Shot Hole Borer Infestations

How to Recognize, Report, and How You Can Help

 

Sometimes it’s the Smallest Pests that Cause the Biggest Problems

In nature, even the tiniest creatures can wreak havoc. Meet the Shot Hole Borer, a minuscule yet menacing beetle measuring a mere 0.05-0.1 inches long. Despite its diminutive size, this pest, along with its two indistinguishable sub-species – the Polyphagous Shot Hole Borer and Kuroshio Shot Hole Borer, pose a significant threat to trees and ecosystems alike.The Shot Hole Borer is a type of dark brown or black beetle that measures about 0.05-0.1 inches long. It’s two subspecies, the Polyphagous Shot Hole Borer and Kuroshio Shot Hole Borer, which are identical in behavior and appearance and can only be distinguished through their DNA. 

Understanding the Threat

These pests attack and bore holes about 0.85mm in diameter in over 300 species of trees, including the red willow, cottonwood, and castor bean. They also use over 50 tree species as reproductive hosts, introducing pathogenic fungi into the tree that supports more Shot Hole Borer colonies. Despite their small size, these pests can leave a wake of destruction in their path by causing significant damage to trees and ecosystems.

Where Did the Shot Hole Borer Come From?

The Shot Hole Borer is believed to have originated in Southeast Asia, where it evolved alongside natural predators and host trees that could tolerate its presence. It was first identified as an invasive species in Israel in the early 2000s and was later discovered in the United States, with California reporting infestations in 2012. Since then, it has spread rapidly, particularly in warm and temperate regions.

Where is the Shot Hole Borer Causing Problems?

In the U.S., the Shot Hole Borer has primarily impacted California, especially in Southern California counties like Los Angeles, Orange, San Diego, and Riverside. However, infestations have also been detected in Florida and Hawaii. Globally, this invasive beetle has been found in South Africa and parts of the Mediterranean, where it continues to threaten native and agricultural tree species. In California, the invasive shot hole borers (ISHB), specifically the polyphagous (PSHB) and Kuroshio (KSHB) species, have significantly impacted various tree species across the state. Here’s an overview of their infestation:

Affected Tree Species:

ISHB has been known to attack 66 different tree species in California, including native species such as box elder (Acer negundo), valley oak (Quercus lobata), arroyo willow (Salix lasiolepis), black cottonwood (Populus trichocarpa), and California sycamore (Platanus racemosa).

Recognizing the Signs

When a tree is infested with a persistent beetle, it may exhibit a range of symptoms. Spotting an infested tree isn’t always straightforward, but there are telltale signs to watch for. Keep an eye out for bark discoloration, wilting leaves, branch dieback, fungal growth, and the presence of frass – the telltale debris left behind by these invasive pests.The tree may also show signs of gumming, which is a natural response to the threat. Additionally, there may be debris, or excrement present, produced by the insects.

How Can We Combat These Pesky Shot Hole Borers? 

Fortunately, there are proactive steps we can take to combat Shot Hole Borer infestations. The National Park Service’s “Buy it where you burn it” campaign encourages responsible firewood practices, urging campers and homeowners to source firewood locally from their home or camping destination. This will help reduce the amount of infected firewood being transported to other locations to prevent the spread of beetle or fungal infected wood.  

In terms of treating plant material that has already been infected, chipping it to under 1-inch chunks and composting or solarizing it afterward has proven to be an effective strategy. Solarization involves covering the plant material with clear plastic sheeting or tarp and exposing it to the sun for 6 weeks to 6 months (depending on the season) to heat the plant material and kill any beetles or fungi present.

In addition to these strategies, there are a few other ways to manage Shot Hole Borer infestations. In their natural habitats, there are some predators, such as certain species of parasitoid wasps, that can help control the beetle population. However, these natural predators are not always present in invaded areas, making it difficult to rely on them for control.

Other proactive measures include:

• Biological control: Research is ongoing into introducing or enhancing natural predators like parasitoid wasps that specifically target the Shot Hole Borer.
• Chemical treatments: In some cases, insecticides and fungicides may be used, though they need to be applied carefully to avoid harming beneficial species or the environment.
• Tree removal and sanitation: Removing infected trees and properly disposing of them can help limit the spread. Ensuring proper sanitation practices when pruning or removing trees can reduce the risk of further infestation.

Are There Natural Predators?

In its native habitat, the Shot Hole Borer has natural predators, including certain parasitic wasps and fungi that help keep its population in check. However, these biological controls are not present in many of the regions where the beetle has invaded. Researchers are currently studying whether introducing natural predators into affected areas could be a viable solution without harming native species.

Reporting

If you have identified symptoms that are certainly the work of the Shot Hole Borer and suspect an infestation in your area, swift reporting is essential. Suspected tree infestations can be reported to eskalenlab@gmail.com (the County of San Diego Parks and Recreation regional task force), sdcawm@sdcounty.ca.gov, and/or www.pshb.org. 

Conclusion:

In nature, every living organism has a crucial role to play, no matter how small it may seem. We can safeguard our trees and ecosystems from the destructive impact of Shot Hole Borers by being watchful, reporting sightings, and taking preventative measures. 

We extend our gratitude to the biologists at Summit West for their diligent efforts in identifying and addressing the threat of Shot Hole Borers to trees. Their expertise highlights the importance of taking proactive measures to protect our ecosystems. Together we can ensure the protection and preservation of our natural environment for generations to come. 

Sources

Sdparks.org. “Frequently Asked Questions: Shot Hole Borers.” Sandiegocounty.gov, County of San Diego Parks and Recreation, https://www.sandiegocounty.gov/content/dam/sdc/parks/MiscellaneousDocuments/FAQ        %20Shot%20Hole%20Borer.pdf.

“Shot Hole Borer.” Sandiegocounty.gov, San Diego County, 13 Jan. 2015, https://www.sandiegocounty.gov/content/sdc/pds/zoning.html

https://www.arborwell.com/shot-hole-borer/