Ambrosia Beetle Biology, Impact, and Management

Ambrosia beetles are commonly found across the Southeastern United States. Most are secondary pests that only damage weakened, stressed, or dying host plants. Some species, such as the redbay ambrosia beetle (Xyleborus glabratus; RAB), can be primary pests that damage healthy host plants.¹ This article provides an overview of common ambrosia beetles found in the Southeastern United States and will help landowners and land managers recognize common signs of ambrosia beetle infestations and understand management strategies.

Introduction

Ambrosia beetles (Coleoptera: Curculionidae: Scolytinae and Platypodinae) are a globally found type of wood-boring beetle with over 4,800 known species.¹ These tiny beetles are known for their symbiotic relationship with fungi, which they grow as a food source within their galleries (tunnels that the beetle bores into a woody stem or branch). Ambrosia beetles can infest hundreds of different woody plant species, including trees, shrubs, and vines. The majority of ambrosia beetles colonize stressed, damaged, or dying host plants, but a few species can colonize healthy hosts. Infestations can occur in any environment where a host is present, including natural or managed areas, urban or rural settings, in a single tree, or across a large, forested area. Both native and non-native ambrosia beetles can be found in South Carolina.^2–4 While many do not cause noticeable damage, some non-native species can cause significant damage, leading to mortality in healthy host plants.

Description

Ambrosia beetle adults are relatively small, measuring up to 4 mm. long for females and 1.5 mm. long for males.⁵ Due to their minute size, measurement of ambrosia beetles is most accurately represented using millimeters. Most are dark reddish brown to black in color, and their shape is highly variable (figure 1). Ambrosia beetle bodies are generally smooth and shiny, often with sparse setae (hairs) or differently shaped projections on their elytra (hardened wing coverings). Males of some species are flightless and rarely seen, while females are responsible for nearly all the damage observed and recorded.¹ Eggs are relatively small, translucent, and oblong. Larvae are small, white, legless, and grub-like, with distinguishable brownish or reddish mouthparts resembling small beaks. Pupae are similar in color and size to the larvae, though they are more angular in shape rather than grub-like. As a pupa develops, the head becomes more rounded and defined (figure 2).

Figure 1. Ambrosia beetles show a wide variation in size and shape: black twig borer (A), redbay ambrosia beetle (B), granulate ambrosia beetle (C), camphor shot borer (D), fruit-tree pinhole borer (E), and pinhole borer (F). Image credit: Pest and Diseases Image Library, Bugwood.org (A, C, E, F); Joseph Benzel, Bugwood.org (B); and Rachel Osborn, Bugwood.org (D).

Figure 2. Granulate ambrosia beetle (Xylosandrus crassiusculus) life cycle: galleries (A), eggs (B), eggs and larvae (C), larvae and pupae (D), and adult emerging from a gallery (E). Image credit: David Coyle, Clemson University (A); Lacy Hyche, Bugwood.org (B, D, E); Will Hudson, Bugwood.org (C).

Life Cycle

The ambrosia beetle life cycle primarily occurs in the galleries. In these galleries, all life stages can be found (figure 2). Ambrosia beetles are attracted to chemicals produced by host plants ^6,7 and a fungus with which they have a symbiotic relationship.⁸ In most cases, volatiles produced by stressed plants (e.g., ethanol) attract ambrosia beetles, but in rare cases, some species are attracted to living, healthy hosts.¹ When an adult female locates a suitable host, she chews a tunnel, or gallery, into the stem, branch, or vine.^9,10 Ambrosia beetles may consume some phloem or sapwood but mostly survive by feeding on their symbiotic fungi, which are introduced by the tunneling female. The gallery excavation causes external projections called frass tubes or noodles (figure 3), which are comprised of sawdust and beetle waste.¹¹ Frass tubes appear on the outside of the plant, extending from the beetle’s entrance hole. Each tube represents one female beetle’s activity. Ambrosia beetles carry the fungal propagules of symbiotic fungi in specialized pockets called mycangia, which are located on the head or thorax.^1,12 The fungal propagules get rubbed off onto the wood while the beetle is tunneling. This results in a fungal garden that both adults and larvae use for food.⁹ The majority of ambrosia beetles are of no concern because they infest dead or dying host plants, and their symbiotic fungi are non-pathogenic. Their host plant is already dying, and the ambrosia beetle’s symbiotic fungi aid in host plant decomposition. However, a few invasive ambrosia beetle species, such as RAB, stray from their original niche and infest living host plants instead.¹³

