Polyphagous Shot Hole Borer (PSHB)

The polyphagous shot hole borer may be tiny, but it sure has a huge appetite! This bug can damage so many different types of trees because it carries its own dinner from tree to tree--a fungus friend that lives in the beetle's galleries and provides food for the adults and larvae.  Aponi knows that avocado trees are among its favorites to attack, but she loves avocadoes too!  Good thing she also knows that firewood can spread this pest, and shouldn't be taken along on her camping trips.

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  • Side view of a female shot hole borer (Euwallacea sp.). These tiny black beetles are smaller than a sesame seed (2 – 2.5mm or 0.07 – 0.1inches long) and covered with spiky golden hairs.  Photo: Javier Mercado, Bark Beetle Genera of the U.S., USDA APHIS ITP, Bugwood.org

  • Top view of a female shot hole borer (Euwallacea sp.).  Photo: Javier Mercado, Bark Beetle Genera of the U.S., USDA APHIS ITP, Bugwood.org

  • The dark stains in the sapwood of this tree were caused by the fungus associated with the polyphagous shot hole borer, Fusarium euwallaceae, which not only causes the beetle's galleries to be stained black (seen as dots in this picture), but also moves into the tree's xylem, clogging it.  Photo: Akif Eskalen, University of California Riverside

  • Many adult female polyphagous shot hole borers, shown by red arrows, can be seen in this cross section of an infested tree. Their winding galleries can reach to a depth of around 3 inches (8 cm) into the tree's wood.  Photo: Akif Eskalen, University of California Riverside

  • Polyphagous shot hole borer (Euwallacea sp.) eggs are very tiny, oval-shaped, and whitish.  These eggs are seen within a cross-sectioned gallery.  Photo: Akif Eskalen, University of California Riverside

  • Life stages of a female polyphagous shot hole borer (Euwallacea sp.) from left to right: Young larva, older larva, pupa, immature adult, mature adult. Life cycle details  of PSHB are still being researched, but larvae have 3 instars and take about a month to develop into adults. The beetle is probably active year-round and may have between 2 and 4 generations per year in California.  Photo: Michael Lewis, University of California Riverside

  • Larvae of the polyphagous shot hole borer (Euwallacea sp.).  These tiny, white grubs live in galleries dug by their mothers, where they eat their symbiotic fungus (Fusarium euwallaceae).  Photo: Akif Eskalen, University of California Riverside

  • Closeup of a cross section of a polyphagous shot hole borer gallery made by a female beetle.  As she digs the gallery, she 'plants' spores of her symbiotic fungus, which then grows on the gallery walls, giving them a black color.  The beetle will then lay her eggs in the gallery, and the fungus provides a tasty snack for the hatching larvae!  Photo: Akif Eskalen, University of California Riverside

  • This young backyard avocado tree is showing wilting and branch dieback due to polyphagous shot hole borer attack.  Did you notice that it is right next to a big pile of firewood?  You can help stop the spread of PSHB by not moving firewood!  Photo: Akif Eskalen, University of California Riverside

  • Oozing sap (called 'bleeding') and dark patches around polyphagous shot hole borer holes are seen on the bark of some host trees.  Photo: Akif Eskalen, University of California Riverside

  • "Gumming" is a common tree response to attack by insects or pathogens.  Here, you see 'gumming' around polyphagous shot hole borer holes in the trunk of this golden raintree (Koelreuteria paniculata).  Photo: Akif Eskalen, University of California Riverside

  • Some trees really freak out when they are attacked by the polyphagous shot hole borer! Avocado trees (Persea americana) produce 'sugar volcanoes' which are piles of white powder around the beetle holes.  Photo: Akif Eskalen, University of California Riverside

  • Branch of a coast live oak (Quercus agrifolia), a native California species threatened by the polyphagous shot hole borer.  Photo: Joseph O'Brien, USDA Forest Service, Bugwood.org

  • Leaves and acorns of a coast live oak (Quercus agrifolia). This and other oak trees (Q. lobata, Q. engelmannii) are threatened by the polyphagous shot hole borer.  Photo: Wikimedia Commons, www.commons.wikimedia.org

