Appl. Entomol. Zool. 42 (1): 27–33 (2007) http://odokon.org/ Effect of spiders on inoculated populations of the migrant skipper Parnara guttata guttata Bremer et Grey (Lepidoptera: Hesperiidae) in untilled and tilled paddy fields Takashi MOTOBAYASHI,1,* Chikara ISHIJIMA,2,† Mihoko MURAKAMI,1 Motonori TAKAGI,1 Ayame TAGUCHI,1 Kazumasa HIDAKA3 and Yasuhisa KUNIMI2 1 Field Science Center, Faculty of Agriculture, Tokyo University of Agriculture and Technology; Fuchu 183–8509, Japan 2 Department of Bioregulation and Biointeraction, Graduate School of Agriculture, Tokyo University of Agriculture and Technol- ogy; Fuchu 183–8509, Japan 3 University Farm, Faculty of Agriculture, Ehime University; Hojo 799–2424, Japan (Received 19 May 2006; Accepted 21 August 2006) Abstract To examine the effects of spider predation on pest populations in untilled paddy fields, we constructed life tables for immature stages of the migrant skipper Parnara guttata guttata Bremer et Grey artificially inoculated into untilled and tilled paddies in 2000 and 2001, and conducted a spider removal experiment in 2001. The life tables showed that the migrant skipper larval mortality rate was significantly higher in untilled than in tilled paddies. Unknown factors mainly contributed to this high mortality rate in untilled paddies. The spider removal experiment suggested that the presence of spiders was related to the high mortality rate of migrant skippers in untilled paddies. Key words: Life table; migrant skipper; no-tillage; paddy fields; spider assemblage sect pests (Itô et al., 1962; Kiritani et al., 1972; INTRODUCTION Kiritani and Kakiya, 1975; Oraze and Grigarick, Generalist predators, including ground beetles 1989). Planthopper density tended to be lower in and spiders, occur in higher numbers in no- or untilled than in tilled paddies, and the spatial distri- reduced-tillage than in conventional-tillage upland bution of wolf spiders overlapped that of planthop- cropping systems (House and All, 1981; Blumberg pers (Ishijima et al., 2004). Enhancement of the and Crossley, 1983; House and Parmelee, 1985; spider assemblage by no-tillage management prac- Robertson et al., 1994). Furthermore, the full gen- tices may reduce other rice insect pests. Parnara eralist predator assemblage that occurs with no- guttata guttata Bremer et Grey (migrant skipper) is tillage management has reduced pest populations one of the major pests in paddy fields in central and plant damage in upland crops (Brust et al., and western Japan. In our study fields, migrant 1985; Clark et al., 1994). skippers were observed every year; thus, we ana- Hidaka (1993, 1997) showed that Lycosidae lyzed the life tables of immature populations of mi- (wolf spiders) were more abundant in winter mulch grant skippers in untilled and tilled paddy fields. and untilled paddy fields than in conventionally tilled paddy fields. Previous studies have also MATERIALS AND METHODS shown that the spider assemblage, including wolf spiders, was enhanced by no-tillage management in Study fields. The study was conducted in rice paddy fields (Ishijima et al., 2004; Motobayashi et paddies of the Tokyo University of Agriculture and al., 2006). Spiders have been characterized as the Technology on Fuchu campus, Tokyo, Japan, dur- most important biological control agent of rice in- ing the growing seasons of 2000 and 2001. In *To whom correspondence should be addressed at: E-mail: [email protected] † Present address: Laboratory of Entomology, Department of Tea, National Institute of Vegetable and Tea Science, Kanaya, Shizuoka 428–8501, Japan DOI: 10.1303/aez.2007.27 27 28 T. MOTOBAYASHI et al. 2000, we used two paddy fields divided into 12 370-m2 plots. In 2001, we used one paddy field di- vided into six 370-m2 plots. These plots were sys- tematically assigned to one of two tillage treat- ments: untilled or tilled (Fig. 1). All fields were ir- rigated in early May, and 20-day-old rice (Oriza sativa L., variety Tukinohikari) seedlings were transplanted into the field in mid-May at a density of 16.7 hills mϪ2. Plastic flashing (25 cm in height) was placed on the sides of each plot to prevent spi- der emigration and immigration. No insecticides or fungicides were applied during the experimental period. The cultivation procedure is described in Motobayashi et al. (2006) in detail. Life tables of the migrant skipper. To evaluate the effect of spider assemblage on insect pests in untilled and tilled paddy fields, we used life-table analysis for an artificially inoculated larval popula- tion of the migrant skipper. In central and western Fig. 1. Arrangement of experimental plots in 2000 and Japan, the skipper has three discontinuous genera- 2001. NT indicates untilled plots; T indicates tilled plots. tions per year (Ishii and Hidaka, 1979). Adults of the overwintering generation emerge in late May. These adults produce a low-density first genera- and the fifth-instar larvae pupated in the nest. The tion, whose