Walnut twig beetle's origin and spread revealed in genetic studies

Even though the walnut twig beetle (WTB) is likely native to Arizona, California, and New Mexico, it has become an invasive pest to economically and ecologically important walnut trees throughout much of the Western and into the Eastern United States. Through genetic testing, researchers from the Pacific Southwest Research Station (PSW) and partners from the University of California, Riverside and U.S. Forest Service Forest Health Protection have characterized the beetle's geographic distribution and range expansion. Results were recently published in the journal PLoS ONE.

These researchers examined genetic variation in WTB samples from 62 counties in 13 states representing the beetle's current range and concluded that WTB descends from two geographically distinct genetic lineages in the Southwest. The most genetic diversity within these two lineages occurred in WTB populations in Western New Mexico and the Madrean Sky Island region of Arizona and New Mexico, where WTB is believed to be native. In areas where WTB has become invasive, researchers found evidence of hybridization of the two genetic lineages. Hybridization did not occur in areas where WTB is believed to be native.

"The discovery of two genetic lineages for walnut twig beetle is exciting because it implies that this pest may in fact be two species, one that has invaded many parts of the United States and one that is still largely resident to the foothill and mountain canyons of the desert Southwest," says Steven Seybold, PSW research entomologist.

WTB carries the Geosmithia morbida fungus and causes damage to walnut trees when it excavates under bark to feed and lay eggs, and, in the process, inoculates the tree with the fungus. The result is Thousand Cankers Disease (TCD), so named because of the numerous cankers that form and cut off nutrient flow. Infected trees experience dieback of branches and may be killed in as few as three years. TCD is a threat to walnut in most of the western United States and, since 2010, has been detected in seven eastern states.

TCD and its spread may be due to the beetle's relatively recent range expansion to areas where walnut species (particularly eastern black walnut) did not co-evolve with WTB or the fungus. Researchers are concerned about the strong possibility for new associations among walnut trees and other twig beetles and fungi that exist in walnut's range that could result in similar diseases devastating to walnut.

Walnut trees and fruit are an incredibly valuable resource worldwide. "California produces over $1 billion dollars in edible nuts each year, and the value of the eastern black walnut growing stock east of the Mississippi is in the neighborhood of $500 billion. The five species of native black walnut trees and butternut trees are also important ecological components of riparian corridors across the United States where they provide a key food source for wildlife," explains Seybold.

Researchers conducted a similar study on the genetic variation of G. morbida and published a companion paper in 2014. The findings from this research can inform policies to prevent the introduction of agricultural and forest pests during global trade.