This insect has a broad range but is of particular concern in the southern and eastern regions of the US. The term Ambrosia beetle refers to a number of species of the insect. 40 or so are native, with several destructive non-native species as well. Native beetles prefer stressed and weakened trees, while exotic species will attack both stressed and healthy trees. On small caliper trees, the burrowing of multiple beetles can girdle the tree. On larger caliper trees pathogenic fungus introduced by the beetle can damage the tree’s vascular system.
Prevention is the most reliable method available. The beetle does not feed on the host tree, so most systemic insecticides are not effective. Trunk and limb bark sprays have traditionally been the industry standard for this pest, however, arborists have recently begun using Mectinite on high value trees that can be root flare injected. Trees that are adjacent to or near infested trees should be considered for preventive treatment.
Maintaining tree vigor can help. Most native species will not attack healthy trees, but attack weakened, dying, or dead trees with enough wood moisture to support their symbiotic fungal growth. Cambistat can also be used to promote tree health and reduce the incidence and severity of stress mediated diseases and insect problems on trees growing in urban areas.
Some exotic species will attack both weakened and healthy trees.
Other Treatment Practices
- Some species of Ambrosia beetle feed on dead or dying wood. Removal of infested material near the treated tree may help reduce local beetle activity. Maintaining general tree health will help deter native beetles, but possibly not exotic ones.
- Some arborists have been combining 10ml Alamo +30ml of water per inch DBH to manage introduced pathogens and improve efficacy of treatments.
- Adequate water is a key factor in maintaining stress free plants. A slow, deep watering event once every few weeks during dry conditions will help maintain soil moisture levels and minimize the stress that invites ambrosia beetles.
- Mulch is very beneficial for all trees because it reduces competition with turf and moderates soil temperature and moisture levels. The addition of 3 inches of wood chips or shredded bark under the drip line can have a very beneficial effect by holding in moisture and promoting healthy fibrous roots.
- Cambistat can be also be used to promote tree health and reduce the incidence and severity of stress mediated diseases and insect problems on trees growing in urban areas.
Trees that are adjacent to or near infested trees should be considered for preventive treatment. Maintaining tree vigor can help. Most native species will not attack healthy trees, but attack weakened, dying, or dead trees with enough wood moisture to support their symbiotic fungal growth. Some exotic species will attack both weakened and healthy trees. Data from UF and USDA states that no immature phases of sweetbay ambrosia beetle were found after application of emamectin benzoate 4%
Signs of Damage
- 1.4 mm-4 mm dark brown to black beetle
- Eggs and larvae are minute (smaller than a pencil tip) and are rarely seen
- 1” to 2” long frass tubes the thickness of pencil lead are a dead giveaway
- They are rare though; being weak, they will break in wind and rain
- Some species begin feeding at the bottom of the trunk; others can attack near the base of twigs and branches
- “Sawdust” that is actually frass can accumulate at the base of the tree
- Pencil lead sized holes called “shotholes” at the base twigs where the brood has exited the host
Photo: Louis-Michel Nageleisen
Departement de la Sant des Forts, Bugwood.org
Photo: Albert (Bud) Mayfield
USDA Forest Service, Bugwood.org
Photo: Gerald J. Lenhard
Louisiana State University, Bugwood.org
Trees at Risk
Reported to damage over 100 species of trees. Many ambrosia species will preferentially attack smaller trees , orchard or new plantings in particular. Common species include:
- Ornamental cherry
- Japanese maple
- Golden rain tree
- Sweet gum
- Chinese elm
- Ambrosia beetles become active in early spring (forsythia bloom/red bud bloom).
- Beetles remain active through the summer and into the fall.
- Nursery stock is attacked primarily in spring during the first generation but landscape trees may be attacked all summer.
- Females bore into twigs, branches, or small trunks of susceptible hosts. They excavate tunnels in the wood, introduce ambrosia fungus, and lay eggs to produce a brood. It is the growing fungus on which the beetle grubs feed, not the wood.
- Eggs, larvae and pupae are found together. Females remain with their brood until maturity.
- Males are small and flightless and often remain within the gallery. New females mate with their brothers, if present, before emerging to attack a new host.
- On average 55 days for the insect to complete one generation, but can be faster depending upon temperature.
University of Minnesota
University of Florida
North Carolina State University
Always refer to product label for rates and approved uses. Some images courtesy of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments.