JULY 2016 – Since the first sighting in San Diego in 2012, we have been watching the spread of the Polyphagous Shot Hole Borer (PSHB) and the Kuroshio Shot Hole Borer (KSHB) throughout Southern California, affecting the health of native tree species that are vital to the area’s ecosystem.
Considering the way these pests have begun to impact the San Diego landscape, we feel it’s important to share information with you that will help defend against them. In this blog post we will share the insects’ tell-tale warning signs, the damage they inflict, how they spread, and what you can do prevent and treat infestations.
What trees in San Diego County are being impacted?
The Shot Hole Borer has been known to attack over 200 species of trees, and has 38 different hosts. It has been found in tree species native to California, including the Avocado, Castor Bean, Coast Live Oak, California Sycamore, Red Willow, Black Willow, and Cottonwood.
With so many avocado groves in our North County hills, and castor bean farms in the Tijuana River Valley Regional Park, many large swathes of California are being infected. The highest impacted area is the Tijuana River Valley Regional Park in San Diego County.
Silly to Say, Harder to See: Facts about the Polyphagous (PSHB) and Kuroshio (KSHB) Shot Hole Borers
Have you tried pronouncing Polyphagous or Kuroshio out loud? If not, go ahead. Try it. Their names are silly sounding and hard to pronounce, but part of what makes them difficult to fight is that these beetles are tiny and hard to see.
The invasive insects drill holes approximately 1 to 4 centimeters (0.4 to 1.57 inches) in diameter into the wood of tree trunks and branches, and then bore their way through, leaving a random network of tunnels inside.
The beetles are smaller than the tell-tale holes they leave behind. Females are black and about 1.8 to 2.5 millimeters (0.07 to 0.1 inches) long. The males are more rarely found, and are even smaller, at about 1.5 to 1.67 millimeters (0.06 to 0.065 inches) long.
How the Shot Hole Borer Grows, Spreads, and Damages Trees
The Shot Hole Borer burrows into the trunks or branches of trees, creating tunnels. They lay their eggs, and embed a pathogenic fungus as food for their developing young, or larvae. Within one month from hatching, the fungus can provide enough nutrition for the larvae to reach adulthood. Most borers that do reach adulthood are females, who then mate with males and create new colonies of their own on surrounding trees. The Shot Hole Borer can fly 1 to 2 miles to their new home, and can also migrate from fallen tree limbs, water channels, and from wood and chippings.
The tunnels created by the beetles and the fungus that’s deposited, makes trees fragile, which results in damaged and broken trunks, limbs, and branches – and can eventually kill them. It is vital to treat trees at the first sign of infestation.
How to Treat and Contain the Polyphagous (PSHB) and Kuroshio (KSHB) Shot Hole Borer
Call the experts. Infested trees can be treated a variety of ways. Before you begin, we recommend that you consult with your community’s local authorities (Green Waste, Services, Parks & Recreation) to confirm their requirements for treatment and disposal.
Watch for damage. It is important to spot and treat affected trees as early as possible in order to prevent dead and falling limbs, habitat loss, and further devastation to surrounding areas. Trees damaged by the Shot Hole Borer are more susceptible to fires.
Cut off affected areas and solarize. This method takes time, and kills the beetle by increasing the temperature of the wood waste. This can be done during Summer months for best results.
Chip wood into one inch or smaller pieces. Cover chips, both underneath and on top, with tarps. The sun naturally heats the chips to a high temperature, killing the beetle. Make sure the chip layer is thin to ensure even heating. This method can also work on logs.
Keep waste wood on site. Do not carry wood to other locations with active and live beetles. If relocation is absolutely necessary, consult with your local Parks and Recreation or Service departments for guidance, and make sure to cover waste with tarps to prevent beetles from traveling in flight during transit.
Clean all equipment. The beetle can travel on any kind of equipment. Make sure to clean all landscaping and tree care equipment thoroughly after each use, especially in known infested areas.
What You Can Do to Keep the Shot Hole Borer from Spreading
Regularly Maintain your Trees. By simply keeping your trees healthy with regular maintenance, watering, and fertilization, you can increase their ability to naturally defend against infestations. For example, healthy, hydrated trees produce sap, a sticky substance that stops critters in their tracks.
Educate Yourself and Others. Share your knowledge (and this blog post) with others and be aware of surrounding infected areas.
Notify Local Authorities. When you spot signs of the Shot Hole Borer in your area, let your local Parks and Recreation department know as soon as possible. Early treatment can save trees and reduce spreading.
Limit Transportation of Infected Wood. This one is so important that it bears repeating. Never move wood, chippings, or trimmings far from the area. If you must transport infested wood off-site, contact your local green waste authority to find out how they recommend disposing of waste without causing further contamination.
The Takeaway
Prevention and treatment starts with knowledge and requires action. Let’s work together to fight these invasive boring beetles, and stop them from further damaging the landscape that makes San Diego, California such a beautiful place to call home.