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Development of insecticide resistance often changes life history traits of insect pests, because metabolic detoxification of insecticides in insect bodies requires huge energetic reserves. The brown planthopper, Nilaparvata lugens (Stål), an important insect pest of rice crop in East and Southeast Asia, has developed strong resistance to imidacloprid from mid-2000s. The aim of this study was to examine the costs of life history traits and reveal changes in energy reserves with developing imidacloprid resistance. We compared the life history traits (survival time, fecundity, developmental time, and hatchability) and total lipid content between imidacloprid-resistant and imidacloprid-susceptible (control) brown planthopper strains. As compared to the control strains, adults’ survival time of the resistant females was shorter, and their fecundity was lower; the other life history traits did not differ significantly between the resistant and control strains. As the results, net reproductive rates (R0) were lower in the resistant strains than in the susceptible strains. However, the amount of stored lipids was larger in resistant females than control ones. Our findings demonstrated a physiological trade-off between the development of imidacloprid resistance and the reproductive traits of brown planthopper. The imidacloprid-resistant strains are likely to store lipids for metabolic detoxification rather than consume them for reproduction.

Resistant rice cultivars are an alternative of insecticide control for one of the major rice pests: the brown planthopper, Nilaparvata lugens (Hemiptera: Delphacidae). However, N. lugens has obtained virulence to resistant rice cultivars as same as insecticide resistance. The monitoring of virulence to differential cultivars is important for detections of the breakdown of resistance genes ‘BPH’ owned by the cultivars. We conducted long-term virulence monitoring (2001–2019) of differential cultivars in N. lugens immigrant populations in Japan. The virulence to the cultivars IR26 (BPH1), Mudgo (BPH1), and ASD7 (BPH2) was higher than that to other differential cultivars. The virulence to Babawee (BPH4) was lower than that to IR26, Mudgo, and ASD7, but it fluctuated over the immigrant years. The virulence to Kanto-PL7 (BPH3), Rathu Heenati (BPH3, BPH17) and Balamawee (BPH27, Three QTLs) was lower than that to other differential cultivars. The immigrant populations have therefore not developed virulence to three differential cultivars. These results suggest that N. lugens continued to develop virulence to differential cultivars that possess BPH1 and BPH2 after the breakdown. However, our results indicate that N. lugens could not break through the efficacy of differential cultivars possessing multiple BPH resistance genes.

Background The brown planthopper, Nilaparvata lugens (Stal), is one of the most notorious pests of rice throughout Asia. The brown planthopper have developed high resistance to imidacloprid, a member of neonicotinoid insecticides. Several genes and mutations conferring imidacloprid resistance in N. lugens, especially in eastern and southeastern Asia populations, have been reported. Thus, the key mechanisms of imidacloprid resistance need to be examined. Results RNA-seq analyses revealed that only one cytochrome P450 monooxygenase gene, CYP6ER1, was commonly upregulated in the five resistant strains tested. Sequences of CYP6ER1, which were highly expressed in the imidacloprid-resistant strains, contained a three-nucleotide deletion in the coding region, and amino acid substitutions and deletion, compared to that in an imidacloprid-susceptible strain. RNAi-mediated gene knockdown of CYP6ER1 increased imidacloprid susceptibility in a resistant strain. Further, we established two simple and convenient PCR-based molecular diagnostic methods to detected the CYP6ER1 locus with the three-nucleotide deletion. Using these methods, the resistance of F2 progenies derived from the crosses of F1 siblings from susceptible and resistant parents was analyzed, showing that the imidacloprid resistance had a relationship to the CYP6ER1 locus with the three-nucleotide deletion. Conclusion The overexpression of a variant CYP6ER1 with amino acid substitutions and deletion was involved in imidacloprid resistance in N. lugens. Based on these findings, molecular diagnostic methods have been developed and are promising tools for monitoring imidacloprid resistance in paddy fields. This article is protected by copyright. All rights reserved.

