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).