Many invasive non-native pest and pathogens are introduced unintentionally via pathways driven by consumers such as the trade of ornamental plants for planting (Brasier, 2008b; Liebhold et al., 2012). However, many of the stakeholders involved in the “plants-for-planting” pathway are generally unaware of the risk and their possible roles in the introduction and spread of these organisms (Marzano et al., 2015). Therefore, methods that raise awareness of the risks associated with the trade of living plants, for example, could be effective at preventing the introduction of invasive non-native species.

Citizen science programs can increase participant knowledge and awareness. For example, reviews of citizen science projects active in biodiversity and environmental monitoring reported knowledge gain and increased community awareness as main participant learning outcomes (Stepenuck and Green, 2015; Peter et al., 2019; Gardiner and Roy, 2022). However, studies about the learning outcomes of participating, in terms of knowledge or behavior change, in citizen science projects aimed at monitoring invasive non-native species are limited and generally inconclusive (Crall et al., 2013; Bela et al., 2016), although it is assumed that participation increases awareness (Gallo and Waitt, 2011; Bates et al., 2015). Incorporating mechanisms for participant feedback and evaluation within more citizen science initiatives can help inform future citizen science initiatives.

Raising public awareness can increase support for invasive non-native species management (Klapwijk et al., 2016; Novoa et al., 2017) and citizen science can complement these programs (Pocock et al., 2020). For example, resources from dontmovefirewood.org are routinely referenced during training activities in the Forest Health Watch¹ citizen science program, and these resources have increased awareness of firewood as the pathway of invasive non-native species (Solano et al., 2020). Indeed, increasing awareness of the signs and symptoms of biological ivnasions before they arrive can be an effective method to promote early detections. For example, members of the public were responsible for locating and sharing observations and signs of Anoplophora glabripennis (Asian longhorned beetle) infestations in multiple locations of North America (Meng et al., 2015). Therefore, projects that incorporate resources from other awareness campaigns or highlight general signs or symptoms can be an effective pre-border strategy promote the early detection of biological invasions.

Citizen science programs can also contribute to enhanced biosecurity in pre-border situations by providing relevant information on biological invasion history of pests and pathogens that can be used within horizon scanning or risk assessments. For example, Hulbert et al. (2019b) engaged staff in Cape Citizen Science to detect and sample plant pathogens affecting exotic and native plants in botanical gardens. These surveys identified novel plant-microbe interactions and detected pathogen species outside of their previously known distributions. Botanical gardens and arboreta are hubs of non-native plant species that provide a unique opportunity to integrate citizen science (Martellos et al., 2016) with initiatives aimed at monitoring sentinel trees such as the International Plant Sentinel Network (Barham et al., 2016). Planting non-native species can serve as a source entry point for non-native pests or pathogens (Kirichenko and Kenis, 2016; Eschen et al., 2019). For example, the quarantine pathogen Lecanosticta acicola which causes brown spot needle blight was recently detected on exotic Pinus mugo planted at an arboreta in southern Sweden and represents a new risk for commercially important Scots pine (Cleary et al., 2019). Therefore, conducting citizen science to monitor tree species planted outside of their native range can generate information pertinent for horizon scanning and risk assessment.

Urban forests and urban green spaces also provide important opportunities for collecting data useful to horizon scanning (Paap et al., 2017) and there are many citizen science initiatives and tools that have been designed to monitor street trees (Hawthorne et al., 2015; Roman et al., 2017). For example, the Healthy Trees Healthy Cities program was initiated to engage citizen scientists to establish and monitor urban trees (TNC, 2020). Urban forests are also recognized as places to connect people to nature (Gulsrud et al., 2018) and programs such as the Urban Forest Visual program in Melbourne (City of Melbourne, 2017) have generated and engaged citizens in extensive datasets of urban forest health with novel forms of engagement such as giving individual trees email addresses. The extensive potential to incorporate public engagement and surveillance of tree health in urban settings is widely recognized (Meentemeyer et al., 2015; Hulbert et al., 2017; Hallet and Hallett, 2018).