The recent spread of Oryza sativa L. (rice) ssp. indica-derived cultivars in East and Southeast Asia increases the risk of white-backed planthopper, Sogatella furcifera, and Southern rice black-streaked dwarf virus (SRBSDV) outbreaks. We compared the degree of ovicidal response to S. furcifera and susceptibility to SRBSDV among two Oryza sativa ssp. japonica, two Oryza sativa ssp. indica and one Oryza sativa ssp. indica-derived rice cultivars. The ovicidal response of the O. sativa ssp. indica-derived cultivar was significantly higher than that of the tested O. sativa ssp. indica cultivars and was equivalent to that of the O. sativa ssp. japonica cultivars that we used, supporting previous findings that the ovicidal response is higher in O. sativa ssp. japonica cultivars than in O. sativa ssp. indica cultivars and is a quantitative trait controlled by a few relevant genes. Contrary to the varied ovicidal response, the SRBSDV infection rate among the cultivars and the height of SRBSDV-infected plants between most pairs of cultivars did not differ significantly, although twisting of the leaf tips, a typical symptom of SRBSDV, was not observed in the O. sativa ssp. japonica cultivars. These results indicate that damage by S. furcifera and SRBSDV can occur in O. sativa ssp. japonica as well as in O. sativa ssp. indica and O. sativa ssp. indica-derived cultivars; therefore, in the cultivation of O. sativa ssp. japonica, measures should also be taken against white-backed planthopper during high levels of emergence.

Pymetrozine is a promising chemical agent for controlling the brown planthopper, Nilaparvata lugens (Stal), after resistance to some neonicotinoid insecticides developed nearly a decade ago. Susceptibility tests by assessing mortality resulting from insecticidal activity are not available for pymetrozine because its key biological action suppresses the reproductive performance of treated N. lugens females. In this study, a new simple method for monitoring the susceptibility of N. lugens to pymetrozine was developed by combining topical application and assessment of the number of offspring using young rice seedlings in a test tube. The ED50 values of the Osaka strain (collected in 1971) and Kumamoto strain (collected in 2011) of N. lugens against pymetrozine were 0.036 and 0.091 μg/g, respectively. In the future, this method will be used to monitor the susceptibility of N. lugens to pymetrozine.

In migration analysis of the brown planthopper, Nilaparvata lugens (Stal), in a tropical region, it is difficult to know the arrival timing of the immigrants because rice plants in different growth stages are simultaneously cultivated, and local insects may already occur in destination areas. Due to this analytical difficulty, additional information is necessary to discriminate between immigrants and locally originated individuals in the light trap catch. This study applied rice phenology maps retrieved from satellite images to analysis of N. lugens’s migration in the Vietnamese Mekong Delta. The analysis also employed occurrence data by dense light traps and trajectory analysis. The result showed that the occurrence of N. lugens peaked periodically at an interval of 27.2–30 days, changing in association with the rice cropping area in the harvesting stage. For an N. lugens occurrence in late July 2009 in the delta, the rice phenology suggested catch increases in light traps in a destination area could likely be attributed to immigrants from outside the area. Therefore, the migration analysis suggested that N. lugens that emigrated from paddy fields in An Giang and Kien Giang provinces possibly moved to east-neighboring provinces. The migration distance was estimated to be about 100 km at a maximum.

The brown planthoppers (BPH)Nilaparvata lugens(Stål) and the white-backed planthoppers (WBPH)Sogatella furcifera(Horváth) annually migrate from tropical and subtropical regions to temperate regions in Asia, including Japan, Korea and northern China. To elucidate the genetic divergence based on geography of planthoppers and to estimate their migration route on the basis of molecular data, we analysed a part of their mitochondrial genome sequences. Sequences of cytochrome oxidase subunit I (cox1) – transfer RNA for Leu (trnL2) –cox2were determined for 579 BPH (1,928 bp) and 464 WBPH (1,927 bp) individuals collected from 31 and 25 locations, respectively, in East and Southeast Asia. Thirty and 20 mitochondrial haplotypes were detected for BPH and WBPH, respectively. Single populations of both planthoppers included multiple haplotypes, and many haplotypes were shared in some populations and areas. The most frequently detected haplotypes accounted for approximately 50% of all BPH and WBPH individuals. To evaluate gene flow among planthoppers in different regions in Asia, pairwise fixation index (Fst) values were calculated. For BPH, highFstvalues (0.580–0.926) were shown between planthoppers in Papua New Guinea (PNG) and the other areas and moderateFstvalues (0.176–0.362) were observed between those in southern Philippines and other areas. For WBPH, theFstvalue was the highest between Taiwan and southern Vietnam (0.236), and low among the other areas. AMOVA indicated no genetic structure among eight areas, excluding southern Philippines and PNG, for BPH, and among ten areas for WBPH. These data indicate that both planthoppers do not show much differentiation of local populations and/or have genetically intermixed Asian populations. These data also indicate that it may be difficult to distinguish regional planthopper populations on the basis of differences in mitochondrial sequences.

