Spreading, seasonal migration and population. fluctuations of the rice water weevil (Lissorhoptrus oryzophilus) (Coleoptera:

Transcription

1 , Spreading, seasonal migration and population fluctuations of the rice water weevil (Lissorhoptrus oryzophilus) (Coleoptera: Curculionidae) in central Taiwan Chung-Ta Liao*, and Ching-Chung Chen Crop Environment Division, Taichung District Agricultural Research and Extension Station, Changhua 515, Taiwan (ROC) (Accepted for publication: Oct. 24, 2005) ABSTRACT Liao, C. T.*, and Chen, C. C Spreading, seasonal migration and population fluctuations of the rice water weevil (Lissorhoptrus oryzophilus) (Coleoptera: Curculionidae) in central Taiwan. Plant Prot. Bull. 47: The rice water weevil (RWW), Lissorhoptrus oryzophilus Kuschel, was first observed in Taichung County in the second-crop rice of 2000, and soon spread to the coastal towns of Taichung County in central Taiwan in the first-crop rice of Subsequently, RWW further invaded the northwestern part of Changhua County in the first-crop rice of 2002, and the infested area reached to 8,100 ha, although the southern spreading of RWW has been retarded in subsequent years. This study investigated the inhabiting sites of adult RWWs during rice fallow periods in the winter and summer seasons to better understand their migration characteristics. In the period when rice is not planted in the winter season, adults were detected in 16.7%~33.3% of samples from rice stubble. The results indicated that adult RWWs hibernate at the base of rice stubble during fallow winter periods. Otherwise, adult RWWs also hibernate in the surface soil and leaf litter of bamboo groves, woodland groves, and at the base of grasses clumps and ground trash on levees. After harvesting the first cropping rice, adult RWWs were detected in 50%~75% of samples from rice stubble and 25%~58.3% from levees. Furthermore, during the fallow periods of the * Corresponding author. liaoct@tdais.gov.tw

2 summer season, adult RWWs were not detected in the soil or leaf litter of bamboo groves or woodland groves; but fed on Gramineae grasses on levees and newly germinated rice seedlings from dropped grain in paddy fields. This indicates that newly emerged adults do not have an obvious dormant behavior at this time. The RWW has 2 generations annually in central Taiwan. The density of adult RWWs reached its peak in mid-march and larvae were observed on the root portions in late March. The maximal larval density occurred in late April. Pupae were found primarily on the root portions in early May. The first-generation adults began to pupate in late May. During the second cropping rice season, larvae were observed mostly in mid-august, with maximal densities from late August to early September. Second-generation adults began to pupate in mid-september. (Key words: rice water weevil, spread, seasonal migration, population fluctuation) INTRODUCTION The rice water weevil (RWW), Lissorhoptrus oryzophilus Kuschel, which is the most-destructive insect pest of rice in the US, now has the potential to become a global threat because it was accidentally introduced into Japan in 1978 (15), Korea (8) and China (11) in 1988, and Taiwan in 1990 (6). In Taiwan the RWW was first discovered in 1990 in Taoyuan County of northern Taiwan (12), and it rapidly spread southward to Hsinchu and Miaoli Counties in 1992 (13), becoming a primary pest of rice in these areas. However, some environmental factors might be blocking the southward spread of RWW (1). In late August 1997, the RWW was confirmed to be prevalent in Renai ( ) Township, Nantou County, and a 2-year control scheme was implemented by the government to decrease the density of this pest. In late August 2000, the spread of RWW was confirmed in the border area between Wuchi ( ) and Shalu ( ) Townships, Taichung County. At the same time, RWW was also found for the first time in Fuli ( ) Township of Hualien County, in eastern Taiwan (8). Adult RWWs feed on the upper surface of rice plant foliage, leaving narrow longitudinal scars, which are considered to be of little economic importance. Females oviposit in the leaf sheaths below the water surface (2, 10). Larvae enclose within 4 to 9 days after eggs are laid and migrate to the roots and feed on them (2, 10). Chronic injury caused by root pruning results in retarded plant growth, fewer tillers, and reduced grain yields (5, 18). In Louisiana, yield losses amount to 10% and can exceed 25% under heavy weevil pressure (16). RWW has 1 generation per year in areas of single rice cropping (3, 17, 19), but has 2 generations annually in areas of double rice cropping (7, 13, 14, 21). Adult RWWs hibernate on the levees or under fallen leaves of groves adjacent to paddy fields. After transplantation of rice seedlings, overwintering adults then migrate to paddy fields and feed on the expanding leaves of rice seedlings. New-generation adults emerge and migrate to the hibernating sites

