Introduction
Biological invasions are major contributors to environmental or ecosystem change with invasive alien species driving this change by challenging the conservation of biodiversity and natural resources (Simberloff & Von Holle, 1999). This is achieved through significant interactions e.g. interspecific competition which can influence habitat selection, avoidance behaviour, and niche overlap (Davis, 2003; Martin et al., 2010; Rosenzweig, 1987; Young, 2004). Indeed, the least desirable outcomes of these interactions are habitat alteration (Burks et al., 2011; Carlsson et al., 2004) and in extreme circumstances, extinctions do occur (Clavero et al., 2009).
Among the notable invaders throughout the world are species from the Pomacea genus, including Pomacea canaliculata (Lamarck, 1829) (channelled apple snail or Golden Apple Snail (GAS)) (Cowie & Hayes, 2012; Hayes et al., 2012, 2015; Naylor, 1996). GAS is an invasive exotic gastropod invading freshwater aquatic systems throughout the tropics and is listed among ‘100 of the world’s worst invasive species’ (Lowe et al., 2000). It is native to South, Central and North America (Hayes et al., 2008) and has gained pest status outside its native ranges, becoming a serious agricultural and ecological pest, that has caused significant economic losses in wetland rice cultivation including threatening biodiversity (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). As with many other successful invasive species, several factors contribute to the invasiveness of GAS, especially in the invaded ranges. They include high adaptability to stressful environmental conditions, high reproductive rates and a lack of potential and effective natural enemies (Cowie, 2009; Yusa, 2006) in the invaded ranges. Also, sufficient irrigation water (Cowie, 2009; Salleh et al., 2012a; Teo, 2003) and food sources- rice and wild hosts e.g. Taro, (Sanico et al., 2002) have facilitated the spread of this pest, especially in rice fields. The amphibious respiration, capacity to aestivate during dry periods, as well as cold acclimation and tolerance, increase the resilience of GAS to rice farming practices, including agrochemical applications, intermittent drainage and crop rotations – under a wide range of climatic conditions (Horgan, 2018). Damage and economic losses continue to be reported in rice-producing areas as a result of replanting to replace damaged crop and due to increased production costs occasioned by the application of management practices (Halwart, 1994; Ranamukhaarachchi & Wickramasinghe, 2006; Salleh et al., 2012) in addition to the non-crop impact to human health and natural ecosystems (Joshi, 2007).
Rice is the third most important crop in Kenya and plays a critical role in increasing household food security and farmers’ incomes. It is mainly produced under irrigation with production expected to increase with the construction of Thiba dam in Mwea. In the past decade, rice consumption has increased at a rate of 12% annually, compared to 4% for wheat and 1% for maize (Atera et al., 2018). However, rice demand in the country exceeds production (Kenya only produces 19% of the rice consumed within the country) with imports filling the gap between production and consumption. In addition to production constraints related to water shortage, pests and diseases (Watanabe et al., 2021), GAS has added to the long list of challenges in rice farming threatening the entire value chain. As such, further invasion of GAS in the already thin rice industry would be detrimental to the country’s economy and livelihoods. Previous studies have found that GAS can reduce rice yields by as much as 50% with economic impacts running into millions of US dollars (Djeddour et al., 2021; Halwart, 1994). Thus, working out where GAS has spread to, and might spread to, could be used to give early warnings to decision-makers to try and limit the effects of any potential invasion. This will also help in contingency planning in case of invasion.
Since its first report in Kenya in 2020 (Buddie et al., 2021), GAS continues to expand its range from the invasion point where damage to rice crop continues to be reported by farmers. Its arrival or introduction pathway in Kenya is however still contentious. Unconfirmed media reports in Kenya suggest that the snail was introduced as a weed biocontrol, but no import permit to introduce the species has been issued by authorised organisations in the country. In Kenya, the management of this pest has majorly relied on physical and cultural practices with the desperate use of broad-spectrum synthetic chemicals. Other available strategies employed by farmers include hand-picking of adults and crushing of egg masses and water/flood management (alternate wetting and drying). However, most of these practices have proved ineffective to curb the spread of GAS. This is further exacerbated by the fact that the country has no pesticides (Molluscicides) registered specifically for the management of GAS.
Following a status survey in September 2020, samples of GAS were collected and molecular work to barcode was carried out to ascertain the true identity of this snail species. After confirmatory work of the identity, CABI and Kenya Plant Health Inspectorate Services (KEPHIS) undertook a delimiting survey in the five major rice production areas i.e. Mwea, Ahero, Bura, Hola and West Kano irrigation schemes in Kirinyaga, Tana River and Kisumu Counties to establish the boundary of spread since its first report and to help in the management and development of quarantine strategies to limit the spread of this pest. In addition, species distribution modelling using the Ensemble model approach was conducted to model the potential distribution of GAS in East Africa.