Poor plant defenses promote invasive beetle's success, not lack of predators

Invasive species cost more than $100 billion a year in damages in the United States, according to the United States Department of Agriculture. While most researchers attribute their success to a lack of natural predators in their new territory, Cornell researchers offer proof for a less popular explanation: Invasive species fare so well in their new digs because their host species lack an evolutionary history with -- and defenses against -- the new invaders, making the hosts especially vulnerable to attack.

Their study, appearing in the April 11 issue of the journal Proceedings of the National Academy of Sciences, specifically examines the relationship between the viburnum leaf beetle (Pyrrhalta viburni), which originated in Europe and was first discovered in North America in 1924, and viburnums, common woody shrubs found in gardens and forests, with native species in North America, South America, Asia and Europe.

"North American viburnum species have not evolved with a leaf beetle," said Anurag Agrawal, professor of ecology and evolutionary biology, and the paper's senior author, whose postdoctoral associate Gaylord Desurmont is the lead author. Co-authors include Michael Donoghue, a Yale professor of ecology and evolutionary biology, and Wendy Clement, a lecturer in the same department.

Agrawal and his colleagues used phylogeny -- the study of evolutionary relatedness among groups of organisms (e.g., species, populations) using molecular sequencing data and morphology -- to better understand the relationships between viburnum beetles and North American viburnums.

Using phylogenetic data, the researchers found that there were at least three events, millions of years ago, where Eurasian viburnums colonized North America. During each of those events, a single colonization diversified into several North American species, but without a predatory beetle putting adaptive pressures on them.

Meanwhile, European viburnums continued to evolve with such beetles and developed a defense strategy: The female beetle lays her eggs in twigs in summer, but they don't hatch until the following spring. During that time, the twig tissue of European viburnums grows over the eggs and crushes them. "This remarkable form of plant behavior seems to be a very effective defense," said Agrawal.

The researchers studied controlled applications of beetle eggs on 16 viburnum species in field conditions in upstate New York. They found that while the European species grew tissue and crushed the eggs, this defensive response was greatly reduced in the North American species.

"In the absence of beetle pests, the North American species have evolved low levels of defense," said Agrawal. As a result, "our native viburnums are like candy for the beetles," resulting in high egg survival, extensive defoliation first by the larvae and then by adults, which can kill shrubs in two to four years, Agrawal said.

The study challenges the perspective that it is the lack of natural enemies that promotes invasive species success, partly because the viburnum beetle's natural predator, a parasitoid wasp, is not a major threat to the beetle in its native European range.

"In many other invasions, scientists have argued that enemies -- like the wasp -- do not follow the invader, thus giving the invader freedom from parasitism and the opportunity to be successful," said Agrawal. "That does not appear to be the case for the viburnum leaf beetle. It seems that host quality, or lower plant defenses, is the main driver of this pest's success," he added.

The study was funded by the National Science Foundation and Cornell University Agricultural Experiment Station federal formula funds.


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