3 Spreading, seasonal migration and population fluctuations of the rice water weevil 307 adjacent to paddy fields. In northern Taiwan, places under fallen leaves of bamboo groves and levees were considered to be more-favorable hibernating sites for adult RWWs (4, 13, 21). However, since there are few bamboo groves adjacent to fields in central Taiwan, the migration behavior of adult RWWs during fallow periods merits further investigation. For this study, we conducted a long-term survey to understand the spread and distribution of RWW in central Taiwan and its population dynamics. The hibernation sites of RWW during the fallow periods of paddy fields were examined to better understand the seasonal migration characteristics of this pest. MATERIALS AND METHODS Spreading of the RWW This investigation was conducted from August 2000 to 2004 in areas of Taichung City, as well as in all townships of Taichung, Changhua, and Nantou Counties. Within 30 d after transplantation of rice seedlings, 5~10 fields were randomly selected in each township, and 200 rice hills along the sides of levees were examined for the presence of adult RWWs. We searched for the longitudinal scars on rice leaves, which are indicative of RWW infestation, in each field. Inhabiting sites of adult RWWs during fallow periods To study the hibernating and estivating densities of adult RWWs, we examined their inhabiting sites at different cultivated management stages in winter and summer fallow periods between cropping seasons. Two different adjacent environments, namely areas covered in bamboo and woodlands, of paddy fields were investigated in this study. Paddy fields covered in bamboo denote bamboo planted on levees, while woodlands indicated woody plants on the levees between rice paddy fields. According to agricultural practices, before the first cropping rice season, farmers usually plow the dry fields in advance, followed by plowing the flooded fields, and then transplanting the rice seedlings. Therefore, before rice transplanting, the sampling period was from December 2001 to February In each sampling location, 12 core samples of 25 x 25 cm by 5 cm deep were randomly selected. In fields adjacent to bamboo groves, before plowing the dry field, samples were taken from various places including the bamboo grove soil and residuals, surface soil on levees, rice stubble, and surrounding soil. After plowing the dry fields, only the soil and ground trash on the levees and the paddy field soils were sampled. After irrigating the paddy fields and rice was transplanted, the soil and plant materials on the levees were examined. For woodland fields, soil samples were taken from woodland groves rather than bamboo groves, while other samples were the same. During fallow periods of the summer season, samples were taken in July However, before transplanting of the second cropping rice, farmers generally plow the flooded fields. Therefore, before plowing the flooded fields, bamboo grove and woodland grove soils and rice stubble were sampled. When paddy fields were irrigated and after rice seedlings were transplanted, the soil and plant materials on the levees were examined.

4 Sampled soil and plants were first sieved with a 5-mesh screen to leave the residue and big clusters of soil, and then the sieved material was washed through a 20-mesh screen which retained adult weevils. Population fluctuations in paddy fields Changes in the number of adults, larvae, and pupae in the rice plants or roots in paddy fields were respectively investigated every 10 days after transplanting the rice seedlings. Investigations were conducted in Taichung County in 2001 and The date of planting in both years was March 3 for the first cropping rice and July 25 for the second crop. During the investigation period, no insecticide was applied to the fields. Two days after transplanting the rice seedlings, farmers permanently flooded the paddy fields. Forty-five days after transplanting the first crop and 30 days after transplanting the second crop, farmers drained the fields followed by allowing the soil to dry out as a method for blocking additional tillers on rice plants. On each sampling date, 200 rice hills were examined to count the number of adults and the population and development of larvae and pupae. Twelve samples were collected from each plot for examination. The root core samples, consisting of a single plant, its roots, and the surrounding soil, were collected with a metal sampler (25 cm in diameter by 5 cm deep) and held individually in plastic bags until processing. The soil and plants were sieved through a 5-mesh screen to remove the residual matter and big clusters of soil, after which the sieved material was washed through a 35-mesh screen that retained larvae and pupae. When the screen was submerged in water, the larvae floated to the surface, where they were counted and collected. RESULTS Spreading of the RWW In late August 2000, the occurrence of RWW was confirmed at the border between Wuchi ( ) and Shalu ( ) Townships in Taichung County, where the infested area amounted to 3 ha. Larvae, pupae, and adults were observed in rice paddy fields. The following year (2001), the RWW had spread to the first cropping rice in coastal towns of Taichung County, including Dajia ( ), Chingshui ( ), Wuchi, Shalu, Lungjing ( ), Waipu ( ), and Houli ( ) Townships. The distribution pattern of the RWW was restricted in some areas in those townships, and the overall area infested amounted to 2600 ha. At the same time, the occurrence of RWW was confirmed in Shianshi ( ) Township, which is on the northwestern side of Changhua County. In the second crop of rice of 2001, the infested area had substantially increased to 7200 ha, and although the infested area had increased in each township, the density of adults was only higher in Wuchi and Waipu Townships. In the first cropping rice of 2002, the RWW had moved north to Daan ( ) Township; east to Daya ( ) Township, Taichung County; and south to the northwestern side of Changhua County, including Shengang ( ), Lugang ( ), and Hemei ( ) Townships. Furthermore, the infested area had increased to 8100 ha. The following year (2003), no new areas were discovered to be infested by the RWW. By the time of the second cropping rice of 2004, the occurrence