We analyzed overseas immigrations of rice planthoppers in Kyushu, Japan, based on trap data collected during June–July in 2000–2011. The immigrant density was high in 2006, whereas it was low in 2008 and 2011. To understand these annual fluctuations, we investigated the relationships among trap catches and the following three meteorological conditions: (1) the average temperature during January–February in North Vietnam (T NV), where planthoppers successfully overwinter; (2) the strong upper wind from North Vietnam to South China in April–May (UWVC), when the first stage of migration occurs; (3) the strong upper wind from South China to Kyushu in June–July (UWCJ), when the second stage of migration occurs. In 2008 and 2011, T NV values were 2.4–3.0 °C below the 2000–2011 average of 17.4 °C, and there were 9–13 fewer days with a strong upper wind (UWVC + UWCJ) in April–July compared with the 2000–2011 average of 25 days. This study showed that the rice planthopper immigrant density during the last 12 years correlated significantly with T NV and the number of days with a strong upper wind (UWVC + UWCJ) in April–July. Thus, the meteorological conditions affected the immigrant density of rice planthoppers in Kyushu.

A method has been developed for predicting the overseas migration of Laodelphax striatellus (Fallen) from eastern China to Japan and Korea. The method consists of two techniques: estimation of the emigration period in the source region and simulation of migration. The emigration period was estimated by calculating the effective accumulated temperature for the insect by use of real-time daily surface temperatures at the source. During the emigration period, migration simulations were performed twice a day, at every dusk and dawn. The prediction method was evaluated, by cross-validation using migrations in the 4 years from 2008 to 2011. The results showed that the emigration periods included the mass migrations, and that the method successfully predicted those migrations.

The population of Nilaparvata lugens (Stal) in Taiwan is believed to belong to the East Asian population of the species, which has low susceptibility to the insecticide imidacloprid. The population in the Philippines belongs to the Southeast Asian population, which has high susceptibility to imidacloprid. In this study, long-winged adults of N. lugens collected in paddy fields in Taitung, southeastern Taiwan, just after a typhoon, were tested for imidacloprid susceptibility after rearing for 4–17 generations. The migration source was also estimated by backward trajectory analysis. Our findings were, first, that its curved dosage–mortality plots for imidacloprid suggested that the Taitung population is heterogeneous in its susceptibility to the insecticide. Source estimation found areas on Luzon Island of the Philippines, mainly, and southern China, partially, as possible sources. Before the typhoon approached, however, light trap monitoring data indicated some local populations might have occurred in the survey area. Therefore, with other pieces of circumstantial evidence, this study suggested that migration of N. lugens to Taiwan from the Philippines, and, simultaneously, possibly southern China occurred because of the typhoon, with the insects partly crossing the boundary of the Asian populations, and that a mixture of individuals with different susceptibility to the insecticide happened to form the Taitung population at the time of collection.

Cytoplasmic incompatibility (CI) is a reproductive phenotype induced by bacterial endosymbionts in arthropods. Measured as a reduction in egg hatchability resulting from the crossing of uninfected females with bacteria-infected males, CI increases the frequency of bacteria-infected hosts by restricting the fertilization opportunities of uninfected hosts in populations. Wolbachia, a type of alpha-proteobacteria, is well known as a CI inducer in a wide range of arthropod species, while Cardinium, a member of the phylum Bacteroidetes, is known to cause CI in one wasp and three spider mite species. In this study, dual infection with Cardinium and Wolbachia induced strong CI in a single host, Sogatella furcifera (Horvath), a planthopper species that is naturally infected with both bacteria. Specifically, infection with Cardinium alone was found to cause a 76 % reduction in egg development, and dual infection with Cardinium and Wolbachia a 96 % reduction, indicating that Cardinium induces CI and the dual infection raises the CI level. This study was the first to document reproductive alteration by Cardinium in a diploid host species.

Nucleotide sequences of the internal transcribed spacer 1 (ITS1)–5.8S–ITS2 region of the nuclear ribosomal RNA gene were determined in the white-backed planthopper (WBPH) Sogatella furcifera (Horvath) to detect molecular variation among regional populations in Asia. We analyzed 932 sequences from 172 individuals (4–9 clones per individual) of 33 populations collected in 1987–2008 from six countries, Japan, China, Taiwan, Vietnam, Philippines, and Papua New Guinea. WBPH showed intra-individual variation in ITS1, which is mainly attributable to the frequency (0–10) of the 66-bp repeat sequence in ITS1. Among the examined clones, the sequences of 5.8S were mostly identical and those of ITS2 were similar. A single planthopper had a maximum of 6 different variants in the number of ITS1 repeats, suggesting highly varied repeat numbers in individual planthoppers. The ITS1 with four repeats was the most frequently (64%) detected. Such a repeat was not observed in two other economically important planthopper species, Nilaparvata lugens (Stal) and Laodelphax striatellus (Fallen). The ITS nucleotide sequences in the WBPH populations in Asia were genetically close and some variations in the sequences were not related to regional populations, indicating that the nucleotide sequences of the ITS region are not useful for geographical discrimination of the WBPH. This closeness seems to be caused by long distance migration and genetic exchange among populations.

Particular alkaloids produced by Neotyphodium endophytes show toxicity to invertebrates. Italian ryegrass (Lolium multiflorum Lamarck) cultivars and strains that are symbiotic with Neotyphodium endophytes have been recently established in Japan. N. uncinatum-infected Italian ryegrass lines accumulate N-formylloline, a type of loline alkaloid (1-aminopyrrolizidine) showing neurotoxicity to herbivorous insects. This study investigated the toxicity of N-formylloline and resistance of N. uncinatum-infected Italian ryegrass to vascular-sap feeding Clypeorrhynchan pests. When four vascular-sap feeding insects: Laodelphax striatellus (Fallén) (Homoptera: Delphacidae), Sogatella furcifera (Horváth) (Homoptera: Delphacidae), Cicadulina bipunctata (Melichar) (Homoptera: Cicadellidae), and Nephotettix cincticeps (Uhler) (Homoptera: Cicadellidae) fed on N. uncinatum-infected Italian ryegrass, significant decreases in survival rate were observed for three phloem-sap feeders but not for a xylem-sap feeder, N. cincticeps. This result suggests an uneven distribution of N-formylloline among plant tissues. A potency assay for N-formylloline using a Parafilm feeding sachet and a quantitative analysis of N-formylloline in plant showed a concentration-dependent lethal effect of N-formylloline on all four tested vascular-sap feeders. Our results strongly suggest that N. uncinatum-infected plants can control some Clypeorrhynchan pests in crop fields.

The monitoring of insect pests in fields of forage maize is difficult because plants are tall and grow at a high density. We investigated the effectiveness of colored sticky traps and appropriate conditions for monitoring insect pests in forage maize fields. Large numbers of the maize orange leafhopper, Cicadulina bipunctata Melichar (Hemiptera: Cicadellidae), and the small brown planthopper, Laodelphax striatellus Fallen (Hemiptera: Delphacidae), were collected during the experimental period with yellow and blue sticky traps placed in summer crop forage maize fields. A greater number of insects were trapped in yellow traps relative to blue traps. Traps located at a lower height (40 cm above the ground) attracted larger numbers of C. bipunctata, whereas L. striatellus did not demonstrate a height-dependent preference. These results indicated that yellow-colored sticky traps located at low height are effective for collecting C. bipunctata and L. striatellus simultaneously. Seasonal occurrence data obtained by the yellow sticky traps showed clearer seasonal occurrences than that obtained by two previously developed methods, suction and light traps, indicating that sticky traps are effective for monitoring the seasonal occurrence of these two insects in forage maize fields.