Citizen science programs can also contribute important baseline data on the distribution, and host range, of invasive non-native pests and diseases. For example, the discovery and subsequent description of two Phytophthora species was possible because of the Cape Citizen Science program (Bose et al., 2021). Such information is critical for local post-border management, but also for informing pre-border biosecurity within uninvaded regions. The general requirement within current regulatory frameworks for a non-native species to have been previously identified based on taxonomic information can be a major pitfall because only a small proportion of microbes have been found or adequately described (McTaggart et al., 2016; Roy et al., 2017). Therefore, projects that aim to explore the distribution and diversity of pests or pathogens, such as the Phytophthora Citizen Science project in Sweden, Cape Citizen Science in South Africa² or the Backyard Bark Beetles program in the USA,³ are important because of the potential to discover and describe novel species. Taken together, there are many citizen science projects generating data about potential forest pests and pathogen species that could emerge as major threats within different environments or hosts. Sharing data rapidly, openly and widely (e.g., Findability, Accessibility, Interoperability, and Reusability (FAIR) Principles; Wilkinson et al., 2016) is critical to maximize the benefits of such information.

Programs that provide baseline distribution data, host susceptibility, and environmental/climate suitability with respect to imported commodities and potential diseases are critical for understanding the potential risks associated with imports (Hulme, 2009; Webber, 2010). For example, once an invasive non-native species is known to occur in an exporting country, the importing country can include it in risk assessments or specifically screen imported material for the organism during inspections. However, distribution data is often limited because the origins for most pathogens are unknown (Stukenbrock and McDonald, 2008) and the increasing connectivity of previously separated biogeographical areas and phenomena such as “the bridgehead effect” (Lombaert et al., 2010) are increasing the spread of forest pests and pathogens (Hulme, 2009; Wingfield et al., 2015; Hulbert et al., 2017; Prospero and Cleary, 2017). Indeed, efforts to improve understanding of the distribution of potential threats are critical because invasion risk is strongly linked with the distribution of non-native species within a country’s trade network (Chapman et al., 2017). Thus, citizen science programs that increase information about the distribution of potential pests and pathogens provide important data that can enhance pre-border biosecurity measures in other parts of the world.

Datasets derived from citizen science platforms like iNaturalist can also contribute to the passive surveillance for invasive non-native species. For example, citizen scientists may add observations of Thaumetopoea processionea (oak processionary moth) out of curiosity or simply to document the biodiversity. This passive surveillance can lead to the first detection of an invasive non-native pest in a new area (Brown et al., 2020) and provides a potential long-term and widescale dataset at a relatively low cost (Pocock, 2013). These datasets can also be actively monitored for priority pests.⁴ However, these occurrence datasets are usually derived from opportunistic data collection which can result in ambiguity of whether the organism is truly absent from an area without recorded presence data (Pocock et al., 2017). These challenges can be overcome through active recruitment and training of citizen scientists for structured monitoring that records failure to detect the target organism (Cooper et al., 2012), but this design depends on the resources and objectives of the project.

Immense value can be realized in training citizen scientists to recognize specific symptoms and signs of damaging agents on trees to effectively inform and implement control measures. For example, several “first detector” programs include training via the National Plant Diagnostic Workshop⁵ or state and country specific programs such as the Oregon Forest Pest Detector training⁶ or Observatree⁷ in the United Kingdom. These projects raise awareness by offering training sessions, materials, and online courses to first detectors.

Human activities drive biological invasions (Santini et al., 2018), and consequently involving citizens in surveillance efforts increases the chance of detecting an outbreak or epidemic sufficiently early to ensure effective response. Detecting a new incursion shortly after introduction is critical to limiting subsequent impacts because eradication and containment have higher levels of success for biological invasions which are locally established (Liebhold et al., 2016; Hansen et al., 2019; de Groot, 2020). In many cases, biological invasions are first detected by landowners or homeowners making initial reports to local officials that are then amplified. For example, sudden oak death was first detected by a California homeowner and then amplified by an extension specialist (Garbelotto et al., 2001; Hulbert et al., 2017). Training citizens as first detectors can be key to making early detections for responding rapidly and reducing the impacts of biological invasions.

