By James A. Bethke.
Ambrosia beetles are a group of beetles in the bark beetle family, Scolytidae. However, there is some argument by taxonomists that the group actually belongs to the Family: Curculionidae (weevils and snout beetles) as a subgroup Scolytinae. That’s for the taxonomists to argue about. Ambrosia beetles can have profound effects on plants, mostly tree species, by feeding on the cambium layer or sapwood. They tend to attack smaller branches in the upper crowns of trees, but they have also been found on the main stems of large trees. A characteristic indicator of an infestation is the presence of small toothpick like structures, which are frass or excreta of feeding beetles being pushed out of feeding galleries (Figure 1). The toothpicks radiate from the trunks and branches of infested trees. If the inside of the trunk is dissected, it usually reveals galleries and staining caused the symbiotic fungus.
Ambrosia beetles (Figure 2) are small (2mm, 0.08 inches) and they are usually associated with one species of fungi. The beetles don’t really eat wood, but instead excavate tunnels in trees in which they lay eggs and cultivate the fungi as food. Both adults and immature beetles feed on the fungi. The fungi stain the wood blue, black, or dark brown (Figure 3). The fungal spores are carried from tree to tree on the adult beetle’s exoskeleton near the mouthparts. This makes sense because they were associated with a fungal infection caused by their parents during development to the adult stage. Once galleries are started and spores released, the spores penetrate and destroy the plants vascular system. If enough of them circle the tree, some of these fungal gardens can girdle and kill the tree.
Ambrosia beetles are very common in California and regularly infest deciduous fruit trees, landscape trees and oak and pine forests. Some of the more common pest species include: Striped Ambrosia Beetle, Western Hemlock Wood Stainer, the Lesser Shot Hole Borer, Red-shouldered Shot Hole Borer, Wilson's Wide-headed Ambrosia Beetle, the Large California Oak Ambrosia Beetle, and many others. The ones you are probably most familiar with are the Polyphagous and Kuroshio shot hole borers (Euwallacea sp.), which have been destroying avocado trees in Southern California, willows in the San Diego River bed, and many large mature trees in urban areas such as the box elder trees in Long Beach.
Here locally, I have seen dracaena stock that was infested with ambrosia beetles. This is not an uncommon occurrence. There have been reports of ambrosia beetles infesting dracaena for 50 years. It is usually a localized occurrence, which means not every shipment or every stalk of dracaena coming from other countries is infested. However, these infestations should be taken seriously, and infested dracaena should be bagged and destroyed so that they do not persist.
As I mentioned, ambrosia beetles are very common in California, and they can infest just about every tree you can think of. They are a big part of the decomposition of forests, but they can be serious pests, especially when they are introduced as an invasive species. Keep your eye out for the toothpicks on tree trunks. The next time you see them, you will know more about what causes them than just about anyone.