They may seem too tiny to do much damage to a mature, healthy tree, but invasive shothole borers (ISHB) are responsible for the deaths of tens of thousands of trees in Southern California. These beetles bore into trees and infect them with Fusarium dieback, a fungal disease that kills the trees. Many native California trees like California sycamore, valley oak, and arroyo willow can be killed when invasive shothole borers attack them.
While these pests are currently only found in Southern California, they could spread to many other parts of the state. Limiting the infestation will reduce their impact. Controlling the beetles is difficult but includes regular monitoring of trees to quickly identify sources of beetles, disposing of infested cut wood, and appropriate pesticide treatments.
What can you do to help?
- Don't move firewood around the state. These beetles and other potentially damaging beetles are easily moved on cut wood. Buy it where you burn it.
- Learn more about host trees, symptoms of infestation, and what to do.
UC IPM's new publication, Pest Notes: Invasive Shothole Borers is written by various state experts on this pest and contains everything you need to know about the beetle. Visit the UC IPM website for specific management recommendations, identification of the beetles, and lists of trees affected.
- Author: Beatriz Nobua-Behrmann
- Posted by: Karey Windbiel-Rojas
When tiny tree-killing beetles first arrived in Southern California several years ago and began destroying urban and riparian forests, they raised widespread concerns among both tree experts and affected communities. More recently, invasive shothole borers have captured far less attention, and many people may think the pest threat is over. Unfortunately, it's not!
While significant progress has been achieved in invasive shothole borer research, surveying, trapping, and management programs, these beetles are still an ongoing threat to the state's urban and wildland trees. Continue reading to find out what you can do to be part of the solution to this invasive pest issue.
What are invasive shothole borers?
Invasive shothole borers (ISHB) (Figure 1) are sesame seed-sized beetles. They tunnel into trees and introduce a fungus that they use as their food source. As the fungus grows, it causes a plant disease called Fusarium dieback that leads to branch dieback, tree decline, and, in many cases, tree death.
The beetles and fungi can live and reproduce in a wide range of tree species including more than 65 types of trees found in California. The most highly susceptible trees include many species that are commonly used for landscaping like sycamores, some oaks, cottonwoods, and box elder. Invasive shothole borers can attack healthy, stressed, or diseased trees.
What's the problem?
Urban trees provide us with many benefits to our health and our economy. The trees around us reduce our stress levels, provide shade, allow for energy conservation, improve air quality, reduce stormwater runoff, and provide habitat for wildlife. It is important to protect them from invasive pests, like invasive shothole borers, which could potentially kill one out of three urban trees in California.
ISHB-infested trees can quickly become a public safety hazard. Trees with heavily infested branches can be especially hazardous. The combined damage of the fungal disease and the beetle's tunneling activity weakens the wood, causing limbs to break and fall (Figure 2). In addition, severely infested trees will become a constant source of beetles that can disperse and infest neighboring trees.
Where are they found?
These non-native beetles are now established in many areas of Southern California and the Central Coast. Female beetles are capable of flight over short distances, allowing the pest and its associated fungi to spread into new areas. Beetles can also be transported in infested firewood and green waste, leading to dispersal over much greater distances.
What to look for
Because they are very small and spend most of their lives inside their tree hosts, you probably won't see the beetles themselves, but there are several common signs and symptoms associated with their infestations: ?Beetle entry holes: When the beetles tunnel into the trees, they make small, perfectly round holes, each about the size of the tip of a medium ballpoint pen (Figure 3).
- Tree response symptoms: One or more of the following symptoms usually accompany the presence of entry holes (symptoms vary by the tree species): dark, wet staining; thick gumming; sugar-like buildup; or boring dust (resembles fine sawdust).
- Dieback: Dead or wilting branches can be a sign of a severe infestation. If you see dieback on trees, check for entry holes on the trunk or the branch collars.
What you can do
Several steps can be taken to prevent pest problems and manage infestations.
- Keep your trees healthy. Proper irrigation and maintenance will keep trees strong and help protect them from ISHB and other pests.
- Check your trees. Look for the common signs and symptoms listed previously. Regular monitoring ensures that infestations are managed early, before they cause dieback or tree death.
- Confirm suspected infestations using the detection tool on www.ishb.org. ?Know your management options. When possible, pruning infested branches is recommended. Low and moderately infested trees can be treated. You will need to contact a licensed professional to apply the treatments. Severely infested trees may require removal.
