Granulate Ambrosia Beetle: Pest Alert
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A entry hole left behind by a granulate ambrosia beetle. This was a stressed tree due to drought in Buncombe County.
Granulate ambrosia beetles are a common pest in nursery and landscape trees throughout the Southeast. Introduced in the early 1970’s via wood shipments from Asia, granulate ambrosia beetles damage trees by boring into the wood leaving toothpick-like strands of boring dust emerging from the trunk to lay eggs while also transmitting diseases (ambrosia fungus) within the tree itself. These pathogens can clog vascular tissue of the tree leading ultimately to death of the tree infested by ambrosia beetles. Trees that are impacted include styrax, dogwood, redbud, maple, ornamental cherry, Japanese maple, crapemyrtle and others. So what can be done to manage the ambrosia beetle?
Monitoring:
Tracking Growing Degree Days (GDD) is the most accurate and reliable method for determining when adult beetles are active, which is typically somewhere between 75-80 GDD. It can also be useful to track GDD in the Eastern and Central parts of NC as well since those regions traditionally reach peak beetle activity 2-3 weeks before us here in the mountains.
Growing degree days is a measurement of heat accumulation and is gathered by taking the daily average temperature minus a base temperature where insect development can occur. Typically this baseline temperature is 50 degrees for most insects, and is a useful tool to accurately estimate when insects emerge not based purely on calendar dates but on specific trends in temperature observed that can be linked to events in the insect or disease life cycle. GDD can easily be tracked by using calculators such as the Growing Degree Day Explorer offered by the North Carolina State Climate Office. This tool uses daily average temperature minus a base temperature to determine the GDD units for a particular day. If this measurement is negative, there are no units added cumulatively. For more information on GDD, you can visit another resource here.
An inexpensive way to monitor for ambrosia beetles is a simple trap design using soda bottles.
You can also monitor onsite using ethanol-baited traps. Ambrosia beetles are attracted to ethylene, a chemical produced by stressed plants in the landscape. Ethyl-alcohol formulated hand-sanitizers or ethanol can be a great way to bait ambrosia beetles. It is crucial to know when flights occur so that preventative chemical treatments will be most effective. Once ambrosia beetles infest a tree, there is no treatment that is effective against the beetle or the ambrosia fungus. Therefore knowing precisely when beetles are active is important. These traps can be homemade, and inexpensive. See a publication here to learn more about the construction of these traps.
Cultural Control:
Ambrosia beetles are attracted to ethylene, a chemical that is produced by stressed plants in the landscape. Typically this is related to stress induced by drought, overwatering/wet conditions, injury, or a soil nutritional issue. By providing plants with the necessary conditions needed to grow and thrive, we can mitigate the possibility of ambrosia beetle infestations at the source; however, it is always safe to assume that trees coming out of winter dormancy will be stressed. For growers this means actively monitoring nutritional amounts by using pour-thrus, or leaching fraction tests to optimize nutrition, monitor irrigation scheduling, and properly pruning trees/shrubs.
If ambrosia beetles persist in the residential or commercial landscape, you can also purchase plant material that is more tolerant to ambrosia beetles. For landscapers, this means proper plant establishment/planting, soil nutrient management, and irrigation scheduling are all critically important.
Chemical Control:
If cultural conditions are in place to mitigate damage, for high value trees another tool in management is a preventative chemical treatment. Control measures are typically contact sprays of pyrethroids such as permethrin or bifenthrin that need to be reapplied at intervals of 2-3 weeks during active flights. These chemicals are toxic to insects, beneficial and otherwise, and therefore should be applied in a targeted manner that minimizes drift. It is critical to apply these sprays prior to the majority of ambrosia beetle flights because once ambrosia beetles have bored into a tree, chemical control has little to no impact. It is important to direct these sprays on the main trunk in areas of diameter breast height or 4.5’ down to the surface of the pot or ground. Systemic products such as imidacloprid do not work because ambrosia beetles do not actively feed on the vascular tissue of the plant where these products work. For specific recommendations for your operation, reach out to your local extension office.
To learn more about ambrosia beetle management, visit these other helpful publications:
Ambrosia Beetle Pests of Nursery and Landscape Trees- NCSU Extension