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The month of May is when two of our more common ambrosia beetles here in the Northeast start to become noticeable in landscapes. Ambrosia beetles are a group of small beetles that attack trees by boring into tree wood producing tunnels (galleries) that are used for rearing of young. Adult beetles carry a fungus that is transmitted to the wood of a tree as tunneling is occuring. This fungus colonizes the wood of a tree and serves as a food source for both young and adult ambrosia beetles. Trees are damaged by both the boring activity of the beetles, which often occur in large numbers, and destruction of tree conductive tissue by the fungus.
Two of the more common ambrosia beetles here are the black stem borer and the granulate ambrosia beetle. Both become active in early spring as buds are swelling and leaves begin to emerge. Their presence is indicated by collection of “sawdust” (results of their tunneling activity) on tree wood. Holes are generally 1-2mm depending on insect species with black stem borer producing somewhat smaller holes. When attacks are extensive there may be an accumulation of “sawdust” at the base of trees. Both of these ambrosia beetles produce thin, toothpick-like extrusions of compacted wood borings that may approach an inch or more in length.
They seem to be attracted more to stressed or declining trees but there are reports of healthier trees also being attacked. While there are several generations of the insects here, much of the damage takes place in the early spring. Treatments consist of bark-applied insecticidal sprays that should be initially applied when early activity is beginning. Follow-up treatments may need to be applied based on degree of control and infestation, chemical and rates used and thoroughness of coverage. Once activity has produced numerous “toothpicks” control is not very effective as the insects are protected from the chemical. Weakened or severely attacks trees may not survive.