After completing the gallery, the female will lay up to sixty eggs. Most ambrosia beetles are parthenogenic,¹ meaning the female beetle can reproduce asexually (i.e., without a male) and create haploid (containing a single set of chromosomes) male individuals with which they can later mate. Reproduction with males occurs to create more female individuals. The eggs hatch and the larvae spend several days consuming the fungus that is growing inside the gallery. Larvae then pupate and emerge as adults, which may or may not mate with beetles already in the galleries. Female ambrosia beetles leave the gallery to start the cycle over again. Ambrosia beetles can produce multiple generations each year and can be active year-round in warmer climates or coastal areas. However, high humidity is required for successful reproduction.¹⁴ Beetles overwinter as adults inside their galleries.

General Signs and Damage

The best indicator of ambrosia beetle presence is the unique frass tubes found on infested trees, shrubs, and vines (figure 3). Frass tubes are delicate and easily destroyed by rain, hail, and wind. Other signs of ambrosia beetles are sawdust at the base of the tree or small entrance holes on the host plant. On live trees, sap leaking out of the tree, stained bark, dieback of terminal buds, or wilting leaves can also be indicators of ambrosia beetle activity. When young plants are infested with the ambrosia beetle and its fungal symbiont, the damage can be fatal to the immature host. This infestation can lead to stunted growth and premature defoliation.¹⁶

Figure 3. Ambrosia beetle frass tubes: frass tubes from a branch on the ground (A), frass tubes on a potted plant (B), and frass tubes on a root (C). Image credit: Katy Crout, Clemson University (A); David Coyle, Clemson University (B, C).

Common Ambrosia Beetle Species in South Carolina

Black Twig Borer (Xylosandrus compactus)

First detected in Florida in the 1940s, the black twig borer is an invasive ambrosia beetle that utilizes healthy host plants. Native to Asia, it has now spread to several southeastern states and Hawaii.¹⁵ Black twig borers have a stout, cylindrical appearance and are tiny, ranging from 0.4 mm. long by 0.4 mm. wide in males to 1.9 mm. long by 0.8 mm. wide in females (figure 1A). They are often brown to black in color. This species tends to infest ornamental plants and has become a nuisance in nursery production, including orchids (Dendrobium spp., and Cattleya spp.), flowering dogwood (Cornus florida), redbud (Cercis canadensis), and red maple (Acer rubrum), as well as avocados (Persea americana) and coffee (Coffea canephora).¹⁵ In South Carolina, this beetle is also known to infest plants in the families Buxaceae, Cornaceae, Fabaceae, Hydrangeaceae, Magnoliaceae, Moraceae, Myricaceae, Pinaceae, and Rubiaceae.¹⁷ Although this beetle-fungi complex does not often cause mortality in hosts, aesthetic damage can occur. It most commonly infests smaller-diameter materials, leading to damage that can occur as branch flagging, where only the end of a branch is killed and is indicated by dead or dying leaves (figure 4). Mortality is possible if the beetles colonize the main stem of a smaller-diameter host plant.¹⁵

Figure 4. Entrance hole (A) and flagging damage (B) caused by black twig borer. Image credit: David Coyle, Clemson University.

