Research sheds light on ambrosia beetles

WASHINGTON, DC and COLUMBUS, OH — Research funded in part by the Horticultural Research Institute has shed light on ambrosia beetle biology and ecology and management options for growers.

Ambrosia beetles are not new insects to the U.S.; many native species exist here that are considered minor pest problems. Many exotic species can be found here too, 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 US (29 states for the granulate ambrosia beetle and 32 states for the black stem borer). 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.

Figure 1A-B. Two highly destructive ambrosia beetles in nurseries are the (A) black stem borer Xylosandrus germanus and (B) granulate ambrosia beetle Xylosandrus crassiusculus.

Ambrosia beetles are often overlooked as being a primary cause of plant loss, but research and experience are proving otherwise.

“Coming into Willoway, I was unaware of the full extent of the damage this pest can cause. I’ve learned this is definitely a difficult pest for the nursery industry,” said Matthew Steinkopf, container grower for Willoway Nurseries in Huron, Ohio. “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, leads this collaboration of USDA and university entomologists from Maryland, Mississippi, North Carolina, Ohio, South Carolina, Tennessee, and Virginia. Horticultural Research Institute (HRI) funded a portion of this research through its general grants program with further support provided by the USDA’s Floriculture and Nursery Research Initiative.

Figure 2A-D. (A) Ambrosia beetle attacks can be difficult to detect due to the small size of the tunnel entrance, but symptoms include (B) sawdust-toothpicks, (C) sap oozing from tunnel entrances, and (D) branch dieback.

Research on this subject was recently reported in the Journal of Environmental Horticulture (JEH), the publication of HRI. JEH is the only peer-reviewed, scientific journal dedicated to the green industry and serves as the outlet for research pertinent to our industry.

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.

Figure 3A-C. (A-B) Adult females tunnel into stems and create galleries (C) where they farm a symbiotic fungus that serves as food for the larvae and adults.

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; over 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," said 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 (USDA-ARS) 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. Notably, HRI-funded research by Dr. Steven Frank (NCSU) determined that soil moisture levels should be maintained at <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," Steinkopf said. "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. At Willoway, Mark [Shelton] has done an impressive job being ahead of this pest and knows that an emergence is possible even in winter. Knowledge of pesticides and setting traps when three consecutive 68° days has helped. Further research will take this beetle from being a serious problem to a mere nuisance.”

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," Dr. Ranger said. "Possible spring freezes predispose frost intolerant species to attack. Susceptible trees include eastern redbud, Japanese maple, Japanese zelkova, and Japanese snowbell.”

Figure 4A-D. (A) Spring flight activity of ambrosia beetles can be monitored using simple traps baited with ethanol lures and containing soapy-water as a killing agent. Traps should be placed low to the ground and near woodlots. Spring beetle activity can also be monitored using (B) stems soaked in ethyl alcohol (ie ethanol) or (C) trees injected with ethanol solutions. (D) Flooding of intolerant trees, such as dogwood, can also be used to monitor attacks.

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. 

Top photo: Pest and Diseases Image Library, Bugwood.org 

All other photos: Dr. Chris Ranger, USDA-ARS