Discussion.
Delimiting surveys are important in helping to establish the boundary of the spread of a pest and to help in the management and development of quarantine strategies to limit its spread to other risk areas (McMaugh, 2005). Knowledge of the boundary of invasion is important in appropriate resource (financial, personnel, time etc.) allocation in the surveillance and management of the pest.
GAS is listed among the world’s worst invasive species’ (Lowe et al., 2000) and its impacts have been documented in various parts of the world (Arfan et al., 2014; Correoso et al., 2017; Cowie, 2009; Cowie & Hayes, 2012; Halwart, 1994; Hayes et al., 2008; Martín et al., 2017; Seuffert & Martín, 2013; Win, 2020; Yang et al, 2018). After its first report in Mwea, Kenya in 2020, a delimiting survey to establish invasion boundary was conducted in five leading rice-producing areas in Kenya. Results from the survey showed an increase/expansion in the range from the initial point of infestation (Ndekia) to other sections of the scheme. Indeed, as reported in previous studies, several biotic, abiotic and human activities have aided the spread of GAS and other invasive aquatic snails around the world. The voracious appetite (Carlsson et al., 2004; Qiu et al., 2011), high reproductive rate (Cowie, 2009; Cowie & Hayes, 2012; Yusa, 2006), resistance to desiccation in dry down periods (Havel et al., 2014), lack of effective natural enemies (Cowie, 2009; Yusa, 2006), and the presence of sufficient water (Cowie, 2009; Salleh et al., 2012a; Teo, 2003) and food source- rice and wild host plants have been confirmed to aid spread. In Mwea, as observed, the infestation has followed or taken the route of slope and water flow i.e. along the river and irrigation channels/canals gradient. The flow or movement of water has the potential of spreading juvenile and adult snails downstream from where they are able to begin a new infestation. Martín et al. (2017) reported that the dispersal of GAS within streams depends on both crawling and drifting. Indeed, the infested irrigation channels form favourable habitats for the active and passive dispersal of snails, acting as pathways for the spread of GAS due to the continuous presence of water even during dry periods and supporting of secondary hosts for GAS.
Knowledge of local risks is critical in instituting and implementing any management practices for a pest (Finch et al., 2021). Species distributional modelling for GAS highlighted areas in Kenya and Eastern Africa which have the greatest potential for invasion. Of note, those areas that were modelled as highly suitable, to the south-east of the country and along coastland, harbour the major rice production areas in the region. Besides, it’s likely that these locations could serve as high-risk areas because of the rich agricultural lands that have water sources. While GAS has not been reported in other countries in Eastern Africa, our results suggest large areas of high suitability across the region. Of particular concern is Madagascar as it is one of the largest rice producers in the region (FAO stat, 2020), and has high suitability for GAS in the eastern and central parts of the country, where rice production is at its highest. In addition, were GAS to enter and persist in Lake Victoria, its spread into surrounding rice production in Tanzania and Uganda could be rapid, and ongoing dissemination throughout catchments and into major river systems such as the White Nile could be disastrous for the region (Djeddour et al., 2021).
Sufficient food is an essential requirement for the establishment and further dispersal of any species (invasive or otherwise) (Sanico et al., 2002). Mwea, Ahero, Bura, Hola and West Kano schemes and indeed all the rice schemes in Kenya, have majorly rice crop in the field at any particular time. In instances where the rice crop is minimal or not available and the paddies are dry, the canals hold water and suitable hosts for the proliferation of GAS. Being its primary host, the GAS population increase is inevitable. Time of sampling and as such, the water temperature has an impact on the activity and density of adult GAS recorded (Cameron, 1970; Winterbourn, 1969), and indeed the minimum temperature was the second most important environmental variable for predicting the potential distribution of GAS. During the survey, and especially where sampling was done after midday (when the sun was overhead), few to no adult numbers were recorded with sections sampled in the morning (Kiamanyeki and Ndekia) on the other hand recorded significantly higher adult densities compared to those sampled early morning or late in the afternoon. This has implications for the timing of management practices as efficacy will be achieved when GAS is active as opposed to when they are submerged and inactive. However, on closer observation, egg clutches were observed on plant stems an indication that the adults were submerged below the water surface. Similarly, Barnes et al. (2008) reported that the occurrence of egg clutches on emergent substrates represents the first indicator of snail’s presence in an area, even at low adult density. Also, the high number observed in these two sections could be because the first report of the GAS infestation was confirmed in Ndekia. Taro and arrowroots stems were the common egg laying substrates observed in the field. These findings concur with those of other studies which reported the preference of GAS to Taro (Colocasia esculanta ), arrowroots (Maranta arundinacea ) and other broad-leaved aquatic weeds e.g.Limnocharis flava (Alismataceae), for oviposition (Arfan et al., 2016; Joshi, 2007; Sanico et al., 2002).