The invasive Asian polyphagous shot hole borer (PSHB) - also known as an ambrosia beetle - is a tiny black beetle the size of sesame seed (2mm).

The adult females:

•  Carry a fungus with them from tree to tree.
•  Burrow into a tree - 60 species have been affected worldwide.
•  Make tunnels or galleries in the trunks and branches of the host tree.
•  Deposit the fungus they carry, which then grows into the walls of the wood in the tunnels.
•  Lay their eggs in the tunnels or galleries.

The fungus grows in the galleries. The adult beetles and their larvae depend on this fungus for food. 

Males are much smaller, do not fly and do not leave the galleries.

Symbiotic relationship

The beetles and the fungus have a symbiotic relationship - which means that these two very different species depend on each other to survive:

• The fungus provides a ready source of food for the beetle
• The beetle provides the fungus with a free ride to new trees.

Why do trees invaded by the borer beetles experience die-back or death?

Whilst the fungus feeds the beetles, the fungus is really bad for trees, as it clogs their water and food conducting tissues.

It is the maze of tunnels filled with beetles and fungus inside the trunk of the tree, together with the clogging of the arteries of the tree - that eventually kills the tree.  

What do landscapers 'look for' when inspecting for a shot hole borer invasion?

Signs and symptoms of PSHB attack can vary a lot depending on the type of tree. 

However this is what you need to look for:

• Tiny beetle entry and exit holes (a bit smaller than the tip of a ballpoint pen) are usually present in the tree’s bark. 
• Sawdust is often found around the holes or on the ground around the trunk.
• Sometimes a sawdust “toothpick” can be seen sticking out of the beetle hole. 
• In areas around the beetle holes, trees may have ‘bleeding’ symptoms (liquid oozing out of the bark), ‘gumming’ symptoms (blobs of goo coming out of the bark), or ‘sugar volcano’ symptoms (little cone-shaped piles of white powdery stuff on the bark). 
• PSHB causes branches to die and can eventually kill the whole tree. 
• Branches can also be weakened by the beetles’ tunneling. 
• The branches break off, revealing webs of galleries filled with black fungus. 
• The PSHB and its fungus friend can be moved in firewood. 
• Cutting down and moving dead trees is also a vector for spreading the invasion. 

An academic explanation

The team at the Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria explains the life cycle of the PSHB and how it attacks your trees. 

As adult female beetles burrow into trees to establish brood galleries, they introduce the symbiotic fungus Fusarium euwallaceae which colonises gallery walls, becoming a food source for developing larvae and adult beetles (Eskalen et al., 2012; Mendel et al., 2012).

The fungus then invades tree vascular tissue, causing cambial necrosis, branch dieback and death of a broad range of trees (Eskalen et al., 2013).

If you are worried about the trees in your landscape - contact the professionals - contact a SALI member.