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Ambrosia Beetles

Introduction

Figure 1. Two brown-colored redbay ambrosia beetle adults on a dime for scale. Photo by Kelsea Young, Clemson University.

Figure 1. Two brown-colored redbay ambrosia beetle adults on a dime for scale.
Photo by Kelsea Young, Clemson University.

Ambrosia beetles are small insects (under 1/4th of an inch; Figure 1), that attack more than 100 woody plant species. Both native and non-native ambrosia beetles live in South Carolina, most of which do no measurable damage to living plants – in fact, most of them attack stressed, dying, or dead trees. Adults are cylindrical shaped and range in color from reddish-brown to black, and larvae are small white grubs. The most common sign of attack by ambrosia beetles is a noodle-like waste and sawdust mixture (frass tubes; Figure 2) protruding from the trunk of attacked woody plants and fine sawdust surrounding the base of trees.

Biology

Life Cycle

Figure 2. Noodle-like frass tubes created by the redbay ambrosia beetle and a common symptom of ambrosia beetle attack. Photo by James Johnson, Georgia Forestry Commission, Bugwood.

Figure 2. Noodle-like frass tubes created by the redbay ambrosia beetle and a common symptom of ambrosia beetle attack.
Photo by James Johnson, Georgia Forestry Commission, Bugwood.

Figure 3. Galleries in Norway spruce carved by the striped ambrosia beetle (Trypoendron lineatum). Photo by Petr Kapitola, Central Institute for Supervising and Testing Agriculture, Bugwood.

Figure 3. Galleries in Norway spruce carved by the striped ambrosia beetle (Trypoendron lineatum). Photo by Petr Kapitola, Central Institute for Supervising and Testing Agriculture, Bugwood.

Most ambrosia beetles are attracted to chemicals produced by stressed trees. When an adult female finds a tree, she will carve a tunnel (gallery; Figure 3) into the tree’s stem. Ambrosia beetles do not eat the wood; instead, they eat into the plant, creating galleries with tell-tale frass tubes (Figure 2). Ambrosia beetles carry the spores of the fungi in pockets (called mycangia) where their shoulders would be, and these fungal spores get rubbed off onto the wood while the beetle is tunneling. This results in a fungal garden on which the adult female and larvae are dependent on for food.

After completing the gallery, the female will lay up to 60 eggs. The larvae hatch as small, white grubs that spend several days consuming the fungus that is growing on the inside of the gallery. Larvae then pupate and emerge as adults that may mate – although many ambrosia beetle females do not need to mate to produce viable eggs, a process called parthenogenesis. After mating, the females 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. They overwinter as adults inside their galleries

Hosts

Ambrosia beetles have been recorded to attack many species of hardwood and conifer trees, woody shrubs, and vines. Ambrosia beetles are attracted to ethanol, a chemical that woody plants produce when they are stressed or dying. The redbay ambrosia beetle (Xyleborus glabratus) is the only ambrosia beetle in South Carolina known to attack healthy trees and is a vector of laurel wilt (Raffaelea lauricola), a fungus lethal to members of the laurel family (Lauraceae; Harrington et al., 2008). Laurel wilt has decimated redbay (Persea borbonia) populations across the Southeast and threatens two federally endangered shrubs as well as avocado orchards in Florida (Harrington et al., 2008).

General Signs

The best indicator of ambrosia beetle presence is the unique frass-tubes (Figure 2) found on the trunk of attacked trees, shrubs, and vines. Frass tubes (Figure 2) are delicate and easily destroyed by rain, hail, and wind (Ambrosia beetles- trees). Other signs of ambrosia beetles are sawdust at the base of the tree or small holes in the trunk (Figure 4; Ambrosia beetles- trees). On live trees, look for sap leaking out of the tree, stained bark, dieback of terminal buds, or wilting leaves (Ambrosia beetles- trees).

Figure 4. Symptoms of an ambrosia beetle attack can include accumulated sawdust at the base of the tree, as seen here.

Figure 4. Symptoms of an ambrosia beetle attack can include accumulated sawdust at the base of the tree, as seen here.
Photo by Randy Cyr, Greentree, Bugwood.

Management

There are no effective strategies to manage ambrosia beetles once they attack a tree, shrub, or vine. Insecticides can kill adults, but their use is not practical because they would need to be applied very frequently and over the entire plant. The best way to prevent ambrosia beetle damage is to maintain proper plant care, including adequate mulching, watering during a drought, and preventing construction or landscaping damage to the root system. Once ambrosia beetles are present in a plant, the plant will likely need to be replaced. This presents the opportunity to plant a native tree, shrub, or vine appropriate for the site in its place.

If this document didn’t answer your questions, please contact HGIC at hgic@clemson.edu or 1-888-656-9988.

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