5 Spreading, seasonal migration and population fluctuations of the rice water weevil 309 of RWW had been confirmed in Taichung City. Changes in the area infested and the spread of RWW in central Taiwan are shown in Fig. 1 and Table 1. Fig.1. Spread of Lissorhoptrus oryzophilus in central Taiwan. Table 1. The spread of Lissorhoptrus oryzophilus in central Taiwan from 1997 to 2004 Year Counties and cities infected No. of townships Infested area (ha) 1997 Nantou County Nantou and Taichung Counties Nantou, Taichung, and Changhua Counties 9 7, Nantou, Taichung, and Changhua Counties 14 8, Nantou, Taichung, and Changhua Counties 14 8, Nantou, Taichung, and Changhua Counties and Taichung City 15 8,130

6 Inhabiting sites of adult RWWs during fallow periods In the winter season, before plowing the dry paddy fields, adult RWWs were detected in 91.7% of samples from the bamboo grove soil and residues. The average density of adults was 6.2 individuals /0.1 m 2. Rice stubble and levee samples both had detection rates of 16.7%. After plowing the dry fields, the detection rate of hibernating adults was 16.7% on levees, but none were detected in the plowed soil in the paddy field. After plowing the flooded fields, adults were detected in 25% of levee samples. After transplanting the rice seedlings, no RWWs were detected on levees between paddy fields (Table 2). As to paddy fields covered with woodlands, adult RWWs were detected in 33.3% of samples from the surface soil of levees and rice stubble, but none was found in the woodland grove soil. After plowing the dry fields, the detection rates of hibernating adults were 41.7% on levees and 50.0% in dry soil of the paddy field, respectively. After plowing the flooded fields, adults were detected in 75% of levee samples. Then, after transplanting the rice seedlings, no RWWs were detected on levees of paddy fields (Table 3). However, at the same time, adult RWWs fed on rice leaves causing obvious longitudinal scars. Table 2. Inhabiting sites of Lissorhoptrus oryzophilus adults in paddy fields covered with bamboo groves during fallow periods (from December 2001 to February 2002 in the winter season and July 2002 in the summer season) Sampling places Date of sampling Number of samples Percent (%) of samples with adults Number of adults/0.1 m 2 Range Average Before the first cropping rice (overwintering sites) Bamboo grove soil and leaf litter Dec ~ Rice stubble Dec ~3 0.1 Levees before plowing dry fields Dec ~3 0.3 Levees after plowing dry fields Jan ~2 0.3 Field soil after plowing dry fields Jan Levees after plowing flooded fields Feb ~5 1.2 Levees after rice transplanted Mar Before the second crop of rice (estivating sites) Bamboo grove soil and leaf litter July Rice stubble July ~2 0.8 Levees before plowing flooded fields July ~1 0.3 Levees after plowing flooded fields July ~1 0.4 Levees after rice transplanted July

