Sachiyo Sanada-Morimura

Current status of insecticide resistance in the small brown planthopper, Laodelphax striatellus, in Japan, Taiwan, and Vietnam

The small brown planthopper, Laodelphax striatellus, is one of the most serious pest insects of rice plants. A large migration of the insects from overseas was reported in western parts of Japan in June 2008. Insecticide resistance to imidacloprid, fipronil and BPMC was compared among local populations in these western regions after migration. The insecticides were applied to the insects using a topical application method. In some populations, the resistance status coincided with that of the immigrant insects just after migration, i.e., resistance to imidacloprid but susceptibility to fipronil. In other populations, resistance was observed not only against imidacloprid but also fipronil. It is likely that the status of the latter populations resulted from intercrossing between domestic populations of the insects and migrants. Insecticide resistance was also assessed in other areas of northern and eastern parts of Japan. In general, these populations showed relatively low resistance, although resistance to fipronil was high in the eastern part of Japan where the density of domestic populations has recently increased. Insecticide susceptibilities were also assessed in several sites in Taiwan and the northern parts of Vietnam. Although susceptibilities differed among these sites or countries, they have recently seen a decline for all three insecticides.

Prediction of overseas migration of the small brown planthopper, Laodelphax striatellus (Hemiptera: Delphacidae) in East Asia

A method has been developed for predicting the overseas migration of Laodelphax striatellus (Fallén) 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.

Genetic mapping of the rice resistance-breaking gene of the brown planthopper Nilaparvata lugens.

Host plant resistance has been widely used for controlling the major rice pest brown planthopper (BPH, Nilaparvata lugens). However, adaptation of the wild BPH population to resistance limits the effective use of resistant rice varieties. Quantitative trait locus (QTL) analysis was conducted to identify resistance-breaking genes against the anti-feeding mechanism mediated by the rice resistance gene Bph1. QTL analysis in iso-female BPH lines with single-nucleotide polymorphism (SNP) markers detected a single region on the 10th linkage group responsible for the virulence. The QTL explained from 57 to 84% of the total phenotypic variation. Bulked segregant analysis with next-generation sequencing in F2 progenies identified five SNPs genetically linked to the virulence. These analyses showed that virulence to Bph1 was controlled by a single recessive gene. In contrast to previous studies, the gene-for-gene relationship between the major resistance gene Bph1 and virulence gene of BPH was confirmed. Identified markers are available for map-based cloning of the major gene controlling BPH virulence to rice resistance.

Insecticide susceptibilities in populations of two rice planthoppers, Nilaparvata lugens and Sogatella furcifera, immigrating into Japan in the period 2005–2012

BACKGROUND The brown planthopper Nilaparvata lugens and the whitebacked planthopper Sogatella furcifera are both important pests on rice throughout Asia. The major cause of recent outbreaks is thought to be the development of insecticide resistance. Thus, the authors monitored insecticide susceptibilities in populations of these two insects immigrating into Japan in the period 2005-2012. Ten insecticides were tested, including members of the organophosphate, carbamate, pyrethroid, neonicotinoid and phenylpyrazole groups. RESULTS The LD50 values of N. lugens against imidacloprid increased from 2005 (0.7 µg g(-1)) to 2012 (98.5 µg g(-1)). The resistance ratio (LD50 value in 2012/baseline LD50 value in 1992) was 615.5. In contrast, LD50 values of N. lugens against fipronil were <1.0 µg g(-1) up to 2012, suggesting that N. lugens had developed no insecticide resistance to this insecticide. However, S. furcifera exhibited resistance against fipronil up to 2012. Except for the case of malathion, the resistances of N. lugens against members of the organophosphate and carbamate groups were closely similar in the period 2005-2012 to earlier determinations in 1984 and 1985. CONCLUSION Species-specific insecticide resistance (imidacloprid resistance in N. lugens and fipronil resistance in S. furcifera) is ongoing in populations of the two planthoppers immigrating into Japan.

Period Gene of Bactrocera cucurbitae (Diptera: Tephritidae) Among Strains with Different Mating Times and Sterile Insect Technique

Abstract The efficacy of sterile insect technique (SIT) depends on successful mating of released males with wild females. If the time of mating in a day of mass-reared and released males differs from those of wild females, the efficiency of SIT decreases. Therefore, understanding the molecular mechanisms controlling mating time of the target pests is particularly important for SIT. The period (per) gene, which has been considered as a key clock gene controlling the mating time of the melon fly, Bactrocera cucurbitae (Coquillett) (Diptera: Tephritidae), was cloned from two strains having different times of mating during the day. DNA sites varied in the 5′ and 3′ untranslated regions and at synonymous sites, although protein sequences were identical. We also provide phylogenetic relationships among PER protein sequences of dipteran species including several tephritid pest species. The functional domains of PER in the melon fly are very similar to those in other tephritid species. A luciferase reporter assay showed that the melon fly PER can functionally complement that of Drosophila melanogaster (Meigen). The results implicate that the major genetic cause of the difference in circadian periods, and thus in reproductive isolation, is probably one or more other clock gene(s). Thus, the series of studies may provide a novel factor concerning genetic quality control of mass-reared insect pests for SIT, which depends on successful mating of released males and wild females.

