In an effort to receive advanced warning of destructive pests that could wreak havoc on native plantings, researchers from Europe, the United States and China are growing "sentinel trees" in strategic locations around the world.
"Sentinel nurseries represent one potential mechanism to address the current lack of knowledge about pests in the countries from where live plants are shipped and the threats they represent to native flora and crops in importing countries," researchers from universities in Italy, China and Switzerland said in a study published in Plos One.
As global trade intensifies, the risk for accidental import and exposure to new invasive pests is a constant worry for entomologists and arborists. Cases past and present illustrate the desperate need for new tactics to prevent future losses.
The emerald ash borer, introduced into the U.S. from its native range of northeastern Asia, has killed off hundreds of millions of ash trees throughout the country at an estimated cost of nearly $11 billion. The American chestnut, estimated to have numbered between 3-4 billion trees at the turn of the 20th century, is today represented by only a few hundred specimens due to the accidental import of a destructive bark fungus. The spotted lanternfly, first discovered in the U.S. in 2014 and free from natural predators, continues to feed unchecked on 70 plant species, including grape vines, fruit trees, ornamental trees and woody trees.
A leafy canary in the coal mine
Schematic representation of sentinel planting types in an exporting country, identified by the origin of the planted trees. (Photo: Eschen, R., O’Hanlon, R., Santini, A. et al. Safeguarding global plant health: the rise of sentinels, CC by 4.0)
According to Gabriel Popkin of ScienceMag, scientists have established sentinel groves comprised of European and North American trees in China. Plans are also underway in Europe on a $5.5 million initiative that would fund the collaborative plantings of additional early-warning species in North America, Asia and South Africa. A grove of Asian trees has also been promised for later this year in the U.S.
In addition to gauging the impact of foreign pests on native trees, sentinel nurseries have also helped researchers uncover pests that might arrive with commonly traded species. A 2018 study of two sentinel nurseries in China containing five popular –– and regularly exported –– ornamental plants discovered that 90% of the 105 insects recorded on the species "had not been found in a previous literature survey of insect pests of the five plants."
Eyes on the forest
In addition to international efforts, local initiatives are also underway to monitor native species for any unusual changes or pest stresses. Michigan State University Extension's "Eyes on the Forest" program trains volunteers to monitor "adopted" sentinel trees across the state. Should the characteristics or health of these sentinels take a turn, regular observations would help inform a quick response.
"Hopefully, with a strong enough network of sentinel trees, we can achieve early detection of new tree pests and work to eliminate them before they can become established," the group states.