7 Spreading, seasonal migration and population fluctuations of the rice water weevil 311 Table 3. Inhabiting sites of Lissorhoptrus oryzophilus adults in paddy fields covered with woodland groves during fallow periods (from Dec to Feb in the winter season and July 2002 in the summer season) Sampling places Date of sampling Number of samples Percent (%) of samples with adults Number of adults/0.1 m 2 Range Average Before the first crop of rice (overwintering sites) Woodland grove soil and leaf litter Dec Rice stubble Dec ~6 0.8 Levees before plowing dry fields Dec ~ Levees after plowing dry fields Jan ~8 1.8 Field soil after plowing dry fields Jan ~6 1.6 Levees after plowing flooded fields Feb ~8 2.7 Levees after rice transplanted Mar Before the second crop of rice (estivating sites) Woodland grove soil and leaf litter July Rice stubble July ~ Levees before plowing flooded fields July ~6 2.3 Levees after plowing flooded fields July ~ Levees after rice transplanted July According to the survey information, overwintering adults became active around mid-february, after farmers had completed plowing the flooded fields in early-transplanted paddy fields in Wuchi Township, Taichung County. This is the earliest record of activity for adult RWWs. After plowing the flooded fields, some germinated seedlings, which had sprouted from grains dropped from the former crop in the paddy field, were damaged by the overwintering adults. Prior to this stage, the leaves of Gramineae grasses on the levees and in the paddy fields had longitudinal scars caused by adult RWWs feeding, although this was not further investigated. In the summer season, before the second cropping rice was transplanted, no adult RWWs were detected under the leaf litter of bamboo or woodland grove soil. In the paddy field covered with the bamboo grove, hibernating adults were detected in 50% of samples from stubble in paddy fields, and in 25% of samples from the surface soil of levees before plowing the flooded fields. After plowing the flooded fields, adults were detected in 41.7% of the levee samples (Table 2). For the woodland fields, adult RWWs were detected in 75% of samples from stubble and 58.3% of those from levees. After plowing the flooded fields, 100% of the samples of the surface soil of levees

8 contained adult RWWs (Table 3). Between the first and second rice cultivation periods, estivating adults were very active on the levees and in rice stubble. In the afternoon, beginning around 16: 00, they moved out of their inhabiting sites and climbed up to feed on the leaves of Gramineae grasses and newly germinated rice seedlings. Population fluctuations in paddy fields Results obtained in paddy fields transplanted on March 3, 2001 are given in Fig. 2. As soon as rice seedlings were transplanted in the paddy fields, adults invaded and began to injure the seedlings, while the population slightly increased until it reached a peak in mid-march. Adults disappeared from the paddy fields in mid-april. The number of larvae reached its population peak in late April. Pupae were first observed in early May and reached a population peak in early June, and the new generation of adults had begun to emerge in late May. Fig. 2. Population fluctuations of Lissorhoptrus oryzophilus in Taichung County (2001). The date of planting was March 3 for the first cropping rice and July 25 for the second cropping rice. The date of harvesting was July 4 for the first crop of rice and October 28 for the second cropping rice.

9 Spreading, seasonal migration and population fluctuations of the rice water weevil 313 Fig. 3. Population fluctuations of Lissorhoptrus oryzophilus in Taichung County (2002). The date of planting was March 3 for the first cropping rice and July 25 for the second cropping rice. The date of harvesting was July 6 for the first cropping rice and October 20 for the second cropping rice. During the second cropping rice, rice seedlings were transplanted on July 25, As soon as the rice seedlings had been transplanted in the paddy fields, adults invaded and began to injure the seedlings, reaching a peak in mid-august, until ending their activities in late August. The number of larvae reached a population peak in the period from late August to early September. Pupae were first observed in early September and reached a peak in mid-september, before the new generation of adults began to emerge in mid-september. Survey data from 2002 showed a similar pattern to 2001 for population fluctuations of the RWW (Fig. 3). Moreover, the larval population in the first cropping season was not necessarily higher than that in the second cropping season. DISCUSSION The distribution area of the RWW has rapidly expanded in the temperate zone (1),