  • Leaves of a boxelder tree (Acer negundo). Unlike most other maples, boxelder trees have compound leaves with 3 to 7 leaflets.  The polyphagous shot hole borer attacks this and other maples, which are often planted as landscape trees. Photo: Wikimedia Commons, www.commons.wikimedia.org

  • Seed clusters of the boxelder tree (Acer negundo).  The seeds are in long strings and come in pairs.  Each seed has a little wing attached to help it travel farther from the parent tree.  Photo: Robert Videki, Doronicum Kft., Bugwood.org

  • Leaves and fruit of a sycamore (Platanus sp.).  Also called 'plane trees,' they are often used as landscaping trees and are often found in the wild next to streams.  Several sycamore species are attacked and killed by the polyphagous shot hole borer.  Photo: Allen Dridgman, South Carolina Department of Natural Resources, Bugwood.org

  • Sycamores (Platanus sp.) have very cool multi-colored bark! Photo: Whitney Cranshaw, Colorado State University, Bugwood.org

  • Avocado trees (Persea americana) are medium-sized (can grow to over 60 feet) and have shiny, oval-shaped leaves. They are native to Mexico and central America.  Photo: Forest and Kim Starr, Starr Environmental, Bugwood.org

  • Avocado trees (Persea americana) have small, yellowish-green flowers located on the ends of branches.  You can also see the mature, pear-shaped avocado fruit in this photo.  Photo: Forest and Kim Starr, Starr Environmental, www.starrenvironmental.com

  • The mature fruits of the golden raintree (Koelreuteria paniculata) look like little brown paper lanterns with little black beans inside! Photo: Franklin Bonner, USFS (ret.), Bugwood.org

  • Leaves of the golden raintree (Koelreuteria paniculata) are compound, with 7 - 15 fern-like leaflets. This tree is native to China, but is planted widely in the U.S., and is damaged by the polyphagous shot hole borer.  Photo: The Dow Gardens Archive, Bugwood.org

  • The castor bean plant (Ricinus communis) has become a nasty invasive in many parts of the world, including California. It grows as a tall shrub with star-shaped leaves having 5-12 points, and is a favorite host for the polyphagous shot hole borer.  Beetle attacks may provide some control for this invasive species,but castor bean may also increase beetle numbers in an area, allowing them to have a greater impact on native trees.  Photo: Forest and Kim Starr, Starr Environmental, www.starrenvironmental.com

  • Seed pods of the castor bean plant (Ricinus communis).  The seeds inside these pods are very poisonous!  Photo: Forest and Kim Starr, Starr Environmental, www.starrenvironmental.com

  • Polyphagous shot hole borer galleries (black holes) and black staining from its Fusarium friend on a heavily infested castor bean plant.  Castor bean is one of the beetle's preferred hosts, and is itself an invasive species.  Photo: Akif Eskalen, University of California Riverside

  • Cross section of a tree infested with polyphagous shothole borer (Euwallacea sp.). The branching tunnels made by the beetle weaken the tree's structure, causing branches to break off.  Photo: Akif Eskalen, University of California Riverside

  • Dark staining from the Fusarium fungus can be seen around the beetle holes and deeper in the sapwood of this boxelder (Acer negundo) trunk.  Photo: Akif Eskalen, University of California Riverside

  • This sycamore tree (Platanus sp.) is in bad shape because of the polyphagous shot hole borer.  The brown leaves and dead branches are a result of damage to the tree's water conduction tissue, called "xylem."  Photo: Akif Eskalen, University of California Riverside

  • This castor bean plant has brown leaves and bare branches because its water-conducting tissue, or xylem, is clogged by the Fusarium fungus.  Photo: Akif Eskalen, University of California Riverside

  • Large avocado (Persea americana) trees shade a field and provide fruit.  Avocado is a very important crop in California, where it is grown on nearly 60,000 acres and the is worth around $435 million every year! Unfortunately, the polyphagous shot hole borer is a serious threat to this crop.  Photo: Forest and Kim Starr, Starr Environmental, www.starrenvironmental.com

Scientific Name:  Euwallacea sp.