A Simple Sequence Repeat- and Single-Nucleotide Polymorphism-Based Genetic Linkage Map of the Brown Planthopper, Nilaparvata lugens

In this study, we developed the first genetic linkage map for the major rice insect pest, the brown planthopper (BPH, Nilaparvata lugens). The linkage map was constructed by integrating linkage data from two backcross populations derived from three inbred BPH strains. The consensus map consists of 474 simple sequence repeats, 43 single-nucleotide polymorphisms, and 1 sequence-tagged site, for a total of 518 markers at 472 unique positions in 17 linkage groups. The linkage groups cover 1093.9 cM, with an average distance of 2.3 cM between loci. The average number of marker loci per linkage group was 27.8. The sex-linkage group was identified by exploiting X-linked and Y-specific markers. Our linkage map and the newly developed markers used to create it constitute an essential resource and a useful framework for future genetic analyses in BPH.

Migration analysis of Nilaparvata lugens (Hemiptera: Delphacidae) from the Philippines to Taiwan under typhoon-induced windy conditions

The population of Nilaparvata lugens (Stål) 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.

Duplication of acetylcholinesterase gene in diamondback moth strains with different sensitivities to acephate

This study examined the acetylcholinesterase 1 gene (AChE1) in Plutella xylostella strains with different sensitivities to acephate. Multiple haplotypes of the gene were found in the field-collected strains including distinct haplotypes carrying one or both previously reported mutations (A298S and G324A). Moreover, sequencing results indicated the presence of duplicated copies of the gene in the field-collected strains. No correlation was found between copy numbers of AChE1 and levels of resistance to acephate suggesting that extensive AChE1 duplication is not a major resistance factor at least in some P. xylostella strains. Proportions of the A298S and G324A mutations showed no correlation with levels of resistance to acephate. This suggests that acephate resistance of P. xylostella is complex and cannot be evaluated based on the AChE1 copy number or proportions of the resistance mutations alone.

Passive Long-Distance Migration of Apterous Dryinid Wasps Parasitizing Rice Planthoppers

The wasp family Dryinidae comprises predator and parasitoid wasps of leafand planthoppers (Hemiptera: Auchenorrhyncha). This family is morphologically distinct from other wasps. Females in most subfamilies of Dryinidae have forelegs that are modified into a chela, with an enlarged claw (Fig. 1: green) and 5th tarsomere (Fig. 1: red) that aid in grasping the host insect. The enlarged claw moves widely when the chela opens. Such foreleg morphology is not always the case, such as for females of the subfamily Aphelopinae R.C.L. Perkins and Erwiniinae Olmi & Guglielmino (Olmi & Guglielmino, 2010) that have simple forelegs. Dryinid wasps often show distinctive sexual dimorphism, such as the presence or absence of chela. The subfamily Gonatopodinae Kieffer is one of the extreme cases of sexual dimorphism, but in this case it is because females of most of the species are apterous. The pterothorax becomes so slender in the apterous form (Fig. 2: A, C) that they look like ants. In contrast, males have well developed wings and a pterothorax (Fig. 1: B).

Seasonal occurrence of Laodelphax striatellus (Hemiptera: Delphacidae) in a rice-forage crops mixed cropping area in central Kyushu, Japan

The small brown planthopper, Laodelphax striatellus (Fallén) is an important pest of rice in East Asia because it is a vector of rice stripe virus. In this study, we examined the seasonal occurrence of L. striatellus in an agricultural area in central Kyushu in which cropping systems have been changing since the 1970s. The first generation of L. striatellus emerged in the spring, not only on barley, oats, and Italian ryegrass but also on spring crops of forage maize. The second generation of L. striatellus adults emerged in forage maize fields in July, after harvesting of winter crops of barley, oats, and Italian ryegrass. These results suggest that spring crops of forage maize are important hosts of L. striatellus from spring to early summer. The overwintering generation of L. striatellus was collected from rice ratoon, suggesting this is a favorable host for L. striatellus during winter. The appearance of these novel hosts may be a factor in the recent re-emergence of L. striatellus in central Kyushu.