Invasive Shot Hole Borers Threatening Trees in Southern California

[From the December 2016 issue of the UC IPM Green Bulletin]

The Polyphagous shot hole borer (PSHB) (Fig. 1) and Kuroshio shot hole borer (KSHB) are invasive wood-boring beetles that attack dozens of tree species in Southern California, including commercial avocado groves, common landscape trees, and native species in urban and wildland environments. Both beetles spread a disease called Fusarium Dieback (FD), which is caused by pathogenic fungi. Trees that are FD-susceptible may experience branch dieback, canopy loss, and tree mortality (Fig. 2).

Insect Vector

PSHB carries three fungi: Fusarium euwallaceae, Graphium euwallaceae and  Paracremonium pembeum. KSHB carries two new species of fungi: Fusarium sp. and Graphium sp. Mature females of both species are black and 0.07­ to 0.1 inches (1.8–2.5 mm) long, whereas males are brown and smaller than females at 0.06 inches (1.5 mm) long. The female attacks a wide variety of host trees forming galleries (Fig. 3), where she lays her eggs. Mature siblings inbreed inside galleries and the pregnant females leave to establish new galleries in the same host or nearby hosts; most wingless males, however, remain in maternal galleries.

The tiny beetles tunnel into host trees and spread the fungi that cause FD disease. Beetle larvae within the gallery in infected trees feed on the fungus, forming a symbiotic relationship between the fungus and beetle. Fusarium Dieback stops the flow of water and nutrients in over 48 susceptible tree species, which can lead to the death of individual branches or, in severe cases, an entire tree (Fig. 2).

Symptoms

External: A host tree's visible response to disease varies among host species. Sugary exudate (also called a sugar volcano) (Fig. 4), staining (Fig. 5), gumming (Fig. 6), and frass (Fig. 7) are among symptoms that may be noticeable before the tiny beetles are found. The beetle's entry holes, which are approximately 0.03 inches (0.85 mm) in diameter, can be located beneath or near the symptoms. Advanced fungal infections will eventually lead to branch dieback.

 

Internal: The fungi interrupt the transport of water and nutrients in branches of affected trees, leading to wood discoloration which can vary in color from brown to black. Shaving outer layer bark with a clean knife around beetle entry holes reveals obvious wood discoloration. Cross-sections of cut branches around affected areas show the extent of infection (Fig. 8).

These two beetles and their symbiotic fungi have a wide variety of suitable hosts. This wide host range makes landscape, native riparian, oak woodland, and mixed evergreen communities highly susceptible to invasion and mortality by PSHB/KSHB-FD.

Management on Landscape Trees

Chemical and biocontrol management strategies are currently being investigated for this pest-disease complex. Early detection of infestation and removal of the infested branches will help reduce vector populations and the spread of this pest-disease complex.

If you or your customers suspect PSHB or KSHB is affecting trees in your area, please contact your local Agricultural Commissioner's office or UC Cooperative Extension office.

Cultural/Sanitation Practices

The removal of the heavily infested reproductive hosts will help reduce vector populations and the spread of this pest-disease complex.

• Chip infested wood onsite to a size of one inch or smaller. If the branch is too large to chip, solarize them under a clear tarp:
• Have wood chips composted at a professional composting facility that has earned the U.S. Composting Council's Seal of Testing Assurance at: http://compostingcouncil.org/participants/. Sterilize pruning tools with either 5% household bleach, Lysol cleaning solution, or 70% ethyl alcohol to prevent the spread of the pathogens through pruning tools
• Avoid movement of infested wood and chipping material out of infested areas unless the material is covered or contained during transport.
• Transport wood or chips to a biogeneration facility (biogeneration facilities burn green waste and convert it into energy).
• Transport wood or wood chips to a landfill where it will be used as Alternative Daily Cover.

• July - August: cover chips/logs with sturdy plastic for at least 6 weeks. Temperatures during these months should be regularly above 95°F (35°C)
• September - June: cover chips/logs with sturdy plastic for at least 6 months

For more information, visit the UC Riverside Eskalen Lab website (http://eskalenlab.ucr.edu) or pshb.org.

*Native species to California

~AkifEskalen¹ akif.eskalen@ucr.edu, Joey S. Mayorquin¹, Joseph D. Carrillo¹, Shannon C. Lynch¹, John Kabashima², Tim Paine¹, Richard Stouthamer¹, Frank Byrne¹ and Joseph Morse¹. ¹Department of Plant Pathology & Microbiology and Department of Entomology, University of California, Riverside. ²UCCE Orange County.

-all photos by Akif Eskalen.