USE OF SPIDERS AS NATURAL ENEMIES TO CONTROL RICE PESTS IN KOREA Joon-Ho Lee and Seung-Tae Kim Entomology Program, School of Agricultural Biotechnology, Seoul National University, Suwon 441-744, Korea ABSTRACT Spiders play an important role in regulating insect pests in the agricultural ecosystem. There are a large number of species, many of them with high population densities. There are 22 families, 99 genera and 175 species of spiders in Korean rice fields. They limit the avail- ability of habitats open to insect pests of rice by occupying various microhabitats. They have a wide range of prey species, catch significant numbers of prey and use various foraging strate- gies. Most of the spiders in rice fields seem to evacuate the field after the application of insec- ticides and move back into the field later. Their predatory capacity can have a synergistic effect in suppressing densities of insect pests when they are used to complement the effect of insecti- cides. INTRODUCTION do not damage plants. They can achieve an equilibrium in pest control, after which their own For many decades, insecticides have been numbers are suppressed by their territoriality and widely used to control rice pests. However, the cannibalism. For some time, spiders have been continuous use of a wide range of pesticides has considered important predators which help regulate caused many side effects, including loss of the population densities of insect pests (Pickett et al. biodiversity, the problem of secondary pests, 1946, Dondale 1956, Duffey 1962, Kajak et al. insecticide resistance, residual toxicity, the 1968, Fox and Dondale 1972, Tanaka 1989). In resurgence of insect pests, and environmental particular, spider communities in areas with a pollution. Recently, many efforts have been made temperate climate achieve equilibrium in the control to combine various non-chemical control methods of agricultural pests (Riechert 1982). In spite of with insecticides in systems of Integrated Pest this, they have not usually been treated as an Management (IPM). One such effort is the combined important biological control agent, because there is use of natural enemies with insecticides. Many uses so little information on the ecological role of spiders of parasitic and predatory natural enemies to control in pest control (Turnbull 1973, Riechert and Lockley agricultural pests have been reported (Van den 1984). Bosch et al. 1992). However, research has shown that spiders in Spiders are carnivorous arthropods. They are rice fields can play an important role as predators in classified into 40,000 species, and are found all over reducing the densities of planthoppers and the world in almost every kind of habitat. They leafhoppers (Hamamura 1969, Sasaba et al. 1973, mainly prey on insects, although they may also feed Gavarra and Raros 1973, Samal and Misra 1975, on various other kinds of prey. The population Kobayashi 1977, Chiu 1979, Holt et al. 1987, Tanaka densities and species abundance of spider 1989). In Korea, many studies have been carried communities in agricultural fields can be as high as out to evaluate spiders as a biological control agent. in natural ecosystems (Turnbull 1973, Tanaka 1989, This Bulletin reviews past research into spiders in Riechert 1981). rice fields in Korea, and presents an effective method Spiders consume a large number of prey, and of using spiders as a biological control agent. Keywords: hunters, Korea, pesticides, spiders, rice pests, web builders 1 STUDIES OF RICE FIELD SPIDERS 1999). However, most of these studies were limited IN KOREA to the identification of spiders, and to investigating the dominant spider species, their regional Since E. Strand first described Ganphosa distribution, seasonal fluctuations and the effect of koreae in 1907, there have been many publications insecticides. There were few studies on the spatial about spiders in Korea. Currently, a total of 46 distribution of spiders, how this is related to their families, 222 genera and 626 species of spiders are ecological role, and how many insect pests they recorded from Korea (Namkung et al. 2000). consume in rice fields. Up until 1970, most of the research on spiders concentrated on identification. From the early RICE FIELD SPIDERS AVAILABLE 1970s, researchers began to study the basic IN KOREA ecological and biological characteristics of spiders as biological control agents. Spiders in rice fields The spiders living in rice fields in Korea have been studied more than spiders on other crops comprise 27.96% of all Korean spiders, and can be (Park et al. 1972, Paik and Kim 1973, Paik et al. classified into 22 families, 99 genera and 175 species 1974, Choi and Namkung 1976, Okuma et al. 1978, (Table 1). It has been reported that P. subpiraticus, Paik et al. 1979, Yun and Namkung 1979, Paik and P. clercki and G. dentatum are dominant spider Namkung 1979, Paik