Description:  The polyphagous shot hole borer is a tiny invasive black beetle smaller than a sesame seed (2 – 2.5mm or 0.07 – 0.1inches long) that was recently discovered attacking many different types of trees in southern California. Female beetles make tunnels, or galleries, in the trunks and branches of host trees and lay their eggs inside.  Males are much smaller, do not fly, and don’t leave the galleries. The females vector (or carry from tree to tree) a fungus (Fusarium euwallaceae) that grows in their galleries, and the adult beetles and their larvae depend on it for food.  While this fungus definitely helps the beetles out, it is really bad for the trees and clogs their water and food conducting tissues!

Native Range: PSHB is native to Southeast Asia, possibly Vietnam and Taiwan.

Introduced Range:  PSHB is found in a currently limited range on the coast of California, from Los Angeles south to San Diego. It has also been introduced in Israel.

Distribution: 

 Map From the Eskalen Lab, University of California, Riverside.

Habitat: So far, PSHB has been found infesting trees in neighborhoods, avocado farms, and several public gardens.  Because PSHB can attack so many different types of trees, native California woodlands are also at risk if the beetle continues to spread.

Host Trees: Part of the reason this bad guy is SO bad is that it has a huge appetite.  In fact, it has been known to attack plants in 58 different families! Many of these plants are important parts of native California ecosystems, and others are planted widely across the U.S. in cities and neighborhoods.  A few host species, like avocado and olive, are important in agriculture.  Here is a list of plants that the beetles can reproduce in: 

  • Acacia (Acacia spp.)
  • Box elder (Acer negundo)
  • Big leaf maple (Acer macrophyllum)
  • Evergreen Maple (Acer paxii)
  • Trident maple (Acer buergerianum)
  • Japanese maple (Acer palmatum)
  • Mimosa (Albizia julibrissin)
  • Titoki (Alectryon excelsus)
  • Tree of heaven (Alianthus altissima)
  • White Alder  (Alnus rhambifolia)
  • Camellia (Camellia semiserrata)
  • Moreton Bay Chestnut  (Castanospermum australe)
  • Blue palo verde (Cercidium floridum)
  • Brea (Cercidium sonorae)
  • Coral tree (Erythrina corallodendon)
  • Red Flowering Gum  (Eucalyptus ficifolia)
  • Chinese holly (Ilex cornuta)
  • Sweetgum (Liquidambar styraciflua)
  • Palo verde (Parkinsonia aculeata)
  • Avocado (Persea americana)
  • London plane (Platanus x acerifolia)
  • California Sycamore (Platanus racemosa)
  • Cottonwood  (Populus fremontii)
  • Black cottonwood (Populus trichocarpa)
  • Mesquite (Prosopis articulata)
  • Coast live oak (Quercus agrifolia)
  • Engelmann Oak (Quercus engelmannii)
  • Valley oak (Quercus lobata)
  • English Oak (Quercus robur)
  • Cork Oak (Quecus suber)
  • Castor bean (Ricinus communis)
  • Weeping willow (Salix babylonica)
  • Goodding's black willow (Salix gooddingii)
  • Red Willow (Salix laevigata)
  • Japanese wisteria (Wisteria floribunda)

The Facts: The polyphagous shot hole borer is an ambrosia beetle.  You might think of ambrosia beetles as the “farmers” of the insect world!  Rather than eating bark or wood like many of their cousins, these guys feed on fungus.  But, the cool part is that they grow the fungus themselves!  Ambrosia beetles have special pits or dents in their bodies (called mycangia) that are used to carry the spores (think of these as the farmer’s seeds) of the fungi that they eat.  When a female beetle finds a suitable tree, she chews a tunnel into the sapwood and “plants” the fungus as she digs.  The fungus grows into the tree’s wood and on the walls of the beetle galleries—a fresh crop of fungal food for both the adult and young beetles!  The relationship between the beetle and the fungus is called ‘symbiotic,’ meaning that these two very different species depend on one another—the fungus provides a ready source of food for the beetle and the beetle provides the fungus with a free ride to new trees.  How smart is that?

In their native homes, ambrosia beetles usually only attack sick or dead trees.  However, when they are accidentally moved to new habitats, they may begin attacking healthy trees—this is what has probably happened with the polyphagous shot hole borer.

Signs and symptoms of PSHB attack can vary a lot depending on the type of tree, however, 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 and break off, revealing webs of galleries filled with black fungus. 

As with many tree pests, this bad guy and its fungus friend can be moved in firewood.  You can help stop its spread by never taking firewood with you when you go on a trip!