Management options
Being a new pest in Kenya and indeed Africa, management of GAS has
mainly relied on cultural, physical and on a desperate scale, a
“trial-error approach” on chemical practices. Similar results have
been reported around the world on the control of GAS in rice fields
(Joshi, 2007; Liang et al., 2014; Litsinger & Estano, 1993;
Tangkoonboribun & Sassanarakkit, 2009; Teo, 2003). Unfortunately, most
of these practices have proven to be either cumbersome, impractical,
expensive or ineffective. Worsening the situation is the unavailability
and inaccessibility of registered molluscicides in the country, which
has resulted in indiscriminate trial-error spraying further compounding
negative effects on environmental safety, human health and natural
enemies. Indeed, with the bio-ecology of GAS and its inevitable impacts
following the invasion, instituting an integrated approach that combines
cultural (water management, field hygiene etc.), physical (handpicking,
field screens etc.), biological (predators, snail parasitic
microorganisms, biopesticides etc.) and chemical control on an
areawide-scale is best for the management of this pest.
Material transfer, mechanisation and other human-mediated activities
have been found to aid the dispersal and introduction of pests in new
areas (Gippet et al., 2019; Litsinger & Estano, 1993; Ranamukhaarachchi
& Wickramasinghe, 2006). Whereas we did not find any adult GAS or their
eggs in Ahero, Bura, Hola or West Kano schemes, the authors learned of
the extensive material transfer and exchange (particularly seedlings of
the aromatic Pishori rice) and farm machinery (especially the combine
harvesters and rotavators) from Mwea- an infested area. While the
invasion boundary of GAS was limited to Mwea scheme, the transfer and
sharing of planting materials and machinery could exacerbate infestation
in lowly infested areas or lead to infestation in new areas. This could
pose a serious threat to rice farming in these areas should GAS invade
further compounding the already constrained rice industry. The
distribution model highlights these areas as highly suitable for
invasion. As such, and as the major purpose of a delimiting survey and
species distribution modelling, this information should aid in early
warning and contingency planning. Strict quarantine measures should be
developed and implemented to contain this invasive pest. Planting
material and machinery from Mwea should be thoroughly inspected and
disinfected before they are allowed into other schemes. Also having
records of the entry and exit of people, seedlings and machinery should
be implemented to have a traceable history and knowledge of
introduction.
While delimiting surveys are usually national, the one described in this
current study covered five rice-producing schemes, which represent over
80% of the rice farming areas in Kenya. As such, while the findings are
representative, a further national survey incorporating time and space
variables of all the rice-producing and irrigated areas is proposed. To
manage and curb further spread especially to other risk areas, the
following containment measures are proposed: Undertake training and
awareness of and on GAS through National Irrigation Authority (NIA) with
support of other county teams; prevent field entry using a mesh as
physical barriers especially of the uninfested sections; practice
physical/mechanical control through hand picking of snails and crushing
of egg clutches; community-based snail management through synchronised
farm activities like land preparation and application of control
measures (an areawide management approach); introduce changes to the
cropping system by avoiding ratoon crops to limit host availability for
GAS; practice cultural practices like alternate wetting and drying (AWD)
of paddies; Change planting patterns by encouraging transplanted rice
compared to direct seeding; Manage water in canals by limiting water
levels less than 2 cm above the soil surface to reduce GAS movement and
dispersal; desilting of canals to minimize the habitable areas where the
snails lay their eggs and reduce their populations among other
practices.