Status: Introduced

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The invasive shothole borers (ISHB) are two closely related ambrosia beetle species that form a symbiotic relationship with specific pathogenic fungal species of Fusarium and other fungi that together cause Fusarium Dieback on a wide variety of woody hosts.

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Location: 

PSHB: Southern California, Israel, South Africa, and Australia. 

KSHB: Southern California

California distribution map here.

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Hosts:

The combined effects of ISHB and their Fusarium pathogens have killed or caused dieback on 77 tree species on which the beetles can reproduce. Hosts occur across varied and complex landscapes, including wildland forests, urban forests, and avocado groves. A full and updated list of host species can be found in Supporting information (Table S1) in Lynch et al. 2021.

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Management:

Use the ISHB detection and management assessment tool to determine best steps for managing invasive shothole borers-Fusarium dieback disease suspect trees on your property. A comprehensive set of management guidelines can be reviewed at ISHB.org. In general, the primary management goal for FD–ISHB is to limit its expansion and minimize its impacts in already established areas. To prevent new introductions, do not move logs and firewood more than several miles from their origin. Buy firewood locally or gather on site where permitted. Solarize, compost, or kiln-dry infested material (more about these options here). To prevent pathogen spread, sanitize wood-handling tools (i.e., clippers, saws, chainsaws, wood chipping equipment) that come into contact with infested wood before handling non-infested trees. Ideally, prune non-infested trees in batches before working with infested trees. Regular monitoring is critical to ensure a rapid and appropriate response to new infestations. Strategize initial monitoring efforts by inspecting sycamore trees because they are common in urban and wildland forests and symptoms on the host are easy to identify. Learn how to identify and assess different sycamore varieties here. Learn how to identify signs and symptoms of FD–ISHB and other pest look-alikes overall here and here for willow species. For trap monitoring, use white sticky panel traps with a querciverol lure (not ethanol). In Southern California, monitor traps monthly in Spring (February – April) and early fall (September – October), when beetles fly the most. Adjust trap schedules according to local conditions suitable for beetle flight (68 F – 84 F between 11 a.m. – 4 p.m.). Traps are not effective at preventing or suppressing an ISHB infestation.

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Actions on monitored and evaluated trees depend on host type (“reproductive” or not), severity of ISHB attack (number of entry holes), tree status (health, location, value, hazard concerns), site infestation status, and available resources. Remove actively infested branches from low to moderately infested trees (<150 entry holes) and treat with pesticides as appropriate following guidelines in the management matrix. Pesticides should be applied by a certified pest applicator in early spring and fall when adult beetles emerge. Preventative chemical treatments on non-infested trees or curative treatments on heavily infested trees (>150 entry holes associated with branch dieback) are not effective. Consult a forest health professional or certified arborist on decisions concerning removal of actively heavily infested trees. Pruned infested branches or removed trees must be handled properly to prevent viable beetles from dispersing from cut wood. Chip wood to sizes less than one inch to kill >99.9% of beetles (Jones and Paine 2015; Umeda and Paine 2019). If wood cannot be chipped, tightly cover cut logs immediately and follow directions for solarization or kiln-drying. Grind or bury stumps of removed trees and treat with bifenthrin or Bacillus subtilis. Always cover infested material while in transit.

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Highlights:

The polyphagous (PSHB) and Kuroshio (KSHB) shothole borers (together known as invasive shothole borers, or ISHB) have evolved an obligate mutualism with wood-inhabiting, closely related Fusarium species over the last 21 million years (Fusarium euwallacea and F. kuroshium respectively). The combined effects of the beetles and these fungi cause Fusarium dieback. Genetic analysis indicates that ISHB was introduced into California from Southeast Asia on two separate occasions from Vietnam (PSHB) and Taiwan (KSHB) (Stouthamer et al. 2017), presumably via infested wood packaging or plant material. In 2003, a single PSHB beetle was caught in a CDFA trap in Los Angeles. The species went unnoticed until 2012 when it was found damaging a backyard avocado tree and urban forest trees in the Los Angeles basin (Eskalen et al. 2013). Since 2012, the PSHB beetle-fungus complex has spread throughout Los Angeles and Orange counties, and into certain areas of Riverside, San Bernardino, and Ventura counties. A separate invasion by KSHB occurred in San Diego County in early 2014, and has since been detected in Orange, Santa Barbara, and San Luis Obispo counties (Rios, 2015). Trap data indicate that the ISHB range is rapidly expanding and preliminary research suggests that the beetles can survive as far north as Tehama County (Umeda and Paine 2019).

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Unlike most bark and ambrosia beetles, ISHB prefer healthy, well-watered trees (Boland 2016; Swain et al. 2017). When the female beetles attack the tree, they chew through the wood and make tunnels known as galleries. The females inoculate the tunnel with the fungi they carry, creating a fungal garden in the gallery wall that serves as the sole food source for the resident adults and developing larvae. Young female beetles mate with sibling males and then pack fungal spores in specialized mouthparts (mycangia) before leaving the natal gallery to produce a new gallery on the same or neighboring tree. Males never leave the tree. ISHB females carry other fungi but Fusarium spp. are the most abundant (Lynch et al. 2016). The fungi are also pathogens that slowly spread and kill the plant tissue. Attack symptoms exhibited by infested host trees vary by species and include staining, gumming, sugary exudates, and or frass outside of boring holes, but can be induced by other pest species.

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The beetles alone attack over 300 tree species in 64 families. The beetles and pathogenic fungi can reproduce on a subset of 77 host species and spread to other hosts, which drives the spread of the epidemic. Twenty host species are native to California and are common components to many forest communities throughout the state. California native and ornamental host species comprise 25% to 60% of all individuals planted in the urban landscape in Southern California. For most of the “reproductive” hosts, we observe mostly branch dieback, but not death, but a subset of 18 species can be killed when attacked, which is important in terms of ecological impact. These impacts are predicted by host evolutionary relationships, which can be used as a tool to predict which plant communities worldwide will the complex most likely establish and cause damage.

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In Southern California, FD–ISHB has caused extensive destruction in riparian ecosystems. By October 2015, KSHB infested more than 280,000 willow trees (Salix spp.) in 597 acres of riparian forest in the Tijuana River Valley, with more than 140,000 trees suffering major limb damage (Boland 2016). Many new areas of native vegetation in Southern California continue to be invaded, and many of the affected locations are critical breeding habitats for endangered bird species. In addition to native vegetation, FD–ISHB has established and spread through urban forests and the main avocado-growing regions of California, which produce 90% of the United States domestic crop. Initial estimates suggest that FD–ISHB has the potential to kill roughly 27 million trees (38%) in Southern California’s 4,244-square mile urban region (McPherson et al. 2016). Tree losses and costly large-scale tree removal efforts to manage the problem could have unintended consequences for the environment and public health.

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