Horizon scanning, a method used in prioritization of invasive alien species (Roy et al. 2014, 2017, 2019; Bayón and Vilà 2019) was applied in this study to highlight potential invasive plant pests of economic importance to Kenya. The species were ranked according to their potential threat and appropriate actions were prioritised. Since the exercise was conducted in 2018, two insect species have been found to be present in Kenya. The red gum lerp psyllid, Glycaspis brimblecombei, was found in Kenya in 2018 but reported in 2020 (Wondafrash et al. 2020), so was already present at the time of the assessment. It was scored as high risk (Table 1) because it had recently invaded eucalyptus production areas in different parts of the world, including several African countries. In all these areas, it quickly became a pest of commercially important eucalyptus species, such as Eucalyptus camaldulensis. It remains to be seen how damaging the species becomes in Kenya, as in several regions, the intentional or accidental introduction of its specific parasitoid Psyllaephagus bliteus entirely or partially controlled the pest (Caleca et al. 2018). If the economic impact in Kenya is significant, biological control should be considered.
The second arthropod species that was recorded in Kenya after the assessment is D. suzukii, a fruit pest of Asian origin (Kwadha et al. 2021). In our assessment it only had moderate overall risk score because, despite its rapid worldwide spread in recent years, in Africa it had only been reported in Morocco and Réunion. Also, D. suzukii is mainly a pest of temperate climates that does not cope well with high temperatures (Ørsted and Ørsted 2019) and its main host fruits (small berries and other soft fruits) are not yet major crops in Kenya. So, the likelihood of establishment and potential economic consequences were both scored as moderate. However, several susceptible fruits from temperate climates such as blueberry, strawberry, raspberry and apricot are becoming increasingly grown in the Kenyan highlands (Kwadha et al. 2021), so the potential for economic damage is increasing. The possibilities for containment and or management measures (Kwadha et al. 2021) such as biological control (Seehausen et al. 2021) should therefore be assessed.
A third species now recorded as present in Kenya but not at the time of the assessment is P. parmentieri, a soft rot Enterobacteriaceae (Ma et al. 2007; van der Wolf et al. 2021). Pectobacterium wasabiae was found for the first time in surveys in 2016/2017 (Kamau et al. 2019), but this species had earlier been transferred to P. parmentieri (Khayi et al. 2016), so is now considered as present in the country. It was scored as having a relatively low likelihood of entry, but a high likelihood of establishing and causing significant economic damage. Although potato spindle tuber viroid (PSTVd) was recently reported in S. anguivi, S. anomalum, S. cerasiferum, S. coagulans, S. dasyphyllum, S. incanum, S. macrocarpon and S. virginianum (Skelton et al. 2019), PSTVd has not yet been reported in key value chains, pepper and chili (Capsicum annuum), pepino (S. muricatum), eggplant (S. melongena) and potato (S. tuberosum) which it also naturally infects (Mackie et al. 2002).
The above examples suggest there may be other species present that have not yet been detected. Thus, one of the follow-up actions (Table 2) identified for high risk species, particularly those recorded as present in neighbouring countries, is detection surveys. Examples include B. tabaci (MED species) (Misaka et al. 2020), Anoplolepis gracilipes (Löhr 1992), Bruchus pisorum (Mendesil et al. 2016), Pseudaulacaspis pentagona (CABI 2021b), Trogoderma granarium (EPPO 1981) and pathogenic species such as C. fimbriata (Rouxa et al. 2001), X. citri (Balestra et al. 2008; Derso et al. 2009; Ference et al. 2018), P. atroseptica (CABI 2021b), and Faba bean necrotic yellows virus (Abraham et al. 2000).
Other species were adjudged as having a moderate likelihood of entry, but had a high overall score due to the high scores obtained for likelihood of establishment and magnitude of socio-economic impact. Kenya has prioritized a number of value chains under the Agricultural Sector Transformation and Growth Strategy, 2019–2029 as key to improving livelihoods and supporting economic growth (MoALF 2019), so species which could affect the prioritised value chains are appropriate targets for conducting a full PRA, and, based on the outcome of the PRA, implementation of import controls and the preparation of contingency plans. Species in this category included Thrips palmi (melon thrips), Cacoecimorpha pronubana, Euwallacea perbrevis (tea shot-hole borer), and Peronospora sparsa (cause of downy mildew of roses) among others.
A further group of species scored low on likelihood of entry, but high on likelihood of establishment and socioeconomic impact, so they were not near the top of the overall ranking. Several of the species in this group are potential pests of potato, including D. solani, D. dadantii, D. dianthicola, C. michiganensis subsp. sepedonicus, Phoma exigua f.sp. foveata, and Synchytrium endobioticum). Potato is an important crop in Kenya which has already recently suffered from new pests such as PCN (Mwangi et al. 2015; Mburu et al. 2018, 2020). For several of these species, adjusting the score for likelihood of entry upwards by 1 would put the final overall score in the top 20. This suggests that follow-up activity should not necessarily be limited to the highest-ranking species, and that the scores need to be regularly reviewed (see below).
The methodology for horizon scanning described here is an adaption of previously used methods (Sutherland et al. 2011; Roy et al. 2014, 2019), adjusted for the Kenyan context. Such pest prioritisation schemes have emerged particularly in the last decade to support plant health decision making by risk managers and policy makers in prioritising the large number of potential invasive species (MacLeod and Lloyd 2020). Most of these prioritisation or risk ranking systems have been deployed in high income countries, so the use of this approach in Kenya was novel.
The approach adopted worked well, bringing together experts from a range of organisations. They achieved consensus on modifying and agreeing the exact definitions of the scores, as well as in the final scoring when there were discrepancies between the experts’ scores. As invasive plants were excluded from the exercise, further work would be required to adjust the definitions to cater for plants as pests. The area in which the experts felt least comfortable was in scoring of potential environmental impact. Few species scored highly on environmental impact; three arthropods and two pathogens scored 4. This may be because socioeconomic impacts of invasive arthropods and pathogens are generally better known than their environmental impacts, and confidence in the environmental impact score was often low. Invasive ants are an exception, considered as serious environmental pests worldwide, including in Africa (Mikissa et al. 2013; Mothapo and Wossler 2017; Mbenoun Masse et al. 2017). Two ants in our assessment had a high score for their potential environmental impact, Anoplolepis gracilipes and Linepithema humile. In other continents, many non-native herbivores and plant pathogens are considered as serious threats to native biodiversity and ecosystems (Kenis et al. 2009; Ghelardini et al. 2017).
Above it was noted that a change of 1 point in a score could move a pest many places up the list of priorities, and pest prioritisation schemes are not without shortcomings (MacLeod and Lloyd 2020). Rather than being a one-off activity, the results should be reviewed regularly, particularly in the light of any new information that might arise. Thus, part of the outcome of a horizon scanning process such as this could be systematic monitoring of information sources to detect possible changes to risk, which can be recorded in a risk register. Kenya does not have a risk register or list of prioritised pests for prevention, and thus the horizon scanning process adopted here could provide the basis for such a register. Given the practicality of the approach and the widespread lack of pest prioritisation in SSA, we propose that the approach reported here could be beneficially adopted in many other countries on the continent. It could also be implemented at a sub-regional level, such as by the East African Community (Eastern Africa), Southern African Development Community (Southern Africa) or Economic Community of West African States (Western Africa), and possibly by the African Union, for which a new Plant Health strategy is currently in development.