Population genetic structure and approximate Bayesian computation analyses reveal the southern origin and northward dispersal of the oriental fruit moth Grapholita molesta (Lepidoptera: Tortricidae) in its native range AbstractThe oriental fruit moth (OFM) Grapholita molesta is one of the most destructive orchard pests. Assumed to be native to China, the moth is now distributed throughout the world. However, the evolutionary history of this moth in its native range remains unknown. In this study, we explored the population genetic structure, dispersal routes and demographic history of the OFM in China and South Korea based on mitochondrial genes and microsatellite loci. The Mantel test indicated a significant correlation between genetic distance and geographical distance in the populations. Bayesian analysis of population genetic structure (BAPS) identified four nested clusters, while the GENELAND analysis inferred five genetic groups with spatial discontinuities. Based on the approximate Bayesian computation approach, we found that the OFM was originated from southern China near the Shilin area of Yunnan Province. The early divergence and dispersal of this moth was dated to the Penultimate glaciation of Pleistocene. Further dispersal from southern to northern region of China occurred before the last glacial maximum, while the expansion of population size in the derived populations in northern region of China occurred after the last glacial maximum. Our results indicated that the current distribution and structure of the OFM were complicatedly influenced by climatic and geological events and human activities of cultivation and wide dissemination of peach in ancient China. We provide an example on revealing the origin and dispersal history of an agricultural pest insect in its native range as well as the underlying factors.

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BACKGROUND The diamondback moth (DBM), Plutella xylostella (Lepidoptera: Plutellidae), is a notorious pest of cruciferous plants. In temperate areas, annual populations of DBM originate from adult migrants. However, the source populations and migration trajectories of immigrants remain unclear. Here, we investigated migration trajectories of DBM in China using genome‐wide single nucleotide polymorphisms (SNPs) genotyped using double‐digest RAD (ddRAD) sequencing. We first analyzed patterns of spatial and temporal genetic structure among southern source and northern recipient populations, then inferred migration trajectories into northern regions using discriminant analysis of principal components (DAPC), assignment tests, and spatial kinship patterns. RESULTS Temporal genetic differentiation among populations was low, indicating that sources of recipient populations and migration trajectories are stable. Spatial genetic structure indicated three genetic clusters in the southern source populations. Assignment tests linked northern populations to the Sichuan cluster, and central–eastern populations to the southern and Yunnan clusters, indicating that Sichuan populations are sources of northern immigrants and southern and Yunnan populations are sources of central–eastern populations. First‐order (full‐sib) and second‐order (half‐sib) kin pairs were always found within populations, but ~ 35–40% of third‐order (cousin) pairs were found in different populations. Closely related individuals in different populations were found at distances of 900–1500 km in ~ 35–40% of cases, while some were separated by > 2000 km. CONCLUSION This study unravels seasonal migration patterns in the DBM. We demonstrate how careful sampling and population genomic analyses can be combined to help understand cryptic migration patterns in insects. © 2020 Society of Chemical Industry

SignificanceThe effects of diverse environmental factors on the crop circadian clock have not been systemically studied. We devised a reliable and cost-effective discovery pipeline and demonstrated its feasibility by mapping the abiotic stress “inputome” of the soybean clock. We found that the interfaces between the soybean clock and abiotic stress signals were quite different from those in Arabidopsis. Furthermore, alkaline stress was identified as a circadian clock modulator. These findings highlight the complexity of the soybean clock, which cannot be simply extrapolated from knowledge obtained from Arabidopsis. Our discovery pipeline thus offers a broadly applicable and affordable tool for similar large-scale circadian clock studies in diverse species since it does not rely on transgenic circadian reporters or expensive imaging systems.

Pest species are often able to develop resistance to pesticides used to control them, depending on how rapidly resistance can emerge within a population or spread from another resistant population. We examined the evolution of bifenazate resistance in China in the two‐spotted spider mite (TSSM) Tetranychus uticae Koch (Acari: Tetranychidae), one of the most resistant arthropods, by using bioassays, detection of mutations in the target cytb gene, and population genetic structure analysis using microsatellite markers. Bioassays showed variable levels of resistance to bifenazate. The cytb mutation G126S, which confers medium resistance in TSSM to bifenazate, had previously been detected prior to the application of bifenazate and was now widespread, suggesting likely resistance evolution from standing genetic variation. G126S was detected in geographically distant populations across different genetic clusters, pointing to the independent origin of this mutation in different TSSM populations. A novel A269V mutation linked to a low‐level resistance was detected in two southern populations. Widespread resistance associated with a high frequency of the G126S allele was found in four populations from the Beijing area which were not genetically differentiated. In this case, a high level of gene flows likely accelerated the development of resistance within this local region, as well as into an outlying region distant from Beijing. These findings, therefore, suggest patterns consistent with both local evolution of pesticide resistance as well as an impact of migration, helping to inform resistance management strategies in TSSM.

Historical invasion scenarios based on observational records are usually incomplete and biased, but these can be supplemented by population genetic data. The western flower thrips (WFT), Frankliniella occidentalis, invaded China in the last 13 years and has rapidly become one of the most serious pests in the country. To assess whether this invasion involved a single event or multiple events, we examined patterns of genetic diversity and population structure of WFT across 12 Chinese populations and a native US population based on mitochondrial DNA and/or 18 microsatellite loci. The average allelic richness and haplotype diversity in Chinese populations were significantly lower than in a population from its native range. The distribution of mitochondrial haplotypes suggested multiple independent invasions of WFT into China, including two invasions into the Beijing region. Based on microsatellite data, two distinct clusters were identified, with both of them splitting further into two clusters; in the Beijing region, the microsatellite data also provided evidence for two introductions. Both the absence of isolation by distance and the fact that distant populations were similar genetically suggest patterns of WFT movement linked to human activities. Our study therefore suggests multiple introductions of WFT into China and human‐assisted spread.

Chemical control is important in the management of the tobacco whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). Susceptibility of B. tabaci to insecticides may vary among different developmental stages and geographical populations. In this study, we examined toxicity of seven commonly-used insecticides to B. tabaci MED in four field populations from China. Avermectin has high level of toxicity to all stages of B. tabaci MED in all four populations. Cyantraniliprole and sulfoxaflor have high toxicity to adults. Spirotetramat, cyantraniliprole and flonicamid have high toxicity to nymphs but not adults. Acetamiprid, cyantraniliprole and sulfoxaflor have high toxicity to eggs. However, the relative toxicity of B. tabaci MED to these chemicals varied across different populations, with little consistency in population differences across developmental stages. Our findings together with some instances where LC values were higher than field recommended dosages indicate field-evolved resistance to insecticides (such as thiamethoxam and sulfoxaflor) and stage-specific mechanisms that will influence effective control of B. tabaci MED by insecticides.

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