evening primrose.

During each growing season, half the plots were treated biweekly with an insecticide; the other half of the plots were not.

The offspring of evening primrose are mostly clones of the parent due to self-pollination and other factors in primrose reproduction. This genetic stability allowed the researchers to track the frequencies of the 18 different genotypes in the plots over five generations.

Between 2007 and 2011, researchers found that the number of evening primrose plants decreased substantially in plots without insects when compared with control plots, while dandelions thrived and outcompeted many of the genotypes.

“The plots where we took insects out had double the dandelions,” Agrawal says.

While the genetic structure of the evening primrose populations changed over time, the researchers found that only nine of the original 18 genotypes in significant numbers remained in both insect-suppressed plots and the controls.

Of the genotypes that remained in the plots without insects, the researchers found more plants with relaxed defenses. By 2010 and even more in 2011, there was a shift toward plants that flowered earlier. When insects are present, later-flowering plants do better due to the timing of insect development, where larvae tend to eat the fruits of early flowering plants.

Also, over time, there was a shift toward primroses with lower amounts of insect-deterring chemicals in the fruits, suggesting that in the wild, selection had been strongest for defense against flower- and fruit-eating insects.

Finally, without insects, primroses were better able to compete against dandelions—primrose genotypes that led to larger plants were favored when compared to the controls.

“The effects of insect pests can have immediate consequences for plant health and also sweeping consequences for evolution of entire communities,” Agrawal says.

Agrawal and his colleagues intend to maintain the experimental evolution plots as a long-term living laboratory to decipher the complexities of ecology and evolution.

Co-authors included researchers from the University of Toronto, the University of Montana, and the University of Turku in Finland.

The study was primarily funded by the National Science Foundation.

Source: Cornell University