There are many examples of programs actively engaging the public in surveillance in addition to the first detector programs mentioned previously. Observatree (see text footnote 7) trains volunteers to monitor for priority pests and diseases in the United Kingdom. The UK also has a Tree-Alert system⁸ for untrained volunteers to report concerns or sightings of pests and diseases. Similarly, New Zealand has developed the Find-A-Pest⁹ program and the USA has an initiative called Forest Health Watch (see text footnote 1). Collectively, these projects emphasize the possibility to raise awareness and empower citizens as first detectors.

Citizen science programs are also known to increase the number of observations and their distributions at relatively low costs (Bonney et al., 2009) while also increasing information from private lands (Meentemeyer et al., 2015) because of the inclusion of volunteers. Citizen participation in many of the forest health focused programs led to many first reports or findings. For example, a citizen scientist in the Observatree program found the second site for the oriental chestnut gall wasp outbreak in the UK (Observatree, 2015; Brown et al., 2020), quickly stimulating the response of officials. Similarly, the first report of Phytophthora ramorum from a county connecting the epidemics in California and Oregon was made during a Sudden Oak Death (SOD) Blitz (COMTF, 2019). The SOD Blitz program is a long-standing citizen science program to monitor the spread of P. ramorum throughout the state of California (Garbelotto et al., 2014). Results of the initiative have also contributed to the understanding of the epidemiology of the pathogen (Meentemeyer et al., 2015; Lione et al., 2017). In the Cape Citizen Science program in South Africa, nine Phytophthora species were recovered for the first time that would not have been possible without citizen participation (Hulbert, 2020). Together, these examples demonstrate the merit of engaging the public in detection efforts and the value of having “many eyes” in nature to enhance biosecurity in the post-border stages of an biological invasion framework.

Citizen science programs can increase the chances of successful eradication through early detection and possibly, through direct action to remove invasive non-native pests or pathogens. Early detection of a pest or pathogen from citizen scientists can on occasion directly enable successful eradication in local environments. The report of the oriental chestnut gall wasp by a trained Observatree participant led to rapid intervention measures to eradicate the pest around St. Albans, UK (Brown et al., 2020). Eradication of forest pests and pathogens before they spread is often difficult because of the long lag phase between when the introduction occurred and the first detection, but citizen science programs can increase capacity of early warning systems and reduce the lag phase (Crow and de Groot, 2020). In another example from the Sudden Oak Death (SOD) Blitz program, local citizens took it upon themselves to remove trees in their community confirmed infected with P. ramorum (Pers. Comm. Garbelotto, 2017). Together these examples demonstrate that citizen scientists can increase the success of rapid responses and be empowered to act to reduce the spread of invasive non-native pests and pathogens.

Citizen science programs can also benefit management strategies focused on containment rather than eradication or detection. For example, the SOD Blitz program has provided accurate and up-to-date information about the distribution and epidemiology of P. ramorum in California to be able to predict hotspots of disease emergence (Meentemeyer et al., 2015) and detect relationships between climatic data and the epidemiology of the pathogen (Lione et al., 2017). This information is invaluable for prioritizing targeted areas for intervention and for adaptive management under anticipated changes to the climate. Similarly, Oregon has also adopted a strategy to involve citizens in monitoring for sudden oak death (Kline et al., 2019). The program involves citizens in sampling streams with bait leaves, a method used for early detection of infestations in entire watersheds (Sutton et al., 2009), and provides valuable data for their active containment and site-based eradication strategies (Hansen et al., 2019).