- Take care of green waste. The beetles can survive in cut wood for weeks or even months. Proper disposal of green waste includes chipping infested wood, followed by solarizing or composting the chips.
- Consult a professional. A certified arborist or pest control professional would be able to provide recommendations based on the conditions of your tree. Your County Agricultural Commissioner's office and UC Cooperative Extension office may have more knowledge about current ISHB monitoring and management programs in your area.
- Use locally sourced or heat-treated firewood. These beetles and other tree-killing insects often reach new locations by hitchhiking in firewood. Buy firewood where you will use it, and only buy the amount of firewood you need.
Visit www.ishb.org for more information about invasive shothole borers.
[Originally featured in the Summer 2022 issue of UC IPM's Home & Garden Pests Newsletter.]
![Fig 1 Adult female of Polyhagous shot hole borer Fig 1 Adult female of Polyhagous shot hole borer](/blogs/blogcore/blogfiles/92652.jpg)
![Fig 2 Beetle entry hole on an avocado branch Fig 2 Beetle entry hole on an avocado branch](/blogs/blogcore/blogfiles/92653.jpg)
![Fig 3 a close up picture of broken branch of red willow tree in OC parks Fig 3 a close up picture of broken branch of red willow tree in OC parks](/blogs/blogcore/blogfiles/92654.jpg)
- Author: Beatriz Nobua-Behrmann
- Author: Randall Oliver
- Posted by: Elaine Lander
Two identical looking species of wood-boring beetles, collectively known as invasive shothole borers (ISHB), have killed thousands of trees in Southern California and pose an ongoing threat to California's urban and wildland forests. These beetles, which are not native to the United States, were first identified in Los Angeles County in 2012 and have since spread to six other counties: Orange, Riverside, San Bernardino, San Diego, Santa Barbara, and Ventura.
Beetles, Fungus, and Impact
As the fungus grows, it colonizes the tree's vascular system, blocking transport of water and nutrients. This causes a disease called Fusarium dieback that can kill branches or entire trees. The beetles and their symbiotic fungi have a wide range of suitable hosts, including more than 65 species of trees found in California. The most highly susceptible trees include many of the species commonly used for landscaping, such as sycamores, oaks, cottonwoods, and box elder trees. ISHB beetles attack healthy trees as well as stressed or diseased trees in a variety of urban, suburban, and riparian settings. Visit www.ishb.org to find the full list of reproductive hosts in California.
Female beetles can fly short distances, allowing the pest-disease complex to spread into new areas near already infested trees. Beetles can also be transported in infested firewood and green waste, leading to spread over much greater distances. While beetles have only been identified in Southern California and the Central Coast to date, further spread throughout much of California is possible.
ISHB-infested trees can quickly become public safety hazards. Trees with heavily infested branches can be especially hazardous, since the combined damage of the fungal disease and the beetle's tunneling activity weakens the wood, causing limbs to break and fall. In addition, severely infested trees will become constant sources of beetles that can disperse and infest neighboring trees. Such “amplifier” trees generally need to be removed completely, while more lightly infested trees can be managed or treated without requiring removal. Therefore, early detection and rapid response is the key to controlling ISHB. Substantial recovery in lightly to moderately infested trees has been observed in some areas where amplifier trees have been removed.
Identifying an ISHB infestation
- Beetle entry holes: When the beetles tunnel into trees they make small, perfectly round holes, each about the size of the tip of a medium ballpoint pen (0.8 mm). (Figure 3)
- Additional signs and symptoms: Entry holes are usually accompanied by one or more of the following signs and symptoms, which vary by the tree species: staining, gumming, white powdery exudate, or frass (boring dust).
- Dieback: Dead or wilting branches can be a sign of a severe infestation. If you see dieback on trees, check for entry holes on the trunk or the branch collars.
In addition to visual inspections, traps using quercivorol, a plant-based lure that attracts the beetles over short distances, can help determine the presence and abundance of beetles in an area. Trapping is especially useful for large or inaccessible areas where regular visual inspections of all the trees are not practical. In those cases, trapping can help determine if ISHB is present in the area and can help focus survey efforts on infested trees. Because the lure has relatively low attractiveness, trapping is not an effective control method for ISHB and is only used for detection purposes.
What Can You Do?
