& Key message We assessed the impact of the polyphagous shot hole borer (PSHB), Euwallacea whitfordiodendrus (Schedl), and Kuroshio shot hole borer (KSHB), E. kuroshio Gomez and Hulcr, on hardwood trees in southern California, southwestern China, and northwestern Vietnam. The highest levels of mortality were recorded from 10 of 39 tree species in the survey, and these were primarily native tree species. & Context Two invasive shot hole borers represent relatively recent introductions in southern California, USA, and continue to spread and cause injury and mortality to several native and ornamental tree species. They originate from Southeast Asia. & Aims Knowledge of tree species susceptibility to these wood-boring beetles is essential to inform better pest management and to evaluate future risk for urban and wildland forests. & Methods From 2012 to 2016, ground surveys were conducted in the invaded and native regions at PSHB/KSHB-infested and PSHB/KSHB-uninfested sites to record levels of tree injury and mortality on native and ornamental tree species. & Results In California, several native species of maple, Acer, willow, Salix, and sycamore, Platanus, were infested by either PSHB or KSHB at high rates (> 70%), and comparative rate of infestation by KSHB in all trees and in native trees surpased that by PSHB, whereas rate of infestation by PSHB in ornamental trees surpassed that by KSHB. Mortality of two maple species caused by PSHB exceeded 20%, whereas background mortality rate of hardwoods was 2% in uninfested areas. & Conclusion These data should inform land managers about the tree species at most risk to injury and mortality, facilitate detection ground surveys, and direct prophylactic treatments for these invasive woodborers. Keywords Ambrosia beetles. Hardwood tree mortality. Impact assessment. Invasive species This article is part of the topical collection on Entomological issues during forest diebacks

The invasive ambrosia beetle polyphagous shot hole borer is one member of the cryptic species, Euwallacea nr. fornicatus (Eichhoff), and poses a great ecological and environmental threat to ornamental and native hardwood trees and agriculturally important tree crops in southern California, United States. We monitored the emergence of polyphagous shot hole borer adults from chipped and unchipped cut logs of infested boxelder, Acer negundo L. (Sapindales: Sapindaceae), California sycamore, Platanus racemosa Nutt. (Proteales: Platanaceae), coast live oak, Quercus agrifolia Née (Fagales: Fagaceae), and red willow, Salix laevigata Bebb (Malpighiales: Salicaceae), every week for 4 to 5 mo. No polyphagous shot hole borer adults emerged from chipped or unchipped coast live oak logs, suggesting this species is not a preferred reproductive host. However, following chipping, a small number of polyphagous shot hole borer adults emerged from boxelder (up to 7 wk) and from California sycamore and red willow (both up to 9 wk). A significantly greater number of polyphagous shot hole borer adults emerged from unchipped logs of boxelder (up to 14 wk) and California sycamore and red willow (both up to 5 mo). Chipping of boxelder, California sycamore, and red willow by using common commercially available chippers reduced polyphagous shot hole borer emergence by over 97%. Emerged polyphagous shot hole borer adults were strongly female-biased, regardless of host. Chipping treatments were highly effective, but for the complete elimination of polyphagous shot hole borer from woody material, other sanitation measures such as solarization in conjunction with chipping are recommended. Environmental and ecological impact of polyphagous shot hole borer is reviewed and discussed.

We conducted an experimental evaluation of treatments to limit Heterobasidion occidentale infection of white fir (Abies concolor) stumps and wounds in California mixed conifer forests. We tested the efficacy of urea, borate, and a mixture of two locally collected Phlebiopsis gigantea strains in preventing pathogen colonization of fir stumps and separately, urea and borate as infection controls on experimental stem wounds. These were paired with a laboratory test on ~100 g wood blocks with and without a one-week delay between inoculation and treatment. Urea, borates, and Phlebiopsis treatments all significantly reduced the stump surface area that was colonized by H. occidentale at 84%, 91%, and 68%, respectively, relative to the controls. However, only the borate treatments significantly lowered the number of stumps that were infected by the pathogen. The laboratory study matched the patterns that were found in the stump experiment with a reduced area of colonization for urea, borates, or P. gigantea treatments relative to the controls; delaying the treatment did not affect efficacy. The field wound experiment did not result in any Heterobasidion colonization, even in positive control treatments, rendering the experiment uninformative. Our study suggests treatments that are known to limit Heterobasidion establishment on pine or spruce stumps elsewhere in the world may also be effective on true firs in California.

ver 147 million dead trees were detected in California by the U.S. Forest Service Aerial Detection Survey (USFS ADS) from 2010 to 2018 (USDA 2019). The massive tree mortality, mostly in the Sierra Nevada and evident in swaths of conifers with red needles, resulted from the 2012-2016 drought and subsequent explosions in native bark beetle populations. While levels of mortality have declined in the last 2 years, the consequences will last for decades to come. Trees that died will fall over and surface fuel loads will increase-already the accumulation of millions of tons of dead material on forest floors is vastly outpacing the resources of local, state and federal jurisdictions to remove it. Urgent dialogue has started among UC scientists, forest managers, and public agencies to manage the consequences of the unprecedented tree die-off and increase the resiliency of forests to future droughts. To accomplish these goals, we need data on the rates of ongoing tree mortality and dead tree fall, surface fuel build-up, wildfire hazard, forest renewal patterns, and the course of bark beetle outbreaks. Data are also needed to understand the long-term impacts of the wave of tree mortality on ecological services such as carbon storage and water quality.

The Agaricomycete Heterobasidion annosum sensu lato (s.l.), a species complex of fungal pathogens, causes root and butt rot on conifers throughout the northern hemisphere, thereby shaping structure, composition, and evolution of vast and diverse forest ecosystems. We analyze forest change 48–49 years following Heterobasidion root disease emergence in 63 permanent plots first established in 1970–1972 and measured at least once every decade through 2020. We use this dataset to infer the long‐term consequences of Heterobasidion root disease in a set of common forest types that reflect some of the most important hosts for this set of diseases in western North America. We contrasted three host–pathogen systems located within the Sierra Nevada, southern Cascades Mountain Range and the Modoc Plateau: (1) H. irregulare in host communities dominated by several Pinus (pine) species and Calocedrus decurrens (incense cedar) in Yosemite Valley, on the Sierra Nevada Range western slope; (2) H. irregulare in a largely single species (Pinus jeffreyi) host disease system on the eastern slopes of the Sierra Nevada and southern Cascades as well as the Modoc Plateau; and (3) H. occidentale—also on the western slope of the Sierra Nevada—in stands with a mixture of susceptible Abies (true fir) and a diversity of non‐hosts for this Heterobasidion species. Approximately 50 years after disease emergence, relative basal area and stem density were significantly reduced within disease centres in all three pathosystems, but changes to forest composition and relative species dominance were determined by pre‐disease host and non‐host diversity. In the western‐slope H. occidentale system, the disease increased the dominance of non‐susceptible species, Pinus species and C. decurrens. In the multi‐genus Yosemite pathosystem, H. irregulare did not significantly shift species dominance, and as expected, species shifts did not occur in the largely single‐genus eastern‐slope forests. In these widespread California forest ecosystems, two factors appeared to determine forest conditions almost 50 years after Heterobasidion root disease monitoring: the size of the disease centre and the initial diversity of non‐hosts. Along with pathogen species, these factors appear to affect the local severity of disease as well as the degree of species shifts relative to initial plot compositions, reaffirming host susceptibility classifications associated with these forest types.