adults emerge in early to mid-July. nest was remade at every molt. These habits make These adults produce the second generation in the it easy to estimate the number of insects in each middle of the rice season, and the density of the developmental stage (Nakasuji, 1982). All larvae second generation is considerably higher than that and pupae were counted, and nests were marked of the first generation. Adults of the second genera- using a felt-tipped pen at intervals of 2–3 days until tion emerge in late August to mid-September and adults emerged. migrate en masse. The most damage to rice plants Mortality factors were estimated as follows. Un- by this species occurs during the second genera- hatched eggs were collected and reared in the labo- tion. Yoshizawa (1996) estimated that a 10% yield ratory to obtain egg parasitoids. After egg para- loss of rice was caused by larvae of the second sitoids emerged, the remaining unhatched eggs generation at a density of one insect per hill. were dissected to estimate egg mortality caused by Although skippers occurred every year in the parasitism or physiological factors. First-instar lar- study fields, their density was too low to construct vae that were missing on the first census date after life tables in 2000 and 2001. Thus, skipper eggs hatching were regarded as having died from a fail- were artificially inoculated onto rice plants follow- ure to settle on leaves. Parasitoid pupae and co- ing the methods of Matsumura (1992). Four cages coons discovered during the census were brought (1.8 mϫ1.8 mϫ1.5 m) with 2-mm mesh netting, to the laboratory and reared until adults emerged. which covered 40 hills, were set up on untilled and Mortality caused by physiological death was esti- tilled plots (Fig. 1). Ten pairs of mature skipper mated as the percentage of brownish shrunken lar- adults reared in the laboratory at 25°C and 16L: 8D val corpses. were introduced into each cage on 1 August 2000 Spider removal experiment. To estimate mor- and 10 August 2001. These adults were allowed to tality caused by spider predation, we conducted a lay eggs for 1 day. The adults were then caught and spider removal experiment in mid-August 2001 the cage was removed. All eggs were counted 2 (Fig. 1). We constructed six enclosures (spider re- days after introduction. Leaves with eggs were moval subplot) of plastic plates measuring 75 cmϫ marked using a felt-tipped pen. After hatching, 60 cmϫ25 cm (lengthϫwidthϫheight) smeared each larva constructed a nest of folded rice leaves, with tanglefoot (Fujiyakuhin Industrial Co., Ltd., Effect of Spider Predation in Untilled Paddies 29 Tokyo, Japan). Each enclosure contained ten rice rate due to unknown factors was higher in the un- hills. All spiders were removed from the spider tilled than tilled treatment (Tables 1 and 2). As a removal subplots by hand or by using suction result, the survival curves were similar between apparatus. Spiders were also removed by hand two tillage treatments at early larval stages; however, or three times during the study period. Fourth-in- survival decreased rapidly in untilled plots com- star larvae of the migrant skipper reared in the lab- pared to tilled plots at later larval stages (Fig. 2). oratory were inoculated at a density of three indi- Spider predation was thought to be the main viduals per hill. Rice plants bordering the plots cause of unknown mortality because the only effec- were cut to prevent the immigration of spiders and tive predators we observed in the study fields were emigration of inoculated larvae. Likewise, fourth- spiders; however, we had no direct evidence and instar larvae were inoculated on ten rice hills in thus conducted the spider removal experiment. each unmanipulated subplot, and the neighboring Two-way ANOVA showed significant effects of plants were cut. We counted larval survival in each spider removal (Fϭ72.96, pϽ0.001) and tillage plot one week after inoculation. treatment (Fϭ11.31, pϽ0.01) on the mortality of Data analysis. For life tables, the survival at fourth instar larvae (Fig. 3), but the correlation be- each developmental stage or by each mortality fac- tween spider removal and tillage treatment was not tor was tested using Chi-square tests. significant (Fϭ3.91, pϾ0.05). Dead larvae were For the spider removal experiment, mortality rate rarely found on rice plants, and we did not observe data were arcsine-square-root-transformed prior to any predators other than spiders in the experimen- analysis and analyzed using two-way ANOVA. tal plots. RESULTS DISCUSSION Life tables for the immature stage of migrant Parasitism was the most important cause of egg skippers artificially inoculated in each treatment in and larval mortality in the life table of the migrant 2000 and 2001, and the survival curves of these in- skipper. These parasitoid species have been re- oculated populations are shown in Tables 1 and 2, ported as the primary parasitoids of the migrant and Fig.