The twisted-wing parasite Elenchus japonicus is a major parasitoid of rice planthoppers (Hemiptera: Delphacidae), including the brown planthopper Nilaparvata lugens, the white-backed planthopper Sogatella furcifera, and the small brown planthopper Laodelphax striatellus. Another Elenchus species, E. yasumatsui, which is considered to be synonymous with E. japonicus, has also been described in southern Asia. However, limited biological and molecular data on this important parasitoid of rice planthoppers are available. In this study, E. japonicas-stylopized planthoppers were obtained from various regions in Asia and Japan. Sequences of the nuclear 18S ribosomal RNA gene and the mitochondrial cytochrome oxidase I gene of the strepsipterans were analyzed from 40 samples of E. japonicus: 14 from N. lugens, 6 from S. furcifera and 20 from L. striatellus. Sequence analysis of these samples revealed three genotypes of E. japonicus. Of the three types of E. japonicus, one was isolated only from L. striatellus from northern Japan. The other two types were found in all three rice planthopper species collected from a wide area in Asia. These results suggest that the three different genotypes of Elenchus can be associated with a particular geographical region and/or planthopper species. The relationship between two previously described Elenchus species and the present three genotypes needs to be elucidated.

The effect of acetone solution on the mortality of rice planthoppers, Nilaparvata lugens (Stal), Sogatella furcifera (Horvath), and Laodelphax striatellus (Fallen), was examined, as it is widely used as a solvent solution in a topical application method for monitoring insecticide susceptibility. The mortality of N. lugens and S. furcifera was significantly higher at 0.32 and 0.28 μl or more acetone droplets per insect than in the control treatment (without acetone), respectively. The mortality of L. striatellus was significantly higher at 0.24 μl or more acetone droplets per insect than in the control treatment. Another commonly used solvent solution, methanol, had a similar effect on L. striatellus. The most standard topical application method uses a hand-operated micro-applicator (Burkard Manufacturing Co., Ltd., UK) equipped with a 50-μl micro-syringe, which can deliver a very small amount of droplets of insecticide/acetone solution per insect, 0.08 μl, so there is no effect of acetone solution on the mortality of N. lugens, S. furcifera, and L. striatellus. However, other equipment for topical application, a repeating dispenser (Hamilton Co., USA) attached with a 10-μl micro-syringe, can deliver a 0.24-μl droplet at a minimum. This equipment can be used for N. lugens and S. furcifera, but not for L. striatellus. It is necessary to choose appropriate equipment for each rice planthopper so that they are not affected by the solvent solution.

The pattern of overseas immigration of Laodelphax striatellus (Fallen) into northern Kyushu district in relation to weather conditions was analyzed using trap catch data, weather data, and backward trajectory analysis. The investigation covered the period May 21 to June 10 of each year between 2000 and 2009. One peak trap catch was recorded on May 27–28, 2006 and was associated with strong westerly winds at 850 and 925 hPa levels to the south of a cold vortex that passed over the southern part of the Korean peninsula. The backward trajectory analysis suggested Jiangsu Province, China, as possible migration source. Another peak catch on June 5, 2008 has already been reported as an overseas migration from Jiangsu Province, China. This study revealed that a cold vortex passing the southern Korean peninsula was also associated with insect migration in 2008. Thus, there is evidence of at least 2 overseas migrations from Jiangsu Province, China, over a 10-year period that coincided with strong westerly winds in the lower atmosphere that were associated with cold vortices.

The virulence of laboratory strains of the brown planthopper (BPH), Nilaparvata lugens (Stal), and the whitebacked planthopper (WBPH), Sogatella furcifera (Horvath), collected in Japan between 1966 and 2005, was evaluated using rice differential varieties carrying different planthopper resistance genes. The BPH strain collected in 1966 was avirulent to all the rice varieties tested. In contrast, the 1989, 1999 and 2005 strains were virulent to Mudgo, which carries Bph1. The 1999 and 2005 strains were virulent to ASD7 (bph2). Thus, the virulence status of the laboratory BPH strains was the same as in previous reports. The 1989, 1999, and 2005 WBPH strains were virulent to N22 (Wbph1), Mudgo, ASD7, Babawee (bph4) and Chin Saba (bph8); the 1999 and 2005 WBPH strains were also virulent to ARC10239 (Wbph2). Although the virulence status of WBPH in Japan has not previously been studied, the present results suggest that the effectiveness of the Wbph1 resistance gene broke down before 1989, while that of Wbph2 broke down between 1989 and 1999. The present study showed that long-term mass rearing in the laboratory has not affected virulence status. Thus, these strains will be useful to analyze resistance genes against BPH and WBPH.

