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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, 2001-09-01

Abstract

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 availability 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 strategies. Most of the spiders in rice fields seem to evacuate the field after the application of insecticides 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 insecticides.

Introduction

For many decades, insecticides have been widely used to control rice pests. However, the continuous use of a wide range of pesticides has caused many side effects, including loss of biodiversity, the problem of secondary pests, insecticide resistance, residual toxicity, the resurgence of insect pests, and environmental pollution. Recently, many efforts have been made to combine various non-chemical control methods with insecticides in systems of Integrated Pest Management (IPM). One such effort is the combined use of natural enemies with insecticides. Many uses of parasitic and predatory natural enemies to control agricultural pests have been reported (Van den Bosch et al. 1992).

Spiders are carnivorous arthropods. They are classified into 40,000 species, and are found all over the world in almost every kind of habitat. They mainly prey on insects, although they may also feed on various other kinds of prey. The population densities and species abundance of spider communities in agricultural fields can be as high as in natural ecosystems (Turnbull 1973, Tanaka 1989, Riechert 1981).

Spiders consume a large number of prey, and do not damage plants. They can achieve an equilibrium in pest control, after which their own numbers are suppressed by their territoriality and cannibalism. For some time, spiders have been considered important predators which help regulate the population densities of insect pests (Pickett et al. 1946, Dondale 1956, Duffey 1962, Kajak et al. 1968, Fox and Dondale 1972, Tanaka 1989). In particular, spider communities in areas with a temperate climate achieve equilibrium in the control of agricultural pests (Riechert 1982). In spite of this, they have not usually been treated as an important biological control agent, because there is so little information on the ecological role of spiders in pest control (Turnbull 1973, Riechert and Lockley 1984).

However, research has shown that spiders in rice fields can play an important role as predators in reducing the densities of planthoppers and leafhoppers (Hamamura 1969, Sasaba et al. 1973, Gavarra and Raros 1973, Samal and Misra 1975, Kobayashi 1977, Chiu 1979, Holt et al. 1987, Tanaka 1989). In Korea, many studies have been carried out to evaluate spiders as a biological control agent. This Bulletin reviews past research into spiders in rice fields in Korea, and presents an effective method of using spiders as a biological control agent.

Studies of Rice Field Spiders in Korea

Since E. Strand first described Ganphosa koreae in 1907, there have been many publications about spiders in Korea. Currently, a total of 46 families, 222 genera and 626 species of spiders are recorded from Korea (Namkung et al. 2000).

Up until 1970, most of the research on spiders concentrated on identification. From the early 1970s, researchers began to study the basic ecological and biological characteristics of spiders as biological control agents. Spiders in rice fields have been studied more than spiders on other crops (Park et al. 1972, Paik and Kim 1973, Paik et al. 1974, Choi and Namkung 1976, Okuma et al. 1978, Paik et al. 1979, Yun and Namkung 1979, Paik and Namkung 1979, Paik and Namkung 1979, Kim et al. 1990, Kim 1992, Lee et al. 1993a, 1993b, Song and Lee 1994, Kim and Kim 1995, Kim 1995a, 1995b, Im and Kim 1996, Lee et al. 1997, Yun 1997, Yang et al. 1998, Kim 1998. Im and Kim 1999). However, most of these studies were limited to the identification of spiders, and to investigating the dominant spider species, their regional distribution, seasonal fluctuations and the effect of insecticides. There were few studies on the spatial distribution of spiders, how this is related to their ecological role, and how many insect pests they consume in rice fields.

Rice Field Spiders Available in Korea

The spiders living in rice fields in Korea comprise 27.96% of all Korean spiders, and can be classified into 22 families, 99 genera and 175 species ( Table 1(1168)). It has been reported that P. subpiraticus, P. clercki and G. dentatum are dominant spider species in rice fields (Paik et al. 1979). However, more recent studies have shown that there is no regional pattern in the distribution of rice field spiders, and that P. subpiraticus is the overall dominant species in Korea.

Seasonal Occurrence of Spiders in Korean Rice Fields

Seasonal fluctuations of spiders in rice fields in Korea have been reported to follow an "M" shaped curve (Im and Kim 1996, Choi and Namkung 1976, Hokyo et al. 1976, Kim et al. 1990, Paik et al. 1979, Yang et al. 1998). Other surveys which began at a different time and covered a different period have reported that fluctuations in the density of spider communities followed a curve shaped like a "W" (Choi and Namkung 1976, Yun and Namkung 1979).

