The pests that attack floriculture and horticulture crops often receive the most attention, but those that affect trees and nursery stock should be paid attention as well. Consider exotic ambrosia beetles.

There are currently 30 species of ambrosia beetles in North America, but only five that are serious pests, especially to ornamental nurseries and apple and pecan orchards, according to Dr. Christopher Ranger, a research entomologist with the USDA-ARS Horticultural Insects Research Lab in Wooster, Ohio. He spoke about exotic ambrosia beetles at Cultivate’22.

The two species he focused on included the black stem borer, which is native to southeast Asia and was first reported in New York in 1932. It’s now established in 32 states and some of Canada. Only the females of the species infest trees. The other species is the granulate ambrosia beetle, first reported in South Carolina in 1974. It’s now found in more than 30 states.

The beetles are worrisome because they are borers. An infestation is evident due to very small holes in trees (sometimes hard to detect) and the creation of “toothpicks” of compacted sawdust being pushed out from the trees. They’re not feeding on the tree, Ranger explained, just burrowing into it. The boring also creates the presence of sap, which is often associated with pathogens.

The infestation reduces the aesthetic quality of ornamentals and significantly reduces the marketability of the trees affected. It may also lead to wilting and branch dieback. Growers often won’t notice an infestation until it becomes that dire.

Ambrosia beetles are farmers themselves, propagating and cultivating fungal gardens inside the holes they create. The larvae and adults need to consume what is “farmed” to survive. They grow from egg to adult in about a month; females can reproduce asexually. Females overwinter inside trees and come spring, they emerge to find new homes.

The black stem borer. Photo courtesy of Maja Jurc, University of Ljubljana,

The females are responsible for transporting the specific fungal symbionts (spores) they need. These symbionts are not pathogenic – meaning they likely won’t harm a tree – but a tree may defend itself against them. What is more likely is that secondary microorganisms may also be carried by the beetles which will cause harm like branch die-back and even tree death, such as black mold, Fusarium, coral spot fungus, Phomopsis and more.

Flight activity in spring dramatically increases with two to three consecutive days above 70º F. This usually means March to April emergence in the northern half of the country. The first new generation emerges about mid-July.

Both species prefer a wide range of hosts (over 200 species for the black stem borer, 120 for the granulate ambrosia beetle), but they’re often drawn to deciduous, thin-barked trees, according to Ranger.

“Ethanol is an important attractant of ambrosia beetles. They are very efficient as finding trees that emit ethanol,” he explained. Ethanol, with its wine-like odor, is emitted by plants due to physiological stressors like flooding, frost or freezing injury, drought, excessive heat, pathogens or impaired root function.

“Flooding and low temperature stress are the biggest issues in ornamental nurseries,” Ranger noted. “And trees will start producing ethanol within 24 hours. Even if they’re asymptomatic, they’re still emitting ethanol.” The odor lures in the beetles and the infestation can begin.

Trees that are especially flood-intolerant (Japanese snowbell, dogwood and redbud) should be monitored carefully during wet spring weather. Keeping growing media below 50% moisture during the beetles’ flight periods is an important method of deterring the pests.

Following flood stress, infestations happen closer to the base of a tree; with low temperature and freeze stress, they happen higher up toward the canopy.

Ethanol benefits the fungal farming of the insects. It promotes their respective symbionts and suppresses the growth of unwanted fungal competitors. Growers can monitor their ambrosia beetle levels with ethanol lures and bottle or funnel traps.

Maintaining tree health is the foundation of preventing an infestation from occurring and a key part of any IPM system. Permethrin and bifenthrin-based insecticides are the most effective against the beetles, but not 100% so. Ranger said to make sure you have good trunk coverage with these because 90% of attacks are on the tree stem, not the branches.

He added that it might be worth trying fungicides to help trees recover. All treatments should be applied prior to the insects’ peak flight period.

Those looking to keep track of the ambrosia beetle can report flight activity in near real-time via

Ranger said a four-year project is currently underway, with 27 researchers in 11 states studying the best prevention tools and management for ambrosia beetles.

And, because when it rains, it pours, there’s already a new pest to keep an eye on. Ranger said the redbay ambrosia beetle is spreading. It’s been found in Kentucky and notably is not attracted to ethanol. This one will attack healthy trees and it does carry pathogenic fungi.

by Courtney Llewellyn