Protect young trees from granulate ambrosia beetles

Heather N. Kolich, ANR Agent, UGA Extension Forsyth County

extreme close-up of beetle larvae inside a tree — Granulate ambrosia beetle larvae mature to adult beetles inside trees, living in chambers bored by their mother and eating “ambrosia” fungus she spread into the tree. Photo by Will Hudson, UGA.

While most boring beetles attack only dead and dying trees, non-native granulate ambrosia beetles (Xylosandrus crassiusculus), formerly known as Asian ambrosia beetles, also attack many species of healthy young trees. They tend to attack deciduous trees with thin bark, such as maple, dogwood, redbud, crape myrtle, ornamental cherry, fig, peach, and plum trees. Granulate ambrosia beetles (GAB) can kill trees. Prevention is the key to protecting valued landscape and fruiting trees from GAB damage.

How GABs injure trees

Granulate ambrosia beetles are small – only about 1/8^th of an inch long – but they can cause significant damage to trees. Like other boring beetles, female GABs bore into the trunk of young trees, leaving toothpick-like sawdust extrusions on the tree as signs of their presence. Once inside the tree, the female bores multiple tunnels into which she lay eggs. Along the way, she introduces an “ambrosia” fungus that will grow, spread, and feed the larvae when they hatch. The larvae pupate and mature into adults inside the tree. The new adults mate, and the females emerge from the tree and fly to other trees to start the cycle again.

The fungus continues to grow inside the tree, adding another layer of injury to the boring damage. The fungus clogs the vascular system, preventing the flow of water and nutrients throughout the root and shoot systems, and potentially killing the tree.

GAB preventive management strategies

Maintaining healthy trees is the first line of defense against GAB issues. This begins with selecting the right tree for the planting environment and planting it properly. Management actions that reduce tree stress factors include applying supplemental irrigation as needed, pruning appropriately to balance growth and allow sunlight infiltration and air circulation for fungal disease mitigation, and applying mulch (3-4 inches deep) around the tree and out to the furthest edge of the canopy. The mulch island serves several purposes: It conserves soil moisture, moderates soil temperature, and provides weed control – which helps protect the tree trunk from damage caused by string trimmers and mowers.

toothpick like extrusions coming from the trunk of a tree. — Female granulate ambrosia beetles bore into young, thin-barked trees, leaving toothpick-like extrusions of sawdust sticking out of the trunk. Photo by Byron Rhodes, UGA.

Timely application of pyrethroid insecticides – before GABs become active – can protect trees from infestation. GAB activity begins in the spring; however, since warm days in late winter can stir them up, preventive chemical applications of pyrethroid insecticides (such as permethrin and bifenthrin) to the trunks of young, susceptible trees should start mid-February. Make additional pyrethroid applications at the interval stated on the product label throughout the spring. GAB activity is lower during summer and fall.

Why post-infestation treatments don’t help

Once GABs are inside the tree, chemical treatments are ineffective. Systemic insecticides don’t work because the larvae and adult beetles eat the ambrosia fungus, not the tree. Fungicides cannot stop the spread of the ambrosia inside the tree.

To stem the spread of GABs to other trees, remove and destroy infested wood and trees before females mature.