- Keep trees healthy. Proper irrigation and maintenance will keep trees strong and help protect them from ISHB and other pests.
- Prune out infested branches. Removing branches that have clusters of 50 ot more ISHB holes would help control this pest. For trees that undergo heavy pruning every year, like avocado trees, removal of all infested branches is recommended. Tools should be disinfected after pruning by spraying them with a solution of 5% disinfecting bleach or 70% ethanol to avoid spreading the fungal disease to other trees.
- Remove severely infested trees. Unfortunately, severely infested trees (with more than 150 entry holes and ISHB-related dieback) are not likely to survive. These trees should be removed as soon as possible, and the stump should be ground to one inch or less. (Figure 4)
- Manage downed wood. Green waste generated by branch and tree removals should be properly disposed.
- Chipping/grinding to one inch or less kills 99.9% of the beetles. If that is not possible, chipping to three inches or less still will kill 98% of the beetles in the wood. In already infested areas, chipped wood can be used onsite as mulch. However, if working on a newly infested area or if the wood chips will be moved to another area, chipping should be combined with solarization or composting to kill 100% of the beetles. If chipping is not an option, logs can also be solarized or kiln dried to exterminate the beetles.Figure 4. California sycamore heavily infested with invasive shothole borers. (Credit: B Nobua-Behrmann, UCCE)
- Solarization involves covering the material with clear plastic tarp and letting the heat from the sun kill the remaining beetles. Chips and beetles should be fully contained by wrapping plastic both underneath and over the material. Chips should remain covered for at least six weeks during the summer months or for at least six months between September and June. The depth of the pile should be no more than 30 inches deep, to ensure even heating.
- Composting, when done correctly, should also kill the remaining beetles in the chips. It is recommended to send infested chips to a professional composting facility that has earned the U.S. Composting Council's Seal of Testing Assurance.
- Prevent the spread. Avoid spreading this pest by not moving firewood or mulch that hasn't been properly solarized or composted. If you must move infested greenwaste (for example, to bring it to a composting facility) make sure the load is tightly covered while in transit.
- Consider chemical control.TreesthatarereproductivehostsforISHB and that show signs of active infestations can be treated with a combination of insecticide and fungicide. The decision to treat a particular tree depends, among other things, on the tree's condition, value, and hazard level. Trees that aren't already infested should be monitored but not treated. There are various chemicaloptionsthatcanbeusedagainstISHB-FD.
- Trunk sprays of a contact insecticide, such as bifenthrin, combined with Bacillus subtilis or tebuconazole (which are fungicides) have been demonstrated to offer some degree of control.
- Systemic soil injections or drenching with the insecticide imidacloprid has also provided control, as has trunk injection with emamectin benzoate (insecticide) combined with tebuconazole or propiconazole (fungicides).
- These pesticides should only be applied by a licenced professional following the instructions on their labels to avoid harming non-target organisms.
Biocontrol options are currently under research. They include the use of natural enemies (such as parasitic wasps from the beetles' point of origin), entomopathogenic fungi (which are fungi that attack insects), endophytes (which are microorganisms that live in the tree that may provide protection) and nematodes. But these biological management options might take time before they are tested and available. Until then, prevention, early detection, and rapid response are our best weapons to keep trees healthy and alive. For more information on invasive shothole borers and their management, visit www.ishb.org.
[Article originally published in the Summer 2021 issue of the Green Bulletin.]
/h2>/h2>/h2>- Author: Beatriz Nobua Behrmann
[Originally published in the Fall 2018 issue of the Green Bulletin. Modified slightly from original.]
Invasive wood-boring beetles are attacking hundreds of thousands of trees in southern California, including commercial avocados, and trees within urban landscapes and wildland environments.
The invasive shot hole borers (ISHBs) consist of two closely related and morphologically identical species of beetles in the genus Euwallacea: the polyphagous shot hole borer and the Kuroshio shot hole borer. Despite their small size (1.8–2.5 mm) (Figure 1), these beetles are causing big problems in Southern California: they are responsible for the fast decline and death of thousands of urban trees, riparian natural forests, and avocado groves.
The beetles bore into trees, creating a series of small galleries (Figure 2). Inside these galleries, they lay eggs and “farm” a fungus (Fusarium spp.), which is their main food source. The fungus colonizes the trees' vascular systems, blocking transport of water and nutrients. This causes a disease called Fusarium dieback that manifests as branch dieback, general tree decline and, in many cases, tree death (Figure 3).
