The Granulate Ambrosia Beetle (GAB), or Xylosandrus crassiusculus (Motschulsky) is a destructive insect pest that can cause severe damage to trees. Taking control of this beetle requires a comprehensive understanding of the behaviors of the beetles and a management strategy that is centered on monitoring and insecticide applications. By following the content of this article, you can learn how to protect your trees from the Tree-Killer Granulate Ambrosia Beetle.
Origin and distribution
GAB is among the most destructive ambrosia beetles in the United States. Its native habitat is Asia, but it can also be found in Asia, eastern Africa, India, Sri Lanka, and China. Several states in the United States now have the beetle. The beetle was first discovered in the United States in 1974 when it was found on a peach tree in Summerville, SC. It now exists in 29 U.S. states, including Tennessee. Generally, this species is found between Missouri and Texas, eastward toward Florida, and northward toward northern Indiana. Also found in Oklahoma, Missouri, Mississippi, and Kansas in the west.
Adult beetles have a small, reddish-brown appearance with a head facing down and hidden by their neck region (pronotum) (Fig. 1). The head can only be seen from the side (Fig. 2). From above, they appear round (Fig. 1B). They have dark-brown to black outer wings (elytra) and setae at the back (Fig. 1B). A granulated (rough) surface covers the front portion of the head (Fig. 1). Females are usually bigger than males, measuring about 1.2 mm wide and 2-2.9 mm long, while males measure about 1.5 mm. The female can fly, but the male cannot.
GABs have four life stages: eggs, larvae, pupae, and adults. They complete one life cycle every 55- 60 days, and there are at least two generations per season. Early in spring, when the temperature is 70°F or above for a few days, females mate and disperse from their overwintering galleries to attack vulnerable hosts. They burrow into the host stem, creating clean tunnels, and laying eggs (Fig. 4B). Furthermore, they inoculate the tunnels with symbiotic fungi (ambrosial fungi) that will serve as their food source. Female GABs often lay up to six white, translucent, shiny, oval-shaped eggs per day (Fig. 4B, white arrow), and the eggs may take up to 6 days to hatch into larvae. Eventually, the larvae develop into white pupae with moving legs. Pupae develop into adult beetles at a 1:10 ratio, male to female. Adult females will mate with their male siblings and leave the gallery looking for a new host to attack. Typically, a gallery can generate 8 (peach) to 27 (pear and cherry) or 59 (chestnut) beetles depending on the host.
GABs can infest more than 120 species of trees. In the United States, some of the most common hosts are aspen, beech, cherry, Chinese elm, crape myrtle, dogwood, golden rain tree, hickory, locust, magnolia, maples, mimosa, oaks, peach, persimmon, plus, Prunus spp., redbud, styrax, sweetgum, tulip poplar, and walnut. The GAB prefers to attack trees with smooth bark and those that are under stress. Stressed trees emit volatiles such as ethanol, which attracts adult beetles. Stress factors include floods, drought, soil compaction, mechanical damage, frost/freeze damage, and ozone damage. Trees that are newly planted or have a diameter at breast height (DBH) of 3 inches or less are also more susceptible to GAB infestation.
Signs and symptoms
One of the signs of infestation by ambrosia beetles is the presence of frass tubes on the tree trunks. These are thin, cylindrical structures made of insect waste, sap, and boring dust that extend up to 3–4 cm from the stem (Fig. 5A and B). The frass tubes are produced by the adult beetles as they tunnel into the wood. They can be easily detached by wind or rain, exposing small round holes (Fig. 5C) where the beetles entered. These holes may be hard to notice on trees with rough bark. Some host trees (e.g. Prunus spp.) may also show gummosis or sap staining at the attack sites. The wood near the galleries may be discolored by fungal growth, especially in spring attacks. The holes made by ambrosia beetles are usually deeper and larger (about 1 mm in diameter) than those of other wood borers like shot hole borers.
GAB beetles can cause severe damage to trees. They can affect the growth, beauty, and economic value of the trees. Infection may result in wilting leaves, dying branches (Fig. 6), or even death, especially in spring. The beetle bores holes in the trunk near the soil, or anywhere on the tree when it attacks in large numbers. These attacks can kill the trees quickly in ornamental nurseries, while less intense attacks may not be fatal. Most trees die when they have 5 to 10 beetle holes, especially if the trunk diameter is less than 3 inches. Trees infested by the beetle are usually not allowed to be shipped across states or countries if detected.
The best way to control GAB attacks is to prevent them from happening. Insecticides are ineffective against GAB after infection because they hide inside the host plant. Therefore, keeping trees healthy and free from stress factors, such as drought, flooding, soil compaction, frost damage, ozone damage, pathogens, or other sources of stress that can attract the beetle to trees, is crucial. If stress is unavoidable, there are two alternative control methods that can reduce tree loss due to GAB attacks.
Monitor beetle flights and attacks
To monitor beetle flights and attacks and decide on insecticide application needs, use traps baited with ethanol or bolts. A simple trap made from a soda bottle (Fig. 7) can catch beetles in flight, while a bolt trap (Fig. 8) can detect beetles attacking trees. Place the traps on both sides of the nursery in early March and check them twice or thrice a week. Start spraying insecticides when the trap catches peak. Repeat every two weeks if beetles are still present. This method reduced tree loss from 13% to 0.1% in one year in South Carolina due to GAB (Xylosandrus crassiusculus), a non-native wood-boring beetle that infests many hardwood species. GAB usually stops attacking when trees have leaves and grow actively. For more information on soda bottle traps and bolt traps, click here and here.
Trap and destroy beetles
To use the method to control beetle populations, first, monitor nursery trees regularly and identify the trees that are attacked by beetles. Then, leave those trees in the nursery for about a month to lure more beetles to them, while protecting the rest of the trees with pyrethroid insecticides sprayed every two weeks. Finally, you can dispose of the beetle-infested trees by burning them.
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