Redbay Ambrosia Beetle (Xyleborus glabratus)

The redbay ambrosia beetle (RAB) is another invasive ambrosia beetle species now common across much of the Southeastern United States. These beetles are brown to black in color, with females ranging in length from 2.1 mm. to 2.4 mm., and their flightless male counterparts approximately 1.8 mm. in length (figure 1B).¹⁸ Native to Asia, RAB was first detected in Port Wentworth, Georgia in 2002.¹⁹ Like other ambrosia beetles, RAB has a symbiotic relationship with a fungus (Harringtonia lauricola). However, this particular fungus has proven to be pathogenic to host species in the family Lauraceae, giving this disease complex the name laurel wilt disease. Laurel wilt disease is primarily spread by female RAB and was first noticed in the redbay trees (Persea borbonia) of Hilton Head Island, South Carolina in 2003, where the disease caused mortality in 75% to 80% of the island’s redbays.¹⁹ It affects sassafras (Sassafras albidum) and the economically important avocado (P. americana), but all North American members of the Lauraceae family are susceptible hosts.²⁰ Laurel wilt causes sapwood to become dark in coloration, wilting or discolored leaves, and nearly always host mortality (figure 5).²¹ Although laurel wilt has had an economic impact on the Florida avocado industry,²² the biggest concern is the potential ecological impact. Members of the Lauraceae family are an important food source for several wildlife species. They are critical to species like the Palamedes swallowtail (Papilio palamedes) which has experienced a population decline in recent years from the loss of Persea species due to laurel wilt disease.²³

Figure 5. Laurel wilt can cause damage in different species, including redbay (A) and sassafras (B). Staining occurs throughout the tree, including the stump (C), where the red arrow indicates the dark staining, and along the stem of the tree (D). Image credit: David Coyle, Clemson University (A); Katy Crout, Clemson University (B, C, D).

Granulate Ambrosia Beetle (Xylosandrus crassiusculus)

The granulate ambrosia beetle, also known as the Asian ambrosia beetle, is a non-native ambrosia beetle originating from Southeastern Asia. It was first detected in peach trees near Charleston, South Carolina in 1974, and since then has spread to at least twenty-nine states.²⁴ The granulate ambrosia beetle is reddish brown in color, with a stout, cylindrical appearance and a very prominent pronotum (plate-like structure covering all or part of the thorax, figure 1C). Females of this species range in length from 2.1 mm. to 2.9 mm. and are about 1.2 mm. wide.¹⁰ Males are much smaller and flightless, similar to other ambrosia beetle species. Granulate ambrosia beetles are commonly found in ornamental nursery stock, which seems to be highly susceptible to infestation, but they are also common pests of woody ornamental, fruit, and nut trees across the Southeast. Damage has been reported on over 100 species of host plants including dogwoods (Cornaceae), redbud (Fabaceae), oak (Fagaceae), maple (Sapindaceae), ornamental cherry, plum, and peach (Rosaceae), crape myrtle (Lythraceae), azaleas (Rhododendraceae), fig (Moraceae), magnolia (Magnoliaceae), pecan (Juglandaceae), persimmon (Ebenaceae), sweet gum (Altingiaceae), grape vines (Vitaceae), and sweet potato (Convolvulaceae).²⁵ Signs of damage include wilted leaves, branch dieback, and frass tubes. Granulate ambrosia beetle infestations do not always lead to host mortality on larger plants, but small-diameter nursery stock is highly susceptible to mortality.¹⁰ Boring activity can also open the door for secondary infectors like Fusarium spp., and the combination of primary infestation plus secondary infectors often leads to host mortality. In ornamental plants, growth, aesthetic value, and economic values are often negatively affected by beetle infestation.¹⁰ Granulate ambrosia beetles are most active in spring, but their presence is possible year-round, especially during warmer winters. They are thought to have two life cycles per year in the Southeastern United States,²⁵ but up to four generations per year have been reported in its native range.²⁶

Camphor Shot Borer (Cnestus mutilatus)