Both insect species are believed to have been accidentally introduced into California via wood products or shipping materials from southeast Asia. Since ISHBs were first identified in Los Angeles County in 2012, the infestation has spread to 6 other counties: Orange, San Diego, Ventura, Santa Barbara, San Bernardino, and Riverside. Once the beetles arrive at a new location, they colonize susceptible host trees and spread to neighboring areas, infesting more and more trees. Movement of infested firewood and green waste are additional ways the beetles may be transported, allowing them to colonize new areas.
Currently, there are 64 confirmed species of trees in which the beetles can successfully grow their fungus and complete their life cycle. Susceptible trees include many of the species commonly used for landscaping; like sycamores, oaks, cottonwoods, and box elder, among many others. UC Riverside researchers found that ISHBs can successfully colonize trees that were previously considered non-suitable hosts by entering and reproducing in canker-infested branches. Canker is another tree disease caused by fungal or bacterial pathogens that enter the tree through open wounds; it typically causes localized dead areas on the trunks and branches, with sunken, discolored bark and, sometimes, dark lesions.The ISHB beetles can establish their galleries and grow their population in the weakened margin of the canker-infested tissue of some of their hosts. Regular monitoring and removal of canker-infested branches is recommended for these tree species. To find the full list of ISHB reproductive hosts (including the canker-associated hosts) please visit pshb.org.
How do you determine if a tree is infested with ISHB?
Correct identification of the pest is the first step for a successful IPM program. The following are typical symptoms of an ISHB infestation:
- Beetle entry holes: When the beetles excavate their galleries in the trees they make perfectly round small holes, 0.8 mm wide, each roughly the size of the tip of a medium ballpoint pen. (Figure 4)
- Symptoms associated with holes: Entry holes are usually accompanied by one of these symptoms: wet staining, gumming (Figure 5), white powdery exudate (Figure 6), or frass (boring dust). Each species of tree exhibits different symptoms.
- Dieback: Dead or wilting branches can be signs of a severe infestation. If you see dieback on your trees, check for entry holes on the branches or the branch collar.
Best management practices
ISHB-infested trees can quickly become a public safety hazard. Trees with heavily infested branches are especially hazardous, since the combination of tissue decline caused by the fungal pathogen and the mechanical damage from the beetle's galleries weakens the wood, causing limbs to break and fall.
Early detection is the key to controlling this pest. So far, no effective preventative treatments have been reported, so regular monitoring is recommended to ensure infestations are managed early, before they cause dieback or death. Regular monitoring also ensures that trees get treated when they are lightly infested and have the most chances of overcoming the infestation. Researchers continue to study different methods for chemical and biological control of this pest. If you suspect you are dealing with an ISHB infestation, contact your local Agricultural Commissioner's office or IPM Advisor for treatment advice.
Trees that are severely infested (with more than 150 beetle attacks and ISHB-related branch dieback; Figure 7) are not likely to recover from the infestation and will become a constant source of beetles that can disperse and infest neighboring trees. Furthermore, weakened branches on such trees pose hazards to people and property. Therefore, severely infested trees should be removed as soon as possible and their wood properly disposed of. Even after an infested tree is removed, ISHBs can continue to live and reproduce in the stump, so following tree removal with stump grinding is always recommended.
Disposing of infested wood
Borers can survive in cut wood for weeks or even months. It is vital to take care of green waste appropriately in order to avoid spreading this pest to new areas. The most recommended practice is to chip infested wood to a size of 1 inch or smaller; this will kill 95% of the beetles. To ensure the elimination of all beetles and fungal spores within wood, you must solarize infested wood chips with a clear tarp. Other effective disposal methods for infested materials include composting, burning at a biogeneration facility, and use as alternative daily cover within landfills. Untreated chips can be used as mulch, but only in areas that are already heavily infested with ISHB. If chipping is not possible, logs should be kiln dried or solarized under a clear tarp to ensure total beetle elimination. Visit pshb.org for more information on solarization and composting guidelines.
ISHB and its associated fungal diseases can be accidentally spread into new areas by the same people who are trying to manage the problem. Make sure you disinfect your tools after pruning (spraying them with 70% ethanol solution works well), and always cover infested material when moving it to a different location (for instance, for treatment) to avoid spreading the pest.
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