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Ambrosia beetles are common wood borers that attack host trees weakened by or dying from drought, winter injury or other factor. They get their name from its association with ambrosia fungi it transfers to tunnels it excavates in trees.
Many native and ornamental deciduous trees and fruit trees (apple, cherry and prune), especially adjacent to forests. The species attacking fruit trees rarely conifers.
Leaves - Sudden wilting and death; delayed emergence in spring.
Branches - 1.5 mm-wide tunnels through bark into heartwood (shothole borer does not tunnel into wood) in a circular or cork-screw pattern with the rings; wood shows purple stain extending from tunnels due to fungus released by the adult beetles (Fig. 1). Severely infested branches can break easily due to gusty winds or fruit load.
Figure 1. Adult ambrosia beetle. (BCMA)
Trunk - Tunneling into trunks of young trees can weaken trunks to the point they can break. Adults push out sawdust as they tunnel into the wood (Fig. 2) which can be used to detect their presence.
Figure 2. Sawdust created by ambrosia beetle. (BCMA)
Larva - White, legless, slightly curved, up to 4 mm long.
Adult female - Dark reddish-brown beetle about 3 mm long.
Adult male - Wingless beetle about 1.5 mm long.
Ambrosia beetles overwinter as adults within galleries in the heartwood. Adults emerge in late March and April and fly to new trees where the females bore tunnels in which they lay their eggs. Larvae feed on ambrosia fungus growing in the tunnels. There is one generation per year.
Look for small entry or exit holes (2 mm). Sawdust at the entry of tunnels indicates recent attacks by adults.Sticky material on tree trunks may help detect adults. Use ethanol-baited funnel traps (2 to 4 traps/acre) to monitor adults in the spring. Control beetles if more than 20 beetles are caught /trap/season.
It is important to maintain optimum tree vigor as ambrosia beetles are attracted to trees weakened due to drought, flooding, transplanting, disease, winter and mechanical injury. In areas prone to sun or winter damage, paint trunks with a solution of equal parts of latex paint and water to prevent damage to trees. Do not store wood with bark near orchards as it can remain a source of ambrosia beetles for up to a year. Remove and promptly burn or chip beetle-infested trees or limbs. If the wood cannot be burned, chipped or otherwise destroyed, cover the wood with plastic to prevent beetles from attacking the wood or escaping if already infested. To intercept female beetles flying into an orchard (males do not fly), place bundles of three or four 1- to 2-metre long hardwood logs (harvested the previous year) at 10-20 metre intervals along borders adjacent to deciduous (hardwood) trees. Puncture the bark several times to improve release of odors attractive to the beetles. Destroy the trap logs before new adults appear.
There are no registered products for controlling ambrosia beetles on apples. Good sanitation is the best management strategy.