10 and its potential for damage in the initial stage of invasion has been expressed in northern areas of Taiwan (12). The average number of adults per rice hill was estimated to be 2.6 in late March 1990, and there was an average of 10.1 larvae found in the rice core soil at the tilling stage, which retards rice growth (12). Since the RWW has migrated south to expand its distribution area, the impact of this invasion on the rice crop in the subtropical and tropical zones has become an important research topic. Results showed that the infested area of RWWs in Taiwan expanded rapidly, from 23 ha in 2000 to 8100 ha in 2002 (Table 1), but the southern movement of RWW has been retarded in subsequent years. This expansion situation is similar to that which occurred when the RWW was restricted to the north side of the Daan River between 1994 and 1999 (1). It has been suggested that some limiting factors might be retarding the expansion of RWW in the central portion of Taiwan. However, at the same time, different patterns for RWW expansion can also be seen in eastern regions of Taiwan. After the RWW became established in eastern parts of Taiwan, it took only 3 yr, from 2000 to 2002, generation of adults emerge, most of them migrate to estivating sites adjacent to the paddy fields, and only a few adults damage the second crop of rice. Therefore, there was less damage to rice resulting from the RWW in the second crop than the first crop of rice (14). In the northern parts of Taiwan, more-favorable hibernating places for adult RWWs are under fallen leaves of bamboo groves and on levees (4, 13, 21). Otherwise, during fallow periods, the detection rate of hibernating adults was high at certain places such as under dry weeds on footpaths, ditches, and banks, under fallen leaves of woodlands or bamboo groves, and under the mulch of orchards. However, none was detected in stubble of paddy fields according to Japanese research (17). However, this study showed that the pattern of seasonal movement of adult RWWs differed from those of previous reports on the hibernation places of the RWW during fallow periods. Our results showed that adult RWWs could inhabit the base of rice stubble during fallow periods of winter and summer seasons (Tables 2, 3), and could then also hibernate in moist surface soil in paddy fields after plowing in for it to spread throughout Hualien ( ) dry conditions. In mid-february, adult and Taitung ( ) Counties' rice-producing RWWs moved from their hibernation places regions (Yang and Syu, personal to flooded fields and damaged the rice communication). The seasonal movement patterns of adult RWWs were extensively investigated in previous reports (14, 17, 19, 20). Adult RWWs seedlings from dropped grains. Before rice transplantation occurred, adult RWWs swam or floated by clinging to floating materials to the levees of paddy fields when the plowed hibernate on levees or under fallen leaves fields were flooded. After rice was adjacent to paddy fields. After transplanting the rice seedlings, overwintering adults then invade the paddy fields and feed on the leaves of rice seedlings. Then, when the new transplanted, adult RWWs invaded paddy fields and damaged rice seedlings adjacent to the levees. Moreover, before the second cropping

11 Spreading, seasonal migration and population fluctuations of the rice water weevil 315 rice was transplanted, adult RWWs were not detected in the soil of the bamboo groves and woodland fields, although most of them were inhabiting both rice stubble and levees (Tables 2, 3). Furthermore, damage to Gramineae grasses on levees and the sprouted seedlings of rice was caused by adult feeding. This suggests that the newly generated adults do not have clearly dormant behavior during the summer fallow period in central Taiwan. When the second cropping rice was transplanted, the adults moved into the fields again and damaged the seedlings adjacent to levees. It was noted that there are 2 generation of RWWs in double-cropping rice areas of Okinawa Prefecture in Japan (7), northern areas of Taiwan (13, 14), and Zhejiang Province, China (21). In central Taiwan, the RWW also has 2 generations annually (Figs. 2, 3). The adult population reaches a peak in mid-march, and larvae were observed on root portions in late March. The maximum larval density occurred in late April. The pupae were primarily found on root portions in early May, and the first-generation adults had pupated beginning in late May. After transplanting the second crop of rice, the first-generation adults moved into the fields for oviposition. Larvae were primary observed in mid-august, and reached a maximum density from late August to early September. Second-generation adults pupated starting in mid-september. The larval population in the first cropping season was obviously higher than that in the second cropping season in northern areas of Taiwan (13), Okinawa Prefecture, Japan (7), and Zhejiang Province, China (21). However, this behavioral situation was not necessarily the same as that in central Taiwan, which had an opposite pattern according to the 2001 survey information (Fig. 2). The newly generated adults appeared more active during the summer fallow period in central Taiwan, when adult RWWs moved into the fields immediately after transplanting of the second cropping rice; therefore, the larval population had also maintained a high level corresponding to that for the first cropping rice. However, direct evidence needs to be obtained to further prove this view about different dormant behaviors. The RWW has been present in the central district of Taiwan for 6 yr, and even though the infested area has increased to 8100 ha, it has been only a minor pest during the growth periods of rice plants, seldom causing obvious yield losses in these areas. Therefore, in future studies, we will attempt to determine the limiting factors for the RWW on the southern front in western regions of Taiwan, including the biological characteristics of the RWW, environmental factors, and agricultural practices. LITERATURE CITED 1. Cheng, C. H., and Chiu, Y. I Review of changes involving rice pests and their control measures in Taiwan since Plant Prot. Bull. 41: 9-34 (in Chinese). 2. Everett, T. R., and Trahan, G. B Oviposition by rice water weevils in Louisiana. J. Econ. Entomol. 60: Godfrey, L. D A demonstration of refined pest management strategies for

