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.