Native to parts of Asia, the camphor shot borer was first detected in the United States in Mississippi in 1999.²⁷ Since then, it has become well established in the United States, ranging across the Southeast, west to Texas, and as far north as Pennsylvania.²⁸ It is one of the largest ambrosia beetle species in the United States, ranging in size from 2.6 mm. to 3.9 mm. They are dark brown to black in color and have a distinct look due to their abdomen being shorter than their thorax and having a “squashed” appearance (figure 1D). Camphor shot borers are a generalist species with a wide host range, known to colonize many species of hardwoods, shrubs, and vines, although they seem to favor sweetgum.²⁸ They have been reported in host plants from the families Aceraceae (maples), Fabaceae (mimosa), Lauraceae (spicebush), Arecaceae (Rattan palm), Theaceae (Camellia), Betulaceae (hornbeam, ironwood), Juglandaceae (hickory, walnut), Fagaceae (chestnut, beech), Cornaceae (dogwood), Taxodiaceae (Japanese cedar), Proteaceae (Australian silver-oak), Sapindaceae (golden rain tree), Hamamelidaceae (sweetgum), Magnoliaceae (tulip poplar, sweet bay magnolia), Meliaceae (chinaberry, big leaf mahogany), Oleaceae (fragrant olive), Pinaceae (loblolly pine), Rosaceae (wild plum, black cherry), Ulmaceae (winged elm), and Vitaceae (muscadine) but is suspected to have hosts in more families.²⁹ These beetles only infest stems 1.2 cm. to 5 cm. in diameter.²⁸ Damage is similar to other ambrosia beetles but is distinguishable due to the relatively large size of their entrance holes. Camphor shot borers differ from other introduced ambrosia beetles because their host preference is not healthy trees. Instead, they are more like native ambrosia beetles that prefer stressed or dying host plants that are emitting ethanol. This attraction to ethanol has led to a unique economic problem. Camphor shot borers are known to damage fuel lines and containers as they may mistake these items as a stressed host (figure 6), leading to Florida news stations warning residents to check their boats to prevent being stranded during use.³⁰

Figure 6. Plastic gasoline storage container (23-L) with numerous holes created by female Cnestus mutilatus. White arrows indicate embedded beetles. The black arrow indicates a detailed inset of holes and elytral apices of embedded beetles. Image credit: Chris Carlton, Louisiana State University.³⁰

Fruit-Tree Pinhole Borer (Xyleborinus saxesenii)

Originally from parts of Eurasia, the fruit-tree pinhole borer is thought to be one of the first non-native ambrosia beetles introduced to the United States around 100 years ago, and since then has become one of the most abundant ambrosia beetles with populations established in a majority of the fifty states.³¹ Adults of this species are elongated, cylindrical, and less than 2 mm. in length, with males being smaller than their female counterparts (figure 1E). They are light brown to dark brown in color and have multiple generations per year.³¹ Fruit-tree pinhole borers have a wide host range including species in the families Anacardiaceae (sumac), Annonaceae (custard apples), Apocynaceae (dogbanes), Betulaceae (hazel, alder, birch), Cornaceae (dogwood), Ericaceae (heath), Fagaceae (chestnut, oak), Juglandaceae (walnut), Lauraceae (avocado), Magnoliaceae (magnolias), Rosaceae (pear, plum, peach, apricot, apple), Salicaceae (black poplar), Sapindaceae (soapberry), Taxodiaceae (Japanese cedar), Tilicaceae (basswood), and Ulmaceae (elm).³¹ This species is attracted to stressed or dying host plants, similar to native ambrosia beetles. However, it has been observed colonizing healthy chestnut trees³² and could potentially be a vector for laurel wilt disease due to its association with H. lauricola, the causal agent of the disease.³³ Unlike other ambrosia beetles that feed solely on fungi, the fruit-tree pinhole borer is xylomycetophagous, meaning it feeds on the ambrosia fungi, as well as the wood itself.³¹

Platypus spp.

