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.