The polyphagous shot hole borer beetle: Current status of a perfect invader in South Africa

Harder, better, faster, stronger? Dispersal in the Anthropocene

The dispersal of organisms in the Anthropocene has been profoundly altered by human activities, with far-reaching consequences for humans, biodiversity, and ecosystems. Managing such dispersal effectively is critical to achieve the 2030 targets of the Kunming-Montreal Global Biodiversity Framework. Here, we bring together insights from invasion science, movement ecology, and conservation biology, and extend a widely used classification framework for the introduction pathways of alien species to encompass other forms of dispersal. We develop a simple, global scheme for classifying the movement of organisms into the types of dispersal that characterise the Anthropocene. The scheme can be used to improve our understanding of dispersal, provide policy relevant advice, inform conservation and biosecurity actions, and enable monitoring and reporting towards conservation targets.

Fusarium species associated with Euwallacea xanthopus in South Africa, including two novel species

Ambrosia beetles (Coleoptera: Curculionidae: Scolytinae) are small wood-boring insects that live in an obligate symbiosis with fungi, which serve as their primary food source. Beetles residing in the genus Euwallacea have evolved a unique association with a clade of Fusarium that falls within the aptly named Ambrosia Fusarium Clade (AFC). The discovery of the invasive polyphagous shot hole borer, E. fornicatus, in South Africa, has heightened awareness of ambrosia beetles and their symbionts in the country. In this study, we investigated the Fusarium symbionts of three species of Euwallacea in South Africa, with a specific focus on those associated with E. xanthopus. Isolations of Fusarium strains from both living and dissected beetles yielded nearly 100 isolates. Using multigene phylogenetic analyses, these isolates were identified as six different Fusarium species. Fusarium hypothenemi and F. euwallaceae have previously been reported from South Africa. Fusarium pseudensiforme and Fusarium AF-6 are new records for the country. The remaining two species are new to science and are described here as F. rufum sp. nov. and F. floriferum sp. nov. Targeted fungal isolation from specific beetle body parts revealed that the AFC species collected were typically associated with the dissected beetle heads and helped us identify the likely nutritional symbiont of E. xanthopus. This study highlights the understudied diversity of fungal associates of ambrosia beetles present in South Africa.

Monitoring urban biological invasions using citizen science: the polyphagous shot hole borer (Euwallacea fornicatus)

Benefits provided by urban trees are increasingly threatened by non-native pests and pathogens. Monitoring of these invasions is critical for the effective management and conservation of urban tree populations. However, a shortage of professionally collected species occurrence data is a major impediment to assessments of biological invasions in urban areas. We applied data from iNaturalist to develop a protocol for monitoring urban biological invasions using the polyphagous shot hole borer (PSHB) invasion in two urban areas of South Africa. iNaturalist records for all known PSHB reproductive host species were used together with data on localities of sites for processing plant biomass to map priority monitoring areas for detecting new and expanding PSHB infestations. Priority monitoring areas were also identified using the distribution of Acer negundo, a highly susceptible host that serves as a sentinel species for the detection of PSHB infestations. iNaturalist data provided close to 9000 observations for hosts in which PSHB is known to reproduce in our study area (349 of which were A. negundo). High-priority areas for PSHB monitoring include those with the highest density of PSHB reproductive hosts found close to the 140 plant biomass sites identified. We also identified high-priority roads for visual and baited trap surveys, providing operational guidance for practitioners. The monitoring protocol developed in this study highlights the value of citizen or community science data in informing the management of urban biological invasions. It also advocates for the use of platforms such as iNaturalist as essential tools for conservation monitoring in urban landscapes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10340-024-01744-7.

Efficacy of Commercially Available Entomopathogenic Agents against the Polyphagous Shot Hole Borer in South Africa

The invasive ambrosia beetle, Euwallacea fornicatus, was first reported in South Africa in 2018. The beetle has now spread to eight provinces of the country and has had a devastating impact on both native and non-native tree species. This is especially true for trees located in urban and peri-urban environments. Recent predictions are that the South African E. fornicatus invasion will cost an estimated ZAR 275 billion (approx. USD 16 billion) if it continues to spread uncontrollably, justifying an urgent need for its effective management in the country. One option is biological control, which is preferred over the use of chemicals due to its lower environmental impact. We tested two broad-spectrum fungal entomopathogenic agents, Eco-Bb^® and Bio-Insek, which are commercially available in South Africa, for efficacy against E. fornicatus. Initial laboratory assays yielded promising results. However, beetle infestation trials using treated pieces of woody castor bean stems showed little effect on beetle survival and reproduction.

Forest Insect Biosecurity: Processes, Patterns, Predictions, Pitfalls

The economic and environmental threats posed by non-native forest insects are ever increasing with the continuing globalization of trade and travel; thus, the need for mitigation through effective biosecurity is greater than ever. However, despite decades of research and implementation of preborder, border, and postborder preventative measures, insect invasions continue to occur, with no evidence of saturation, and are even predicted to accelerate. In this article, we review biosecurity measures used to mitigate the arrival, establishment, spread, and impacts of non-native forest insects and possible impediments to the successful implementation of these measures. Biosecurity successes are likely under-recognized because they are difficult to detect and quantify, whereas failures are more evident in the continued establishment of additional non-native species. There are limitations in existing biosecurity systems at global and country scales (for example, inspecting all imports is impossible, no phytosanitary measures are perfect, knownunknowns cannot be regulated against, and noncompliance is an ongoing problem). Biosecurity should be a shared responsibility across countries, governments, stakeholders, and individuals.

An Assessment of the Potential Economic Impacts of the Invasive Polyphagous Shot Hole Borer (Coleoptera: Curculionidae) in South Africa

Studies addressing the economic impacts of invasive alien species are biased towards ex-post assessments of the costs and benefits of control options, but ex-ante assessments are also required to deal with potentially damaging invaders. The polyphagous shot hole borer Euwallacea fornicatus (Coleoptera: Curculionidae) is a recent and potentially damaging introduction to South Africa. We assessed the potential impact of this beetle by working across economic and biological disciplines and developing a simulation model that included dynamic mutualistic relations between the beetle and its symbiotic fungus. We modeled the potential growth in beetle populations and their effect on the net present cost of damage to natural forests, urban trees, commercial forestry, and the avocado industry over 10 yr. We modeled high, baseline, and low scenarios using discount rates of 8, 6, and 4%, and a plausible range of costs and mortality rates. Models predicted steady growth in the beetle and fungus populations, leading to average declines in tree populations of between 3.5 and 15.5% over 10 yr. The predicted net present cost was 18.45 billion international dollars (Int. $), or about 0.66% of the country's GDP for our baseline scenario ($2.7 billion to $164 billion for low and high scenarios). Most of the costs are for the removal of urban trees that die as a result of the beetle and its fungal symbiont, as has been found in other regions. We conclude that an ex-ante economic assessment system dynamics model can be useful for informing national strategies on invasive alien species management.