During the rice growing period in Korea, rice field spiders generally show two peaks. One is when spiders which have overwintered nearby migrate into the paddy, and the other is when the second generation of the year appears. However, annual fluctuations in numbers of rice field spiders showed several peaks when overwintering periods are included ( Fig. 1(1213)).

Spiders in rice fields in Korea start to build up their populations in the middle of July, 40 - 45 days after transplanting, (Kim 1992, Yun 1997). The same pattern is seen in Japan, which has a similar agricultural environment (Kobayashi and Shibata 1973). Hamamura (1969) has reported that the density of spiders in rice fields in Japan is always low when transplanting is taking place. Lee et al. (1997) suggested that the different time taken by different spider species to immigrate from levees into paddy fields is the cause of the low initial population densities, in spite of the abundance of prey. However, Kim (1998) showed that the immigration rate of overwintering spiders from levees into paddy fields was low. He suggested that most of the dominant spider species immigrated from outside areas into the rice fields by ballooning (Note: "Ballooning" is the term used to describe the habit of young spiders of sailing through the air borne by silk strands on wind currents).

He also suggested that the immigration of the spiders was delayed after transplanting because the young rice plants are too small to provide a suitable habitat for either hunters or web builders.

Even though web builders were more numerous than hunters in some areas (Choi and Namkung 1976, Yun and Namkung 1979, Lee et al. 1993a, 1993b, Kim and Kim 1995), seasonal fluctuations in rice field spiders in Korea are thought to depend mainly on the density and species composition of hunting species of spider ( Table 3(1175), Park et al. 1972, Paik and Kim 1973, Paik et al. 1974, Okuma et al. 1978, Paik et al. 1979, Paik and Namkung 1979, Kim 1992, Song and Lee 1994, Kim and Kim 1995, Im and Kim 1996, Yang et al. 1998, Im and Kim 1999). Kim (1998) pointed out that if spiders are being used as biological control agents, it is very important to understand their life styles. Both web builders and hunters follow a foraging strategy. Song and Lee (1994) suggested that web builders such as P. clercki were better able to suppress insect pests than hunters such as P. subpiraticus. However, hunters are usually active predators which follow a "pursue and kill" foraging strategy, while web builders follow a passive "sit and wait" strategy (Enders 1974). Furthermore, the webbing sites of web builders are easily affected by environmental factors. In addition, when the web spaces overlap, there is competition with and between species of web builders. Therefore, hunters probably are more effective predators than web builders.

Suppression of Rice Brown Planthoppers by Spiders

All the organisms in a rice field, as in other kinds of ecosystem, are involved in interactions within and between species, including prey-predator and host-parasite relationships (Hijii 1984). In particular, the interaction of prey and predator show a constant numerical interaction. Information about these relationships is fundamental to biological control.

Yamano (1977) suggested that spiders are the most important biological control agents regulating insect populations in rice fields, including insect pests. Predators tend to cluster in stable sites where many species of prey are maintained at a high density (Schmitt 1987). Seasonal fluctuations in the numbers of spiders in rice fields showed a constant numerical interaction with those of other arthropods, including insect pests.

The brown planthopper ( Nilaparvata lugens Stål) is a pest which feeds only on rice plants (Otake and Hokyo 1976). Insecticides have been widely used to control brown planthoppers in Asian countries, including Korea (Nagata 1985). Integrated pest management programs to control this pest have been proposed, using natural enemies of planthoppers. Studies have reported numerous parasitic and predatory natural enemies of planthoppers. For predators alone, there are 14 species belonging to 13 genera in Korea (Okuma 1958, Suenaga 1963, Gavarra and Raros 1973, Samal and Misra 1975, Hamamura 1971, Chiu 1979, Kuno and Dick 1984, Shepard et al. 1987, Chua and Mikil 1989, Tanaka 1989, Kim et al. 1991, Choi et al. 1992, Choi and Lee 1990, Shepard and Ooi 1991, Ooi and Shepard 1994, Choi et al. 1996).