Platypus beetles are a genus of ambrosia beetles in the family Curculionidae, subfamily Platypodinae. Seven members of this genus can be found in the United States.³⁴ Although closely related to previously mentioned ambrosia beetles in the Scolytinae subfamily, members of the Platypodinae subfamily are easily distinguished by their physical appearance (figure 1F). Platypus spp. differ from other ambrosia beetles in several ways. Males of this genus, not the females, initiate the infestation on host trees. When a male creates the first entry holes in a host, it triggers a mass infestation on the tree by releasing pheromones. Mated pairs continue to excavate galleries into the heartwood of host trees, inoculate with their symbiotic fungi, and reproduce.³⁴ This genus requires relatively large diameter host material to breed and produce only a single generation per host. Although members of this genus are attracted to stressed hosts, they can only breed in recently killed host material with high moisture content. This includes recently cut trees, pulpwood, lumber, and rewetted or freshly stained log homes (figure 7).³⁵ Host trees differ by species. Platypus flavicornis and P. quadridentatus are only found in pines and oaks, while P. compositus and P. parallelus are more generalist species that can find a host from most trees within their range.³⁴ Platypus flavicornis is a species that will infest southern pines and is commonly found in pines colonized by other bark beetles. Damage includes hosts being covered in large numbers of pin-size holes, frass around the base of the tree, and staining from fungal growth (figure 7.³⁵

Figure 7. Examples of damage caused by Platypus spp: pin-size holes (A and B) and frass around the base of a host tree (C). Image credit: USDA Forest Service-Wood Products Insect Lab, Bugwood.org (A), James Denny Ward, USDA Forest Service, Bugwood.org (B), Andrew Boone, SC Forestry Commission, Bugwood.org (C).

Management

Most ambrosia beetles do not typically require management since their colonization indicates a weakened, dying, or sometimes dead plant. Trees infested by invasive ambrosia beetles, such as RAB, can be removed and destroyed to prevent further spread of disease.²¹ For invasive ambrosia beetle species that infest healthy plants, there are few effective strategies to manage them once they colonize a tree, shrub, or vine.²⁵ Insecticides such as permethrin and bifenthrin can kill adults, but their use is not practical due to necessary application frequency (i.e., about every two weeks) and total use since the entire plant must be sprayed. The application of the chemicals must also be timed appropriately. There are few effective strategies to save a plant once it becomes a host.²⁵ Since a stressed plant is most susceptible, any management to reduce tree stress may help, such as watering or removing competition. Fungicides may also be useful since they attack the ambrosia fungus directly, though this can sometimes cost more money than a plant is worth, except in instances with high-value trees.

Nurseries, in particular, are often hit hard by ambrosia beetles. Some species utilize young plants partly due to their vulnerability. One way to prevent infestations is to maintain plant health in nurseries. However, some invasive species of ambrosia beetle colonize healthy plants, so this method is not a foolproof way of protecting host plants. Another way to prevent an infestation is to determine whether the plant is predisposed to ambrosia beetle utilization. For example, if the area has recently, the plant may be in a state of stress. Even though it may not be apparent to humans, some ambrosia beetle species can detect even the slightest signs of early-stage physiological stress.¹⁰ Another way to determine if a plant is susceptible to infestation is to identify if the growing site is suitable for the plant species. Frost damage or poor drainage can significantly affect the health of non-native plants. Nursery stock susceptible to invasive ambrosia beetles, such as RAB, should not be sold or transported if exhibiting signs of disease like laurel wilt.²¹

The best way to prevent ambrosia beetle damage is to maintain proper plant health through appropriate management tactics, including adequate mulching, watering during a drought, and preventing construction or landscaping damage to the root system. Check trees frequently during peak beetle flight activity, and do not move firewood or other woody material to prevent potential spread. A plant will likely need to be replaced once ambrosia beetles are present; however, this presents the opportunity to plant an appropriate native tree, shrub, or vine.

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Additional Resources

Williamson J. Black twig borer. Clemson (SC): Clemson University Cooperative Extension, HGIC (Home & Garden Information Center); 2021. HGIC 2013. https://hgic.clemson.edu/factsheet/black-twig-borer/.

Darr M. Laurel wilt disease. Clemson (SC): Clemson University Cooperative Extension, HGIC (Home & Garden Information Center); 2021. https://hgic.clemson.edu/laurel-wilt-disease/.