The possible domestic migration of the white-backed planthopper, Sogatella furcifera, in Japan was investigated. The next emigrating generation of immigrants during the Bai-u rainy season was selected for analysis. Four criteria were used to determine whether a migration was domestic: 1) a large catch peak in the net trap in northern Japan, 2) in the trap area, the absence of a preceding generation of catch insects based on the analysis of effective accumulated temperature, 3) suitable air currents for insects to migrate from western to northern Japan, as found by backward trajectory analysis, and 4) the presence of emigrating adults in rice fields in western Japan based on a field survey of the population density. The catch data obtained from a net trap in Akita city, Akita prefecture, from 1980 to 2006 showed four clear peaks exceeding 1,000 insects per day, and only one of these, occurring in early August 1987, was found to meet all criteria. The weather pattern was a low-pressure system in the Japan Sea moving northeastward, with a windy region from western to northern Japan. The possible effect of domestic migration in early August on rice crops in northern Japan was discussed.

Biotypes cause a serious problem in controlling brown planthopper (BPH), Nilaparvata lugens Stal (Homoptera: Delphacidae). We characterized four BPH strains, which were collected in different localities in Japan and maintained by random mating within each strain for different periods. The virulence of the BPH strains was evaluated based on their postembryonic development on susceptible and resistant rice lines and inhibition of rice seedling growth. Two single resistance gene introgression lines, “Norin-PL3 (Bph1 carrier)” and “Norin-PL4” (bph2 carrier), and a pyramided line in which both genes were combined were used with a susceptible japonica recurrent parent “Tsukushibare” as a control. Sequence comparison of the mitochondrial cytochrome oxidase-I (COXI) gene was also performed in the characterization. The four BPH strains were classified into two or three groups with unique virulence spectra. The oldest strain showed avirulence against both introgressed resistance genes, while the three younger strains showed strong virulence against one or both genes. The different virulence spectra suggested the sequential adaptation of these BPH strains to the two resistance genes. Pyramiding of the resistance genes had no additive effect on rice seedlings carrying them even against avirulent and less-virulent BPH strains.

BACKGROUND: In 2003 the development of insecticide resistance against neonicotinoids in the brown planthopper (BPH), Nilaparvata lugens (StÅl) (Homoptera: Delphacidae), was first observed in Thailand and has since been found in other Asian countries such as Vietnam, China and Japan. However, the LD 50 values of BPH and the whitebacked planthopper (WBPH), Sogatella furcifera (Horvath), against both neonicotinoid and phenylpyrazole insecticides have been poorly reported in many Asian countries. RESULTS: The topical LD 50 values for imidacloprid in the BPH populations collected from East Asia (Japan, China, Taiwan) and Vietnam in 2006 were 4.3-24.2 μg g -1 and were significantly higher than those collected from the Philippines (0.18-0.35 μg g -1 ). The BPH populations indicated a positive cross-resistance between imidacloprid and thiamethoxam. Almost all the WBPH populations from Japan, Taiwan, China, Vietnam and the Philippines had extremely large LD 50 values (19.7-239μg g -1 or more) for fipronil, except for several populations from the Philippines and China. CONCLUSION: Species-specific changes in insecticide susceptibility were found in Asian rice planthoppers (i.e. BPH for imidacloprid and WBPH for fipronil). Insecticide resistance in BPH against imidacloprid occurred in East Asia and Indochina, but not in the Philippines. In contrast, insecticide resistance in WBPH against fipronil occurred widely in East and South-east Asia.

Rice planthoppers are long-distance migratory insects. The East Asian population is believed to migrate from northern Vietnam to southern China in the spring. To understand its major migration paths, a migration analysis was conducted with catch data by a single light trap located in the Red River delta in northern Vietnam. The catch data showed large peaks in late April to early May, each of which was used as a starting point of a simulation. Destination regions of simulated migrations were found to be distributed over southern Chinese provinces: Guangxi, southern Hunan, Jiangxi, northern Guangdong and northwestern Fujian. The region formed a diagonal belt stretching in the northeast direction. According to Chinese data, many planthoppers were caught in light traps along the diagonal belt region, supporting the simulation results. The planthoppers that arrive on rice plants of the early crop can multiply by one or two generations before their next emigration.

