Granulate Ambrosia Beetle (Xylosandrus crassiusculus)

Introduction

The granulate ambrosia beetle (Xylosandrus crassiusculus; GAB), also once known as the Asian ambrosia beetle, is a tiny beetle that can cause damage in many different woody plant species in nurseries, orchards, or landscapes. It is a non-native beetle that originates from parts of Southeastern Asia, and since its arrival in the US, has spread to at least 29 states. It was first detected in the US in peach trees near Charleston, SC in 1974.

GAB is an ambrosia beetle, which is a group of commonly found wood boring beetles with over 4,800 known species. These beetles have four distinct life stages (egg, larva, pupa, and adult) and appear similar to bark beetles. However, while bark beetles consume the phloem (a living part of a woody host), ambrosia beetles do not consume living parts of their hosts. Instead, they chew a tunnel (also called a gallery) into a tree, shrub, or vine, while passively inoculating it with ambrosia fungi on which both adult and larval beetles feed. Ambrosia beetles are typically a secondary pest that only infests hosts that are already stressed, dying, or dead. Unlike native ambrosia beetles, GAB can attack healthy host plants, and its damage can lead to loss of aesthetic and economic value on ornamental plants and nursery stock.

Identification and Life Cycle

GAB has a stout, smooth, cylindrical appearance and is typically a dark reddish-brown color. Female beetles are typically 2.1-2.9 mm long and are about 1.2 mm wide. Males are smaller (only ~1.5 mm long) and have a “hunch-backed” appearance, making them easily distinguishable from females (though males are rarely seen). Only the females can fly. The larvae are white, chunky, and C-shaped.

Adult granulate ambrosia beetle, view from the side.
Picture credits: Pest and Disease Image Library, bugwood.org.

Adult granulate ambrosia beetle, view from above.
Picture credits: Pest and Disease Image Library, bugwood.org.

GAB is most active in the spring, with lower numbers being observed in the summer and fall. Female GAB bore a series of galleries into the sapwood of a host plant in which all life stages can be found. During the creation of these galleries, spores of their symbiotic “ambrosia” fungi, generally carried in small pouches called mycangia, which are located on the head or thorax, rub off the beetle onto the insides of the gallery wall. As these spores grow, they create fungal gardens on which both adults and their offspring feed. Females lay tiny, white, translucent eggs in a mass within the gallery. Once mature, female beetles leave the gallery and can fly to a new host plant to start the cycle again. Females can mate with males before leaving the original gallery, or reproduce asexually, which is known as parthenogenesis. Four generations per year are possible in native ranges, but they are thought to only have two generations per year in the southeastern US.

Hosts

GAB has a wide host range and damage has been reported in over 100 woody plant species. They can attack healthy or stressed host plants and are typically found attacking host materials from 2 to 30 cm in diameter. They are common pests across southeastern states in woody ornamental, fruit, and nut trees, but ornamental nursery stock has proven to be highly susceptible to attack. They will also attack freshly cut stumps, vines, and shrubs. Common host species include sweetgum, dogwood, persimmon, redbud, oak, pecan, crape myrtle, magnolia, fig, azalea, ornamental cherry, plum, peach, maple, and grape vines. It is thought to be able to infest most trees and shrubs, except for conifers (e.g., pines, spruces, and cedars).

Signs and Symptoms

Boring activity creates visible damage to a host plant. A common sign of ambrosia beetle attack is frass tubes, which are toothpick-like, sawdust protrusions coming from beetle entrance holes. Frass tubes are fragile and easily knocked off by wind or rain, but the beetle entrance hole remains. Pathogenic symbiotic fungi introduced by the adult beetle causes damage by disrupting the host’s vascular system, blocking nutrients from traveling throughout the host. This blockage can cause wilted leaves and branch dieback. Small-diameter nursery stock attacked by GAB is highly susceptible to death, but mortality is uncommon on larger host plants unless another stressor is present. Stunted growth, delayed leaf emergence, and premature defoliation are also signs of GAB infestations. Boring activity can create entry points for secondary infectors like Fusarium spp. and the combination of primary GAB boring activity and ambrosia fungi introduction plus secondary infectors often leads to host plant mortality.

Management

Once adult beetles are in a host tree, there are few options to alleviate their impact. GAB is relatively protected within a host plant and do not eat the wood of the host, so insecticides are of little help if beetles have already attacked the plant and it is showing signs of infestation. Fungicides are also mostly ineffective against pathogenic ambrosia fungi or too expensive to use for long term management. Therefore, prevention is key in managing damage from GAB.

Monitoring activities, such as using alcohol-based traps, can be good indicators for when to start using preventative insecticide applications. Pyrethroid insecticides like permethrin or bifenthrin can prevent beetles from boring into a host plant but must be applied before plants are infested and reapplied every few weeks while beetles are active. However, this management tactic is not cheap and is typically not feasible in most settings. Remember to always follow the label directions and use proper personal protective equipment. If the plant is heavily infested, it should be removed from the site and destroyed to try and minimize beetle spread to other potential host plants.

Healthy host plants can withstand a low level of GAB attack. Therefore, proper tree, shrub, and vine care is crucial. This starts with choosing the right species for the site (i.e., checking for proper sunlight, soil, water requirements, etc.), appropriate planting, and proper tree and shrub care. The less stressed the host plant, the more likely it will be to avoid or survive beetle attacks. Newly transplanted hosts and nursery stock are highly susceptible to GAB attack and should be monitored more closely. High value host plants can potentially be saved if GAB attack is discovered early, and further attack can be prevented.

Additional Relevant Fact Sheets

• Personal Protective Equipment (PPE) for Homeowner Pesticide Usage
• HGIC 1001, Planting Trees Correctly
• HGIC 1052, Planting Shrubs Correctly
• HGIC 1037, Tree Maintenance
• A Routine Check-up of Trees Saves Lives and Property
• Keep Trees Healthy to Stave off Ambrosia Beetles

Additional Resources

• https://extension.uga.edu/publications/detail.html?number=C1160&title=granulate-ambrosia-beetle-biology-and-management
• https://content.ces.ncsu.edu/granulate-asian-ambrosia-beetle
• https://edis.ifas.ufl.edu/publication/IN288
• https://www.fs.usda.gov/nrs/pubs/jrnl/2013/nrs_2013_haack_001.pdf

Originally published 11/23