Many exotic species of ambrosia beetles can be found in the U.S., and they can be very damaging to plants growing in nurseries and landscapes. Two species are of particular concern in nurseries: the granulate ambrosia beetle, Xylosandrus crassiusculus, and the black stem borer, X. germanus.
Both species are native to southeastern Asia and have established populations in the U.S. The granulate ambrosia beetle is more common and problematic in the mid-Atlantic and South, while the black stem borer is more abundant in the Midwest and Northeast.
Ambrosia beetles are often overlooked as being a primary cause of plant loss, but research and experience are proving otherwise.
“At first I was unaware of the full extent of the damage this pest can cause, but I’ve learned this is definitely a difficult pest for the nursery industry,” says Matthew Steinkopf, a container grower for Willoway Nurseries in Huron, OH. “You don’t know that you have a problem with them until it is too late. Usually when a plant dies, we blame it on water in the container, winter damage, or one of the numerous other things that could go wrong in production. When looking over the plant, it’s easy to miss the pinhole damage from the beetle or think it was an after-effect.”
A collection of researchers is conducting a multi-year project studying ambrosia beetle biology and ecology, in addition to management options for nurseries. Dr. Chris Ranger, USDA-ARS, is leading a collaboration of USDA and university entomologists from Maryland, Mississippi, North Carolina, Ohio, South Carolina, Tennessee, and Virginia. The Horticultural Research Institute (HRI) funded a portion of this research through its general grants program, with further support provided by USDA’s Floriculture and Nursery Research Initiative.
Identification of Damage
Ambrosia beetles tunnel into trees, where they create fungal gardens that serve as food for larvae and adults. The telltale sign of toothpick-like strands protruding from host plants often signals infestations. These protrusions consist of sawdust generated as the female burrows into wood. Other symptoms include sap oozing from the tunnel entrances and branch dieback.
Adults overwinter within their host tree, emerge in spring, and then search for a new host to attack. Both species are a challenge to control, partly because of their wide host range (more than 120 hosts for the granulate ambrosia beetle and 200 hosts for the black stem borer). Thin-barked deciduous trees are often targeted, although conifers can also be attacked.
Adults are difficult to control once they have burrowed into a tree. Their fungal symbiont can be problematic for the host tree, and a variety of opportunistic and pathogenic fungi have been isolated from beetle galleries.
“We identified our first attack in a block of dogwoods 10 years ago and quickly learned it can attack other species,” says Mark Shelton, Pesticide Supervisor at Willoway Nurseries. “Ambrosia beetle requires a different control strategy than other chewing insects. Timing of pesticide application to tree trunks is critical because the pesticide needs to be there by the time beetles fly. We currently use a combination of weather monitoring and ethanol traps for guidelines. Additional research to improve timing and reapplication intervals is needed.”
Peak flight activity and corresponding attacks occur during spring months. Degree days are not particularly useful for predicting initial flight, but Dr. Michael Reding of USDA-ARS has determined that spring flight activity increases dramatically after the first two-to-three consecutive days above 70°F.
This group of researchers is filling in the gaps of information to ultimately increase control strategies. A key finding to date has been that these two beetles are opportunistic and only attack living but weakened trees. While such trees may appear healthy, they emit stress-induced ethanol that is wildly attractive to ambrosia beetles. Ethanol is now used as the standard attractant in traps for monitoring purposes.
The best control methods focus on keeping trees healthy; beetles do not attack or colonize healthy trees in nursery production. Applications of insecticides can be used, but they do not completely prevent attacks from occurring on stressed trees that are emitting ethanol.
Flooding/poor drainage, frost injury, and freezing injury can induce ethanol emissions from plants and trees and have preceded large scale attacks by ambrosia beetles in nurseries. HRI-funded research by Dr. Steven Frank of North Carolina State University determined that soil moisture levels should be maintained at less than 50% to minimize ambrosia beetle attacks on flood-intolerant trees, such as dogwood.
“A small amount of stress could cause release of a pheromone within the plant that attracts the beetle,” says Steinkopf. “While one beetle should not kill the plant, it triggers the plant’s release of more stress pheromones as it feeds and attracts more of its friends.
Steinkopf says Shelton has done an impressive job being ahead of this pest and knows that an emergence is possible even in winter.
“Further research will take this beetle from being a serious problem to a mere nuisance,” Steinkopf says.
This winter in particular has raised concerns to researchers.
“Trees in many regions are currently a month ahead of schedule due to the mild 2016/2017 winter,” Ranger says. “Possible spring freezes predispose frost intolerant species to attack. Susceptible trees include eastern redbud, Japanese maple, Japanese zelkova, and Japanese snowbell.”
Researchers are now considering a new strategy in the form of behavior modification, where beetle repellents are used to protect desirable trees and ethanol-based attractants are used to lure beetles to their death. This option needs optimization but shows promise.
HRI and AmericanHort, in conjunction with other interested organizations, supported funding of this research through the USDA-ARS sponsored Floriculture and Nursery Research Initiative. For more information, contact HRI Administrator Jennifer Gray.