1 Migrations of Sogatella furcifera captured in Japan in the 2003 season were analysed using hourly catches and a simulation model. 2 The catch data showed several clear migration peaks, with average durations of 4 h. The peaks were separated from each other by approximately 12 h, corresponding to the observed fact that planthoppers take off at dusk or dawn. 3 The simulation model comprised two components: a numerical weather forecast submodel, MM5, and a particle dispersion submodel, GEARN. The simulation model used three-dimensional meteorological data and calculated the relative aerial density of migrating planthoppers to allow analysis of the timing and area of migrations in Japan. 4 The simulation model, together with the hourly catches, enabled an estimation of migration source regions. 5 Possible migration sources during our hourly observation period were located in the coastal area of Fujian province in China, as well as Taiwan.

Transoceanic migration of rice planthoppers was simulated by incorporating several parameters related to the flight behavior with a high performance atmospheric dispersion model. The model consists of an atmospheric dynamic submodel for calculating meteorological fields and a particle random-walk submodel for atmospheric dispersion. The model also functions in specifying the release area and take-off time of the migration of planthoppers. Using this model, we carried out a case study simulating the migration of planthoppers to western Japan with net-trap catches at 10 locations in mid-June 1998. In the simulation, 56 areas each with a width of 2° of latitude and longitude were set as tentative take-off areas. The take-off duration was assumed to be 1 h at dawn and dusk. To find the migration source areas, the relative density of immigrants originating from each take-off area was compared with each observed density by rank correlation coefficients. Possible migration release areas, which showed a high correlation to observations, were distributed around 23–27°N, including Fujian and Taiwan. An air temperature that allowed the planthoppers to fly continuously and flight duration were critical parameters for the simulation results.

Migrations of the rice planthopper Nilaparvata lugens (Stal) (Delphacidae) from the Philippines to Taiwan, southern China, and southern Japan were analyzed using three-dimensional migration simulations. The results strongly suggested that the Southeast Asian population of N. lugens mixes with the East Asian population. This highlighted the possibility that planthoppers from the Southeast Asian population, which have properties different from those in the East Asian population such as feeding of resistant rice varieties and wing polymorphism, could migrate to Japan via southern China and Taiwan. This study, therefore, emphasizes the special care that should be taken to monitor the properties of immigrants to Japan.

A 3-dimensional backward trajectory analysis was applied for estimating the migration source of the white-backed planthopper that immigrated into Kyushu, western Japan in June from 1987 to 2001. The results showed that the coastal regions of Fujian province in China and Taiwan were found to be possible sources. The migration duration taken to travel between the sources and Kyushu was estimated to be about 36 or 48 h. The comparison between 2-dimensional and 3-dimensional analyses indicated that the former method had a bias toward the southwest in estimating the source.

The white-backed planthopper, Sogatella furcifera, and the brown planthopper, Nilaparvata lugens, are pests of rice and migrate from south China to Japan in the rainy season of early summer. In order to achieve high-precision migration prediction, a real-time prediction system was developed. In this system, the latest meteorological data are supplied online to an advanced numerical weather prediction model, MM5. The model forecasts three-dimensional atmospheric fields at one-hour intervals. In these fields, a planthopper migration simulation model, GEARN, calculates movement of a number of modeled planthoppers and predicts their relative aerial density at three-hour intervals. The results are converted to maps and become available on the Internet. The maps of relative aerial density provide information about the timing and area of migrations over the next two days. During the main migration season in June and July 2003, the system achieved a prediction quality that was comparable to that of rainfall forecasts by the Japanese Meteorological Agency.

The brown planthopper, Nilaparvata lugens , and the white-backed planthopper, Sogatella furcifera , are phloem feeders. Experiments were conducted to compare plant response of japonica and indica rice cultivars to N. lugens and S. furcifera feeding, and quantify the amount of feeding by these hopper species on various rice cultivars. Plant height, tiller number, leaf area and organ dry weights, and leaf photosynthetic rates and organ nitrogen contents were measured to determine plant response to hopper feeding. Hopper dry weights and honeydew weights were measured to quantify the amount of feeding by the hoppers and determine the effects of cultivar on hoppers. Reduction in plant height caused by S. furcifera feeding was greater than that of N. lugens . The effects of N. lugens feeding on roots were greater than those of S. furcifera . These results suggest a difference in the mechanism of damage caused by S. furcifera and N. lugens even though both hoppers are phloem feeders. Regardless of rice type, the reaction of different cultivars to S. furcifera and N. lugens feeding varied. Hinohikari was most susceptible to S. furcifera and N. lugens feeding, whereas Daili Zaoxian and Nipponbare were more susceptible to N. lugens and S. furcifera feeding, respectively. S. furcifera were smaller and excreted less honeydew than N. lugens , but their effects on tillers, leaf area and plant weight were similar to those of N. lugens suggesting that S. furcifera feeding effects at vegetative stage was greater than those of N. lugens . The demand for nitrogen was probably greater for S. furcifera than N. lugens , because the former contains greater nitrogen and thrives on the upper portion of the rice plant; however, this needs further investigation.

