Snails as biological weed control agents in rice paddies of Haean catchment

Controlling insect pests and weeds is crucial in managing rice, the world’s most important food crop. Due to limits of land area available for an expansion of rice production, increases in productivity have to be achieved mainly through higher yields on already existing rice fields. While conventional rice farming involves the application of herbicides as well as pesticides which can have adverse effects on the environment , organic management systems have evolved which depend on little or no application of chemicals. In the Haean-myeon Catchment, South Korea, the introduction of Pomacea canaliculata as a biological weed control agent is being practiced by a number of farmers. Within the scope of this study conventional and organic rice farming practices, the latter using P. canaliculata, are compared in terms of their scales of rice leaf and panicle herbivory as well as weed cover. Moreover, it is analysed if differences in productivity exist between management types and to which extent these can be related to herbivory damage or weed cover. Another integral part of this study is the investigation of population and dispersion characteristics of Pomacea canaliculata within and outside of a rice field which is analysed using a mark-recapture approach. Moreover, as P. canaliculata is a highly invasive species, the distribution of this snail species within the Haean-myeon Catchment, South Korea, is investigated. While no significant difference between management types could be found in terms of herbivory, significant differences could be shown concerning weed cover which was found to be much higher in conventional fields without snails. Productivity was shown not to be significantly different between the two systems, but the most productive field was shown to be an organic one. The population size estimate for the population size of P. canaliculata within a rice field was computed to be 1135.8 ± 100.1 snails with a snail density of 0.78 snails per 0.5 m2 in June. Concerning the distribution of P. canaliculata within the catchment it was found that snails are distributed within most of the river system throughout the catchment excluding the elevated parts. While only adult P. canaliculata were present in June, hardly any adult individuals were encountered in September. Instead young individuals were present in large numbers.