For effective control of brown planthoppers, predators should inhabit the same part of the rice plant, to reduce the time spent searching for prey. Brown planthoppers prefer to live on rice plants about 10 cm above the surface of the irrigation water (Lin 1970). This part of the plant is usually shaded, and has a high relative humidity and a relatively low temperature, both of which are favorable for reproduction of the pest. Leasar and Umzicker (1978) showed that humidity and temperature are the most important factors affecting the distribution of spiders. Most of the dominant species of spiders in Korean rice fields live on the lower 20 cm of rice plants ( Table 5(1324)).

Spiders represent more than 90% of the natural enemies of brown planthoppers living in paddy fields in Korea (Lee et al. 1997). Because of this fact, most of the studies related to biological control of planthoppers have focused on spiders. Table 6(1205) shows the mean number of brown planthoppers consumed each day by major species of rice field spiders. Subadults of P. subpiraticus consumed the greatest number, although the total numbers consumed by C. kurilensis and G. dentatum were as high. Spiders belonging to the species P. subpiraticus showed the greatest variation in daily consumption, while the number of planthoppers consumed each day by C. kurilensis and G. dentatum varied by only 0-4 planthoppers.

It appears that the difference in the numbers of planthoppers consumed by hunters and by web builders is the result of differences in the efficiency of their foraging strategies. Generally, female spiders consumed more prey than males, except in the case of G. dentatum. It is thought that females usually need more energy for oviposition and brood care, while males need only the essential energy for survival. Subadults had a higher consumption rate than adults, which implies that the energy needs for growth are higher than those needed for reproduction.

Compatibility of Spiders and Insecticides

Most studies of the effect of insecticides on natural enemies in the paddy field have focused on spiders. As we might expect, the results have shown that insecticides have a negative effect on the population densities of rice field spiders (Kuno 1968, Kuno and Hokyo 1970, Kawahara et al. 1971, Choi et al. 1978, Paik et al. 1979, Jang 1981, Ryu et al. 1984, Kim et al. 1984, Paik and Hwang 1990, Lee et al. 1993b, Bae et al. 1994).

The relative toxicity to spiders of various insecticides used against the brown planthopper was tested in the laboratory ( Table 7(1127)). Carbofuran, which is widely used in rice fields, is very toxic to spiders (Bae et al. 1994). Choi et al. (1978) have suggested that the root zone placement of Carbofuran reduces the density of spiders in rice fields by direct lethal action, and by poisoning their food chain.

All the insecticides which were most lethal to spiders were dust or granule types. Many of those tested, such as sprays and wettable powders, were selective, in that P. subpiraticus had a higher survival rate after a longer period of exposure. This was probably because of the active behavior of this species. It is recommended that if there is no difference in the effect of different types of insecticides on brown planthoppers, the use of dust or granule type insecticides should be avoided.

Several studies have shown that the application of insecticides reduces the population density of spiders in rice fields (Park et al. 1972, Kawahara et al. 1971, Paik et al. 1979, Kim 1992, Yun 1997). They have a negative effect, not only on numbers, but also on species diversity (Lee et al. 1993a, 1993b). Even insecticides which generally protect natural enemies often have a negative effect on spiders (Clausen 1990). Hunting species tend to suffer more damage than web builders (Specht and Dondale 1960, Legner and Oatman 1964, Bostanian et al. 1984), while active hunters suffer more than less active ones (Bostanian et al. 1984).

In our study, when insecticides were applied to rice fields, the number of many arthropods, including insect pests, fell immediately afterwards. However, the population density of spiders was not affected ( Fig. 3(1322)). This implies that many of the insecticides used in rice fields have some level of selectivity, and also that rice field spiders may have acquired insecticide resistance, as other arthropods have done. Therefore, the reduction of spider populations in rice fields after insecticide applications shown in previous studies was perhaps not because the spiders were killed by the insecticides, but because their prey were killed. Lee et al. (1993a) showed that the density of hunters was higher than that of web builders in the period after insecticides were applied. They suggested that during the recovery period, the active hunters played an important role in rebuilding spider densities at this time.

Factors which produce mortality in brown planthoppers include high temperatures, predation and parasitism by natural enemies, nutritional deficiencies, and the maturity of crops (Cheng and Holt 1990). Table 8(1198) shows the result of various combinations of insecticides, brown planthoppers and P. subpiraticus. The combination of an insecticide and P. subpiraticus gave the highest control value (66.8%). When insecticide was used alone, the control value was 63.4%, while P. subpiraticus used alone gave 51.6% control. This result suggests that there is a positive relationship between insecticide applications and control by P. subpiraticus.