1. A series of laboratory experiments was conducted to explore the effects of inter-specific interactions, both direct interactive effects and those induced through previous feeding, on the dispersal capability (proportion of macropterous adults) and performance (development time and survival) of two wing-dimorphic planthoppers, the whitebacked planthopper Sogatella furcifera and the brown planthopper Nilaparvata lugens, two pests of rice throughout Asia. 2. An asymmetric effect of inter-specific crowding on dispersal capability was detected between the two planthoppers. With density controlled, the proportion of macropterous adults in N. lugens was higher when raised in mixed populations with S. furcifera than in pure cultures comprised of conspecifics, suggesting that interspecific effects on dispersal capability are stronger than intraspecific effects. In contrast, interspecific effects on macroptery (%) were weaker than intraspecific impacts for S. furcifera. 3. This trend was parallelled by adverse competitive effects on survival, with interspecific effects stronger than intraspecific impacts for N. lugens and intraspecific effects stronger for S. furcifera. 4. Crowding also affected the incidence of macroptery indirectly through feeding-induced changes in plant physiology. Rearing N. lugens on plants fed on previously by heterospecifics resulted in a higher incidence of macroptery in females and protracted development than for N. lugens raised on plants fed on previously by conspecifics. By contrast, the intraspecific effect of previous feeding was stronger than the interspecific effect in S. furcifera, with higher macropter production and prolonged development occurring on plants exposed previously to conspecifics. 5. The results suggest that interspecific interactions between herbivorous insects, both direct and via feeding-induced changes in plant physiology, can have negative consequences for performance and survival and promote the production of flight-capable adults that can disperse.

Virulence of the brown planthopper Nilaparvata lugens strains, which immigrated into Japan between 1997 and 1999, was examined on five rice varieties, Mudgo (carrying a resistance gene Bph 1), IR26 (Bph 1), ASD7 (bph 2), Norin PL10 (Bph 3), and Babawee (bph 4). Newly emerging brachypterous females of N. lugens were released on test rice plants at the tillering stage, and we defined the females that became heavily swollen or survived for five days as virulent. Between 45 and 87% of the females were virulent to ASD7, although such high virulence had not been detected before 1997. Between 49 and 98% of the females were virulent to Mudgo and IR26. Virulence to rice varieties carrying Bph 1 in the N. lugens population has continued to become stronger since the 1988-1990 period in which changes in virulence were first found. In contrast, virulence of the N. lugens strains to Norin PL 10 and Babawee was still at a low level, with 5 to 27% of the females being virulent. The results indicate that the resistance of Bph 1 has probably broken down and the resistance of bph 2 may be becoming ineffective for the N. lugens populations immigrating into Japan.

A population census was conducted to describe the effects of the growth stage of rice on the population dynamics ofS. furcifera, in particular, on immigration, seasonal abundance, population growth rate, and wing-form expression. The number of immigrants was highest on rice plants 17 to 30 days after transplanting (DAT), which suggested that immigrants prefer to settle or remain more on rice plants at the tillering stage (approximately 20–30 DAT). Population growth rate from immigration to the 1st generation decreased with an increase in rice plant age. In contrast, population growth rate from the 1st to 2nd generation was not influenced by rice plant age and was negatively density-dependent. The percentage of macropters (flyers) was positively related to the growth stage of rice when rice was in the vegetative or early part of the reproductive stage, and reached 100% at about 10 days before heading (booting stage). Threafter, most adult females molted into macropters regardless of population density. The roles of host plant age and crowding effect on the population dynamics ofS. furcifera are discussed.

Genetic analysis of a threshold trait: density-dependent wing dimorphism in Sogatella furcifera (Horvath) (Hemiptera: Delphacidae), the whitebacked planthopper