Weeds and pests
Fish have been reported to control pests in rice fields — weeds (summarized in Cagauan, 1995a), Nymphula depunctalis (Guenée) (Vromant et al., 1998) and Pomacea canaliculata (Lamarck) (Halwart et al., 1998). As a consequence, many scientists and extension workers claim that fish are effective controllers of rice pests, in general, and planthoppers and leafhoppers in particular. The stocking of fish in rice fields is thought to have many advantages. One advantage among others, is the control of certain arthropod pests. Fish can eat pests that fall into the water or those that pass part of their life cycle in the water or at the base of the rice plant (Cagauan, 1995a). In a general overview of the literature, Cagauan (1995a) mentions only six studies on the control of arthropods by fish, most of them lacking statistical strength. From these studies, only a few groups of arthropod pests seem to be controlled: leafhoppers, planthoppers, leaffolders and stemborers (Cagauan, 1995a; Xiao, 1995). The species eaten and the quantities consumed are related to the life cycle of the species (Xiao, 1992). However, so far, little information is available on this subject. As most research has focused on the integration of fish in transplanted rice fields, little is known about the effect of fish on the arthropod populations of direct seeded rice fields. Transplanting is more common in ricef ish culture as this reduces the competition for space between rice plants and also provides spaces between plants where fish can move freely and feed (Fernando, 1993).Halwart (1992) doubts if fish can effectively control pest populations or reduce damage in direct seeded rice fields as the access of fish into the field is hampered by the dense vegetation. Rice-fish culture demands a higher water level than rice monoculture. Fish need a water depth of at least 10 cm (Singh et al., 1980). Alterations of the rice environment can act to the detriment of some pest species, but can favour others (Loevinsohn, 1994). As such, standing water will favour aquatic and semi-aquatic pests, like the rice caseworm Nymphula depunctalis (Guenée) (Reissig et al., 1985; dela Cruz & Litsinger, 1988).
As pesticides are considered a constraint to raising fish in rice fields (Koesoemadinata, 1980; Cagauan & Arce, 1992), only the fish are left to control pests favored by the prolonged standing water in the field. It is therefore important to know if fish can control such pests under natural field conditions. The rice caseworm occurs in fields with prolonged standing water in the vegetative stage of the rice plant (Pathak & Khan, 1994), as is the case in rice-fish fields. Both authors consider the rice caseworm to be an important pest of rice. The larval stage cuts off the leaf tips and further damages rice leaves by scraping the leaf tissue. Yield loss may occur if other pests infest the crop during the first 30 days after transplanting (Pathak & Khan, 1994). The rice caseworm is an important pest in the Mekong Delta, Vietnam. A survey by Chi et al. (1995) showed that about 40% of the rice farmers interviewed in Thoi Long village, Can Tho province, Mekong Delta, perceived the rice caseworm to be a major pest, and a survey by Cuc (unpublished) in Kien Giang province, Mekong Delta, showed that 16.1% of all interviewed farmers considered the rice caseworm to be an important pest. Neither of these surveys focused specifically on rice-fish fields. Mekong Delta farmers use mainly Methyl parathion, Monocrotophos, BPMC, Methamidophos and Cypermethrin to control rice caseworms (Mai et al., 1993).
So far, there have been no scientific publications on the control of rice caseworms by fish, although Halwart (1992) suggests that the larval stage of the rice caseworm could be controlled by fish. In the dry season of 1996-97, a rice-fish experiment was carried out on 18 direct seeded plots at the experimental rice-fish station of the Co Do cooperative farm, Can Tho province, Mekong Delta. The aim of the experiment was to evaluate the effect of fish and different rice seeding rates on the ecology of direct seeded rice fields. During the experiment, a natural rice caseworm outbreak occurred (with a peak 20 times higher than that reported in previous rice cropping seasons at the Co Do station (N. Vromant, unpublished)). Data on caseworms in fields with and without introduced fish were collected during the outbreak