Ambrosia Beetles Information
Species & Identification
Ambrosia beetles are problematic on many ornamental trees and shrubs, as well as fruit and nut trees. More than 40 ambrosia beetle species are found in South Carolina but the granulate ambrosia beetle (Xylosandrus crassiusculus) is the most damaging species. The granulate ambrosia beetle is about 1/8 inch long with chestnut brown bodies and wing covers that are darker than the body. The black stem borer (Xylosandrus germanus), the black twig borer (Xylosandrus compactus) and the camphor shoot borer (Cnestus mutilatus) can be occasional pests.
Damage & Symptoms
Ambrosia beetles damage trees physically by tunneling and boring through the trunk and main branches. Toothpick-like structures (called frass tubes) are left protruding from the entry points as boring dust is pushed out by the beetles. Sap may also excrete from the entry points, leading to stained bark. Trees infested with ambrosia beetles may exhibit slow leaf development in the spring and leaves may become chlorotic. The most damaging aspect of ambrosia beetle infestation is the introduction of ambrosia fungi by the beetles. These fungi grow through the vasculature of the affected plants, clogging the xylem and phloem and leading to plant death in most cases, especially trees grown in nursery settings. Infestation cannot be cured once the beetles enter the trees.
Life Cycle
Adult ambrosia beetles can be active any time during the year when air temperature is high enough to allow muscle movement and flight. From a management standpoint, late March to early May is when ambrosia beetles become active and damaging in South Carolina. Beetles active during other time periods do not appear to attack trees in high frequency. Trees stressed by flood, drought, fire, deep planting or other biotic and abiotic factors emit ethanol, which is a chemical highly attractive to the ambrosia beetles. Adult female beetles attracted to a tree by ethanol vapor emitted by the tree bore into the trunks and branches and excavate tunnels or galleries in the sapwood and heartwood. The females lay eggs in the tunnels. Larvae feed on the ambrosia fungi and complete their development in about 55 days. Adult males are flightless and remain in the gallery where they mate with the females before the females disperse to find new trees.
Scouting
Timely deployment of monitoring traps is critical to managing the ambrosia beetles because all currently available management tools perform best when used for preventing beetle attacks. Frass toothpicks are a clear indicator of ambrosia beetle activity, but not a good scouting tool for preventative treatments as it is too late to treat for ambrosia beetles at that point. Beetle activity is more effectively monitored using a soda bottle trap baited with a high concentration ethanol lure. The soda bottle trap can be constructed from used 2-liter soda bottles by cutting 1.5” x 5” windows on the sides of the bottle. The ethanol lure, which can be purchased from pest management supply companies, can be hung inside the soda bottle. A small amount of soapy water is used in the bottom of the trap as a killing agent for trapped beetles. Alternatively, beetle activity can be monitored with ethanol-infused bolts. The bolts are constructed of smooth-barked broadleaf trees (such as poplar, magnolia, redbud, or dogwood), at two to three inches in diameter and 10 to 12 inches long. The bolts can be soaked in 40% ethanol for three days, or they can be cored with a drill bit (about 1/3 the diameter of the bolts) and filled with 70% ethanol (a cork will be needed to cap the bore hole). The soda bottle traps and the ethanol-baited bolt traps should be placed 1.5 to 3 feet above the ground along the edge of the woods. A few strategically placed traps along the wood line are sufficient. Check traps weekly from February through April and begin insecticide application as soon as ambrosia beetles are found in the soda bottle traps or when frass tubes are protruding from the bolt traps.
Cultural Control
Ambrosia beetles seek out stressed trees. Removing stress factors is the first step to take. Plant trees in appropriate places. For example, don’t plant trees that prefer drier root zones in low lying or flood-prone areas. Don’t bury the root flare and plant trees too deep. Avoid root bound containers. Trees that are already exhibiting ambrosia beetle damage must be destroyed by burning or chipping within 45 days after attack or before June.
Chemical Control
Preventative applications of insecticides should be made as soon as the first beetle is caught in the soda bottle traps or the first frass tube is found on the bolt traps. Focus on newly planted stock, as well as Prunus species, redbuds, dogwoods, maples, and styrax. Insecticide applications should be made to the trunk and major branches, and complete spray coverage of the trunks and branches must be achieved. Areas on a trunk missed by the spray can still be attacked. Permethrin and bifenthrin applications are currently the most effective management approach and should be repeated every 7-9 days until trees fully leaf-out. There is no curative solution for the ambrosia beetles or the fungi they introduced.