When 16 individuals of P. subpiraticus were inoculated, the control effect was 4.5% higher than when 8 individuals were inoculated. This implies that if the population density of spiders is high enough, their control of brown planthopper can be as effective as that of insecticides, or even more effective.

The control effect of P. subpiraticus varied according to when it was introduced. When brown planthoppers were introduced first, the control value of P. subpiraticus was 51.6%. If P. subpiraticus was introduced before the planthoppers, the control value was 53.3%. Therefore, if the density of rice field spiders is high enough before the immigration of brown planthoppers, the spiders can control the planthoppers effectively and the number and quantity of insecticide applications can be reduced.

Conclusion

Large numbers of a wide range of spider species inhabit agricultural fields. Their presence limits the habitats open to insect pests. Spiders threaten insect pests with various foraging strategies. As well as consuming large numbers of insect pests as prey, they have the trait of killing all insects living in their territory. For this reason, spiders are a favorable biological control agent in the agricultural ecosystem.

Fagan et al. (1998) indicated that treatments which combine the augmentation of natural enemies with insecticide applications may be counterproductive. However, spiders still play an important role in reducing the numbers of insect pests in agricultural fields, even when insecticides are used. In fact, spiders may be responsible for a significant proportion of insect deaths which were thought to be from insecticide applications (Carter and Brown 1973). A quantitative analysis of the capacity of spiders to suppress insect pests, including the spatial distribution of major species of spiders and pests, should be carried out in the field on a large scale, so that spiders can be successfully used as biological control agents.

Other ecological and biological characteristics of spiders also need to be understood. Chiu (1979) suggested that it takes longer for spiders to rebuild their population densities after the application of insecticides than planthoppers and leafhoppers, because spiders have a longer generation interval. Also, the development of selective insecticides, the effect of insecticides on spiders, and appropriate timing and quantities of insecticide applications, should all be considered.

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Index of Images

Figure 1 Seasonal Fluctuations in the Mean Density of Spider Populations in Yoju, Korea, 1994-1997

Figure 1 Seasonal Fluctuations in the Mean Density of Spider Populations in Yoju, Korea, 1994-1997

Table 1 Overview of Spiders in Korean Rice Fields

Table 1 Overview of Spiders in Korean Rice Fields

Figure 2 No. of Spiders in Rice Fields, and Numerical Response of Other Arthropods, in Yoju, Korea, 1996.

Figure 2 No. of Spiders in Rice Fields, and Numerical Response of Other Arthropods, in Yoju, Korea, 1996.

Figure 3 Seasonal Fluctuations in Rice Field Spiders and Other Arthropods after Insecticide Applications in Yoju, Korea, 1996

Figure 3 Seasonal Fluctuations in Rice Field Spiders and Other Arthropods after Insecticide Applications in Yoju, Korea, 1996

Table 2 Important Spider Predators of Rice Pests

Table 2 Important Spider Predators of Rice Pests

Table 3 Comparison of Population Levels and Occupancy Ratio of Web Builders and Hunters in Yoju, Korea, 1994-1997

Table 3 Comparison of Population Levels and Occupancy Ratio of Web Builders and Hunters in Yoju, Korea, 1994-1997

Table 4 Important Arthropod Predators of Brown Planthopper <I>(Nilaparvata Lugens)</I> in Paddy Fields in Korea

Table 4 Important Arthropod Predators of Brown Planthopper (Nilaparvata Lugens) in Paddy Fields in Korea

Table 5 Vertical Distribution of Important Species of Spider on Rice Plants in Korea

Table 5 Vertical Distribution of Important Species of Spider on Rice Plants in Korea

Table 6 Predation by Major Rice Field Spiders in Korea on Brown Planthoppers

Table 6 Predation by Major Rice Field Spiders in Korea on Brown Planthoppers

Table 7 Relative Toxicity of Insecticides to the Spider <I>Pirata Subpiraticus</I>, and the Brown Planthoppers, <I>Nilaparvata Lugens</I>

Table 7 Relative Toxicity of Insecticides to the Spider Pirata Subpiraticus, and the Brown Planthoppers, Nilaparvata Lugens

Table 8 Control of Brown Planthopper (BPH) (<I>Nilaparvata Lugens</I>) after Different Treatments

Table 8 Control of Brown Planthopper (BPH) ( Nilaparvata Lugens) after Different Treatments

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