and Namkung 1979, Kim et species in rice fields (Paik et al. 1979). However, al. 1990, Kim 1992, Lee et al. 1993a, 1993b, Song more recent studies have shown that there is no and Lee 1994, Kim and Kim 1995, Kim 1995a, regional pattern in the distribution of rice field 1995b, Im and Kim 1996, Lee et al. 1997, Yun spiders, and that P. subpiraticus is the overall 1997, Yang et al. 1998, Kim 1998. Im and Kim dominant species in Korea. Table 1.Overview of spiders in Korean rice fields 2 Table 2.Important spider predators of rice pests SEASONAL OCCURRENCE OF SPIDERS IN (1997) suggested that the different time taken by KOREAN RICE FIELDS different spider species to immigrate from levees into paddy fields is the cause of the low initial Seasonal fluctuations of spiders in rice fields population densities, in spite of the abundance of in Korea have been reported to follow an “M” prey. However, Kim (1998) showed that the shaped curve (Im and Kim 1996, Choi and Namkung immigration rate of overwintering spiders from 1976, Hokyo et al. 1976, Kim et al. 1990, Paik et al. levees into paddy fields was low. He suggested that 1979, Yang et al. 1998). Other surveys which began most of the dominant spider species immigrated at a different time and covered a different period from outside areas into the rice fields by ballooning have reported that fluctuations in the density of (Note: “Ballooning” is the term used to describe the spider communities followed a curve shaped like a habit of young spiders of sailing through the air “W” (Choi and Namkung 1976, Yun and Namkung borne by silk strands on wind currents). 1979). He also suggested that the immigration of the During the rice growing period in Korea, rice spiders was delayed after transplanting because the field spiders generally show two peaks. One is young rice plants are too small to provide a suitable when spiders which have overwintered nearby habitat for either hunters or web builders. migrate into the paddy, and the other is when the Even though web builders were more second generation of the year appears. However, numerous than hunters in some areas (Choi and annual fluctuations in numbers of rice field spiders Namkung 1976, Yun and Namkung 1979, Lee et al. showed several peaks when overwintering periods 1993a, 1993b, Kim and Kim 1995), seasonal are included (Fig. 1). fluctuations in rice field spiders in Korea are thought Spiders in rice fields in Korea start to build up to depend mainly on the density and species their populations in the middle of July, 40 - 45 days composition of hunting species of spider (Table 3, after transplanting, (Kim 1992, Yun 1997). The Park et al. 1972, Paik and Kim 1973, Paik et al. same pattern is seen in Japan, which has a similar 1974, Okuma et al. 1978, Paik et al. 1979, Paik and agricultural environment (Kobayashi and Shibata Namkung 1979, Kim 1992, Song and Lee 1994, 1973). Hamamura (1969) has reported that the Kim and Kim 1995, Im and Kim 1996, Yang et al. density of spiders in rice fields in Japan is always 1998, Im and Kim 1999). Kim (1998) pointed out low when transplanting is taking place. Lee et al. that if spiders are being used as biological control 3 agents, it is very important to understand their life SUPPRESSION OF RICE BROWN styles. Both web builders and hunters follow a PLANTHOPPERS BY SPIDERS foraging strategy. Song and Lee (1994) suggested that web builders such as P. clercki were better able All the organisms in a rice field, as in other to suppress insect pests than hunters such as P. kinds of ecosystem, are involved in interactions subpiraticus. However, hunters are usually active within and between species, including prey-predator predators which follow a “pursue and kill” foraging and host-parasite relationships (Hijii 1984). In strategy, while web builders follow a passive “sit particular, the interaction of prey and predator show and wait” strategy (Enders 1974). Furthermore, the a constant numerical interaction. Information about webbing sites of web builders are easily affected by these relationships is fundamental to biological environmental factors. In addition, when the web control. spaces overlap, there is competition with and between Yamano (1977) suggested that spiders are the species of web builders. Therefore, hunters probably most important biological control agents regulating are more effective predators than web builders. insect populations in rice fields, including insect Fig. 1. Seasonal fluctuations in the mean density of spider populations in Yoju, Korea, 1994-1997 Table 3.Comparison of population levels and occupancy ratio of web builders and hunters in Yoju, Korea, 1994-1997 4 pests. Predators tend to cluster in stable sites where (Lin 1970). This part of the plant is usually shaded, many species of prey are maintained at a high and has a high relative humidity and a relatively low density (Schmitt 1987).