Sampling procedure
We sampled 3- 10 evenly distributed sampling sites in the rice paddy and along the irrigation canals/feeder canals, depending on their length and accessibility; the sites considered as inhabited by GAS were those where eggs masses or live snails were found/observed after a preliminary inspection from the shores. At sites with GAS populations, we sampled 3 points of ca . 10 m located within less than 100 m; at sites without GAS only one section was sampled. Sampling comprised a total of 401 points from 68 georeferenced sites distributed across the five schemes (Table 1).
At each section, two people carefully inspected rice paddies, terrace walls and irrigation canal shores while wading upstream in search for egg masses on the emergent aquatic vegetation and other substrata; GAS were searched for among the submerged vegetation, under stones or buried in the substratum. Start and end points were marked as 10 x 10 m on rice paddies and 10 m along canals for eggs count. A 1 m2wooden frame quadrat was laid in three selected sampling points of 10 m. All visible adults and eggs were counted and recorded. At each sampling point, information on the number of adults, egg masses, crop variety, other hosts infested, GPS points and photographs, and time of sampling were recorded. Where the common white garden snails were found, this was recorded as well. The starting points for sampling were considered based on trace-back information that confirmed that the upper sides were the invasion points. Therefore, sampling was started from the lower (low infestation) to the upper ends to avoid any chance of spreading the GAS to lower un-infested sides.
Special attention was given to drainage and irrigation canals as they are significant for the dispersal of the pest. All the sections of the Mwea irrigation scheme (including outgrowers) were surveyed i.e. Mwea, Tebere, Kiamanyeki (outgrower), Ndekia (outgrower), Wamumu, Thiba, Karaba, Kandongu/Ngothi (outgrower) and Mwea Irrigation Authority Development (MIAD) centre area. A similar procedure was used in Ahero, Bura, Hola and West Kano irrigation schemes. Sampling was done in 68 geo-referenced farms using a global positioning system device Garmin, eTrek© 20x (Garmin, USA) across five major rice-cultivating locations of Kenya.