Citizen science has also been used in conjunction with statutory surveys to map the distribution of disease epidemics. Brown et al. (2017) evaluated the value of incorporating pre-existing citizen reports (e.g., TreeAlert) to increase the efficiency of statutory surveys of acute oak decline in England; the study indicated that incorporating citizen reports maximized the use of available resources to focus on defining boundaries of the affected area. Crow and de Groot (2020) also highlight how Observatree and LIFE ARTEMIS have been integrated in official monitoring systems in the UK and Slovenia, respectively. Therefore, citizen observations can provide important distribution information that can be further used to amplify ongoing regulatory surveys to determine boundaries of biological invasions. These data can then be used to inform interventions in areas near the boundaries similarly to the strategy used to manage the sudden oak death epidemic in Oregon (Hansen et al., 2019).

A final example of a citizen science project actively engaging citizens in the containment stage of a forest epidemic is the Kauri Rescue Program in New Zealand.¹⁰ The program invites citizens to test treatments of varying phosphite stem-injection doses to reduce the impacts of a root disease caused by Phytophthora agathidicida on the culturally important kauri (Agathis australis) tree (Bradshaw et al., 2020). As of 2019, about seventy citizens had treated more than a thousand trees using treatment kits provided by the program and many were on private lands (Kauri Rescue, 2019). This program therefore demonstrates the possible engagement of citizens in strategies to reduce impacts and contain the expansion of forest disease epidemics.

Citizen scientists can also play a role in documenting the spread and impacts of invasive non-native species and contributing to understanding of biological invasions or measuring the success of management approaches. Indeed, forest pests or pathogens provide an opportunity to convey the ecological complexity of biological invasions and specifically the importance of documenting interactions amongst species to contribute to assessment of impacts on biodiversity and ecosystems (Groom et al., 2021).

Options to restore ecosystems back to a resilient state after an invasive non-native forest pest or pathogen has been contained or eradicated are limited. Re-planting non-host species is often a strategy used in urban environments (Liu, 2018), but efforts to breed resistant varieties may be the best solution to restore natural ecosystems (Sniezko, 2006). However, the availability of genetic material with potential for resistance and suitable for breeding can be difficult to find.

Citizen science can aid restoration efforts with increased numbers of people searching for naturally resistant material and providing a low-cost means to maximize search efforts (Ingwell and Preisser, 2011). The Save the Ash Citizen Science program¹¹ in Sweden aimed to encourage citizens to help locate rare “vital” ash trees within landscapes devastated by ash dieback. European ash populations have been significantly reduced following the introduction of the non-native fungus Hymenoscyphus fraxineus, and in Sweden the tree species is endangered (Hultberg et al., 2020), though a small proportion of the natural population shows high resistance to the disease (Cleary et al., 2017; Liziniewicz et al., 2022). Citizen participation was ideal for identifying resistant phenotypes for breeding because ash in Sweden is found at very low proportions within forests (<1% of the forest inventory stock), usually mixed with other temperate broadleaved tree species, and is sparsely distributed across the country (Cleary et al., 2017). Widespread publicizing of the project in media (radio, TV, newspapers, nature/forestry magazines, social media, and the website) and instruction on specific criteria for identifying healthy trees, generated hundreds of responses from the public and informed subsequent surveys. Consequently, more than 900 ash genotypes are now included in the resistant inventory database and currently undergoing screening by established field trials to identify tolerant or resistant ash populations suitable for restoration. The initiative provided a low-cost means of maximizing search efforts across wide geographic areas, raised awareness of the concern regarding the loss of this important keystone species and risk to other associated biodiversity, and has even resulted in wider spreading of awareness regarding ex-situ conservation of ash by the citizens themselves, through online blogs which they themselves manage.

Similar citizen science approaches are underway with a number of tree species in the United States using a mobile application called TreeSnap.¹² The application was created for citizen scientists to contribute to research by locating seemingly resistant trees (Crocker et al., 2019). The application invites citizens to contribute observations of nine focal tree species, including tanoak, one of the species critically threatened by Phytophthora ramorum (Cobb et al., 2012). The application has received more than 2500 observations from at least 1600 users (Crocker et al., 2019), demonstrating the value that citizens can contribute to research and improvements for restoration through breeding with naturally resistant genotypes.