12 rice water weevil in California rice. Pest management grants final report, No University of California-Davis, Davis, CA, USA. 16 pp. 4. Grifford, J. R., Oliver, B. F., and Trahan, G. B Rice water weevil with pirimiphosethy seed treatment. J. Econ. Entomol. 68: Grigarick, A. A General problems with rice invertebrate pests and their control in the United States. Prot. Ecol. 7: Heinrichs, E. A., and Quisenberry, S. S Germplasm evaluation and utilization for insect resistance in rice, pp In: S. L. Clement & S. S. Quisenberry [eds.], Global plant genetic resources for insect-resistant crops, CRC Press, Boston, MA, USA. 320 pp. 7. Kinjo, T., Shimada, T., and Yamanchi, S Occurrence of the rice water weevil, Lissorhoptrus oryzophilus Kuschel, in Okinawa Prefecture. Proc. Assoc. Plant Prot. Kyushu. 32: Lee, Y. I., and Uhm, K. B Landing, settling and spreading of rice water weevil in Korea, pp In: K. Hirai [ed.], Establishment, Spread and Management of the Rice Water Weevil and Migrating Rice Insect Pests in East Asia. NARC, Tsukuba, Korea. 274 pp. 9. Liao, C. T., Chen, C. C., Lin, C. S., and Fang, M. N Population fluctuation of rice water weevil (Lissorhoptrus oryzophilus Kuschel) (Coleoptera: Curcuilonidae) and assessment of rice yield loss in central Taiwan, pp In: F. M. Wei, M. Z. Hong, & R. W. Chen [eds.], Proceedings, Agricultural Technology Symposium, October 30, 2002, Taichung District Agricultural Research and Extension Station, Changhua. Taiwan, ROC 188 pp. (in Chinese). 10. Raksarart, P., and Tugwell, P Effect of temperature on development of rice water weevil eggs. Environ. Entomol. 4: (in Chinese). 11. Shang, H. W., and Zhai, B. P The spread and control strategy of rice water weevil in southern China, pp In: X. K. Yang & H. Wu [eds.], Advance in entomological researches. China Forestry Press, Beijing, China. 336 pp. (in Chinese). 12. Shih, H. P The newly found rice water weevil (Lissorhoptrus oryzophilus Kuschel) on rice plant in Taiwan. Bull. Taoyuan Agric. Improv. Stn. 7: Shin, H. P Population fluctuation and control of the rice water weevil. Bull. Taoyuan Agri. Improv. Stn. 11: Shin, H. P., and Lee, P. H Immigration of the rice water weevil and assessment of rice yield loss in Taoyuan district. Bull. Taoyuan Agric. Improv. Stn. 27: Smith, C. M The rice water weevil, Lissorhoptrus oryzophilus (Kuschel), pp In: K. G. Singh [ed.], Exotic plant quarantine pests and procedures for introduction of plant materials. Asian (Planti), Selanyor, Malaysia, 333 pp. 16. Stout, M. J., Rice, W. C., Riggio, M. R., and Ring, D. R The effects of four insecticides on the population dynamics of the rice water weevil, Lissorhoptrus oryzophilus Kuschel. J. Entomol. Sci. 35: Toshikazu, I Invasion of the rice

13 Spreading, seasonal migration and population fluctuations of the rice water weevil 317 water weevil, Lissorhoptrus oryzophilus Kuschel, into Japan, spread of its distribution and abstract of the research experiments conducted in Japan. Jpn. Pestic. Inform. 36: Way, M. O Insect pest management in rice in the United States, pp In: B. T. Grayson, M. B. Green, & L. G. Copping [eds.], Pest management in rice, Elsevier Science Publishing, New York, USA. 536 pp. 19. Yamashita, I., Horiuchi, T., and Kawamura, M Occurrence time of the rice water weevil adult, Lissorhoptrus oryzophilus Kuschel, in overwintered first and second generations in Nangoku City. Kochi Prefecture. Proc. Assoc. Plant Prot. Shikoku. 20: Zhai, B. P., Cheng, J. A., and Huang, E. Y Population fluctuation of rice water weevils in double cropping rice area of Zhejiang Province, China. China Agri. Sci. 30: (in Chinese). 21. Zhai, B. P., Shang, H. W., Cheng, J. A., and Zheng, X. H Constitution of the autumn population of rice water weevils in double cropping rice area of Zhejiang Province, China. Acta Phytophyl. Sinica. 26: (in Chinese).

14 * Lissorhoptrus oryzophilus Kuschel %~33.3 % 50 %~75 % 25 %~58.3 % ( ) * liaoct@tdais.gov.tw