Tiny but highly destructive beetles that bore into trees creating tunnels where they and their offspring spend most of their lives. The beetles do not eat wood, but as they tunnel, they introduce a fungus that grows within the tree’s inner layers. The fungus is the food source for adult beetles and their larvae.
Invasive Shot Hole Borer Beetle (Photo by John Kabashima, UC Cooperative Extension)
Why are they called invasive?
The term invasive refers to non-native plants and animals that arrive in a new environment with some human assistance – intentional or unintentional – and that are harmful to their new location, either to the environment, the economy or to human health. With invasive insects, those introductions are often on nursery stock, packing material such as wood pallets or crates, or through the movement of firewood.
What type of harm do invasive shothole borers cause?
The beetles and the fungus they carry pose an imminent threat to California’s urban and natural forests.Over time, as the fungus spreads within the tree’s inner layers, it disrupts movement of water in the tree. Deprived of water and nutrients, the tree suffers from branch dieback and, eventually, death. This pest/fungal disease complex has killed hundreds of thousands of trees in Southern California since its arrival in the early 2000s.
Trunk of a heavily infested California sycamore (Photo by Beatriz Nobua-Behrmann, UC Cooperative Extension)
Why are these beetles such a threat to California?
Like other invasive pests, invasive shothole borers have no natural enemies to keep them in check here in California and are able to multiply rapidly. Also, trees here that have not co-evolved with the beetles may lack natural defenses to fight off beetle infestations. These beetles are happy to attack healthy as well as stressed or diseased trees.
What types of trees are at risk?
Unlike many insect pests, these beetles do not just attack one type of tree. Invasive shothole borers affect a wide variety of tree species, including many common ornamentals and California natives. They are equally happy to attack trees in urban and suburban neighborhoods as well as parks and wildlands. Some of the most preferred and affected tree species include box elder, sycamores, cottonwoods, willows and valley oak (you can find the complete list of host species at www.ishb.org)
Are invasive shothole borers in the Bay Area?
Yes, beetle infestations were recently detected in San Jose. Based on some of the infested trees, experts believe the beetles may have been in the area for five or more years. Surveying and trapping efforts are just starting up to determine how widespread the beetles are in the Bay Area.
Can the beetles be eradicated?
If the beetles have already become established in the area, as seems to be the case, eradication is probably not possible. However, experience in Southern California has led to the development of strategies and treatments that can be effective in managing the infestations. Early detection and ongoing monitoring are critical steps in that process.
How can I identify infested trees?
Although you probably will not find a beetle on the tree bark, there are a number of signs and symptoms to alert you of the beetles’ presence. The main giveaway of an invasive shothole borer infestation is the presence of small, round entry-holes that are about the size of the tip of a medium ballpoint pen. In addition, trees under attack can exhibit a variety of other signs of infestation, like staining, gumming, presence of thin sawdust and sugary secretions. Each tree species reacts differently to attacks, so the combination of accompanying symptoms can be unique to that species.
Female ISHB in entry hole (Photo by Monica Dimson, UC Cooperative Extension)
Do infested trees need to be removed?
A number of factors go into the decision of whether or not a tree should be removed, including the type and size of the tree, the tree’s location and the severity of infestation. Heavily infested trees may not be able to be saved and should be removed on a timely basis. For low to moderately infested trees, removal of actively infested branches and treatments with a combination of insecticide and fungicide have proven effective control measures (hence the importance of pinpointing infestations as early as possible). Treatment options include systemic pesticides that can be applied as a tree injection, soil injection or soil drench and pesticide bark sprays (ideally timed in early spring and fall, when adult beetles are emerging). Pesticide treatments must be performed by a licensed Pest Control Applicator.
Who is addressing this issue in the Bay Area?
A coalition of city, county, state and federal agencies are working together to identify the extent of infestation and to build awareness.
