by Gabrielle LaTora
Agriculture and Natural Resources Agent
Ambrosia beetles are tiny, bark-boring insects that pose a threat to over 100 species of trees and shrubs. We have 173 species of bark and ambrosia beetles in Georgia, with 32 of those species being introduced and non-native. Despite our large number of native species, at least 90% of the captures in commercial nurseries have been of a single, non-native species: granulate ambrosia beetle, Xylosandrus crassiusculus.
Ambrosia beetles are considered secondary pests, meaning they focus their attacks on trees and shrubs that have already been weakened by environmental stressors, physical damage, or primary pests. By understanding their natural history and life cycle, property owners can implement sustainable actions to prevent infestations.
Life Cycle and Natural History
Ambrosia beetles are tiny (1.6-4 mm), reddish-brown to black, and have stout, cylindrical bodies. Ambrosia beetles become active as temperatures rise in spring or even late winter, depending on the year. Their emergence coincides with the outside temperature increasing to at least 68°F for 2-3 days.
When adult female beetles emerge from the trees where they have spent the winter, they are looking for a new tree to colonize, in which they can reproduce. They are attracted to semiochemicals, such as ethanol and quercivorol, that are produced by trees in distress. Ambrosia beetles also require wood with internal moisture content of at least 48%. So, while they may attack dying or recently dead trees, they will not attack completely dead trees or dry firewood.
The colonization process begins when the female beetle lands on the bark and bores through the inner bark and softwood, finally reaching the heartwood. The boring of the female beetles produces the tell-tale sawdust plugs that we use to identify their activity.
Inside these galleries, the female carves small niches to lay her eggs, in which the larvae hatch and develop. Each ambrosia beetle has four life stages: egg, larva, pupa, and adult. All life stages can cohabitate inside the galleries, and the adults are the only ones to leave the tree.
The females care for the larvae until they are fully grown and ready to pupate, i.e., to form cocoons and later emerge as adults. The females keep the larval niches supplied with fresh fungus and prevent the niches from being cluttered with excrement or excess fungal growth.
Ambrosia beetles have several generations each year in Georgia, so we see an overlap in the life stages throughout the spring, summer, and early fall. The final generation of adults will overwinter inside the tree until the following spring, when the cycle begins anew with mated females emerging to find new hosts.
The Role of Symbiotic Fungi
What sets ambrosia beetles apart from other bark beetles is their unique, symbiotic relationship with their only food source: fungi. Adults introduce symbiotic “ambrosia” fungi (Ambrosiella spp.) into the galleries and actively tend to the fungal garden to keep it clean and encourage it to grow. Grazing and tending by the beetles makes the fungi grow more vigorously.
The beetles do not actually consume the wood of the host tree as they excavate their galleries. Instead, they dig tunnels specifically to create a microenvironment suitable for the growth of the ambrosia fungi. This is a mutualistic relationship where both species benefit. The fungus is distributed and cultivated by the beetle, and the beetle gains a reliable food source.
When females move to a new tree, they carry fungal spores in a specialized structure called a mycangium. These mycangia are diverse in shape—some are deep pockets on the beetle’s back or mouth, sometimes surrounded by hairs that help scrape spores into the structure.
Signs of Activity
Because of their small size and hidden lifestyle, the beetles themselves are rarely seen. Instead, identification relies on their physical impact on the tree. The most recognizable sign is the presence of sawdust plugs, also known as “toothpicks,” “noodles,” or “frass tubes.” These are spaghetti-like projections that stick out from the bark, each representing an entry point of a single female beetle, Fine powdery sawdust may also accumulate at the base of the tree or within bark grooves.
Severely infested trees may exhibit signs of decline, such as stunted growth, delayed leaf emergence in the spring, or premature defoliation in the summer. This decline is caused by the fungi, which multiply and plug the tree’s nutrient and water-conducting tissues, causing the tree to wilt. While a large tree might take months to show these symptoms, smaller trees can be affected in just weeks.
Trees at Risk
Ambrosia beetles have an incredibly broad host range, attacking species including crape myrtles, redbuds, dogwoods, oaks, tulip poplars, figs, Japanese maples, Styrax, flowering cherries, sweetgum, magnolia, rhododendron, azalea, pecan, peach, apple, plum, and persimmon.
While most are secondary pests, one notable exception is the red bay ambrosia beetle (Xyleborus glabratus), which is a primary pest that attacks healthy trees. Since its introduction to the US around 2007, it has killed over 500 million trees in the Lauraceae family, such as sassafras and red bay, throughout the Southeast.
Protecting Your Trees
The most effective way to prevent damage is through proper tree care, as healthy trees are less attractive to beetles and can better withstand low-level infestations. Prevention starts at planting – property owners should select trees suited for our USDA hardiness zone and site them appropriately. Irrigate as needed during extended periods of drought, and avoid wounding trees with maintenance equipment. These wounds put further stress on trees and could serve as entry points for the ambrosia beetles.
Even with the best protection, trees naturally decline and may still be attacked by ambrosia beetles. Once beetles have attacked a tree, early detection and removal of infested plant materials is the best solution. Prune with with discretion – we never want to remove more than one third of the canopy at a time.
Managing an existing infestation is difficult because the beetles are protected inside the tree and do not consume the wood, making systemic insecticides, like neonicotinoids or diamides, largely ineffective. While topical treatments, such as like permethrin or bifenthrin, can repel females, the timing of application must be very precise. In residential landscapes, insecticides are rarely recommended. Monitoring can be done using a wood bolt trap baited with ethyl alcohol, which simulates a dying tree.
If a tree is infested, removing the affected material is the best solution. For vigorously growing plants, like figs, only the damaged trunks need to be removed, as they can regrow from the stump. Importantly, infested wood does not need to be burned. The beetles are already well-established in the ecosystem, and burning will not significantly reduce their populations. Infested wood can be composted, chipped, or left to dry.
Key Takeaways
- Ambrosia beetles are natural parts of our Georgia ecosystems, taking advantage of stressed or dying trees, inside which they build complex galleries and reproduce.
- Ambrosia beetles grow gardens of fungus in the galleries they excavate inside trees. These fungi are their only food source.
- Ambrosia beetles only attack trees that are under stress from primary pests or environmental stressors.
- Ambrosia beetle infestations are best detected by the sawdust “toothpicks” left on the bark.
- Maintaining healthy trees is the best prevention for ambrosia beetle infestations.
- Early detection and removal of infested plant materials is the best solution after an infestation occurs. Removed wood can be composted or chipped and does not require burning.
