Polyphagous Shot Hole Borers

Your tree may house a tiny, fungi-farming beetle called the polyphagous shot hole borer, but I hope not.

Polyphagous shot hole borer (Rachel Osborn, Ambrosia Beetle ID, USDA APHIS PPQ, Bugwood.org)

Native to southeast Asia, these invasive beetles are threatening trees in Israel and California with Fusarium dieback. Fusarium dieback is a fungal disease that blocks the flow of water and nutrients through a tree’s vascular system. And polyphagous shot hole beetles actively farm those particular fungi. We will get to that in a minute.

Polyphagous shot hole borer identification

Polyphagous shot hole borers (Euwallacea fornicatus) are smaller than a sesame seed. You could fit 6-10 females, end-to-end, across a dime. Females are black and males are brown and wingless, but you will probably never see a male. Sightings are rare and no wonder. Males stay in the galleries and you could fit 12-18 of them across the face of a dime.

Polyphagous shot hole borers look identical to another invasive borer called the Kuroshio shot hole borer, or tea shot hole borer (Euwallacea fornicatus). The tea shot hole borer prefers tea plants in Sri Lanka, while the polyphagous shot hole borer appears to have a voracious appetite for over 110 tree species. [The word polyphagous means eats many things.]

Adult female polyphagous shot hole borer (Dgomezuf) CC BY-SA 4.0

Host trees and signs of infestation

Traditionally, polyphagous shot hole borers tended to only infest dead or dying trees. Having been accidentally introduced to new regions, these pests have developed a taste for healthy trees. Once trees are infected, they can die. Host trees include:

California sycamore with extensive polyphagous shot hole borer damage (Trish Gussler, Public Domain)

• acacia (Acacia spp.)
• avocado (Persea americana)
• blue palo verde (Cercidium floridum)
• box elder (Acer negundo)
• California buckeye (Aesculus californica)
• California sycamore (Platanus racemose)
• camellia (Camellia semiserrata)
• castor bean (Ricinus communis)
• coral tree (Erythrina corallodendron)
• cottonwood (Populus fremontii)
• Japanese wisteria (Wisteria floribunda)
• maples (Acer spp.)

• mesquite (Prosopis articulata)
• mimosa (Albizia julibrissin)
• Moreton Bay chestnut (Castanospermum australe)
• Mulefat (Baccharis salicina)
• oaks (Quercus spp.)
• olive (Olea europaea)
• palms
• silk tree (Albizia julibrissin)
• Sweetgum (Liquidambar styraciflua)
• Titoki tree (Alectryon excelsus)
• Tree of Heaven (Alianthus altissima)
• willows (I spp.)

External symptoms of infestation often look innocuous. Slightly weepy, small damaged areas of the bark, the presence of white frass, maybe a little sawdust or sugar volcano action is all you can see from the outside. If you look very closely, you may see several exit holes, about the size of the tip of a ballpoint pen. The inside of an affected tree is something else entirely.

Polyphagous shot hole borers chew holes that penetrate 1/2” to 1-1/2” into the wood. Then they start burrowing, creating galleries. Black flecks and tunnels can be seen throughout an infested tree. These black areas indicate where Fusarium fungi are being farmed.

Polyphagous shot hole borer galleries (Spidra Webster, Public Domain)

Polyphagous shot hole borer as farmers

Polyphagous shot hole borers are a type of ambrosia beetle. Rather than feeding on bark or wood or sap, ambrosia beetles eat fungi that they grow for themselves. Polyphagous shot hole borers have tiny pockets on their exoskeleton. In these pockets, they carry spores of the Fusarium euwallaceae fungi. After burrowing into a tree, the borer starts growing these fungi along the walls of the burrowed galleries. The fungi provide adult and larval forms of polyphagous shot hole borers with food in a protected environment and the borers carry the fungi to new trees. It's a win-win situation for them. The problem is, this fungi causes Fusarium dieback. Fusarium dieback causes branch dieback, canopy loss, and it can kill trees.

Polyphagous shot hole borer management

Yellow sticky cards, purple prism traps, and multiple funnel traps have been used with some success. Because polyphagous shot hole borers have no natural enemies here in California, and because they live inside the tree, safe from insecticides, prevention is worth the effort.

Polyphagous shot hole borers are most commonly spread on firewood. If infested trees are chipped into mulch, the borers can catch a ride to your trees, so always inspect wood chips before accepting them. Wood chips cut into pieces smaller than 1” are generally considered safe because the borers get chopped up too. Personally, if I saw black galleries, I would refuse delivery just in case.

If you suspect polyphagous shot hole borers have found your trees, please contact your local County Extension Office right away.