Seasonal and Year-Round Distributions of Bactrocera dorsalis (Hendel) and Its Risk to Temperate Fruits under Climate Change.

Bactrocera dorsalis (Hendel) is an important pest to fruits and vegetables. It can damage more than 300 plant species. The distribution of B. dorsalis has been expanding owing to international trade and other human activities. B. dorsalis occurrence is strongly related to suitable overwintering conditions and distribution areas, but it is unclear where these seasonal and year-round suitable areas are. We used maximum entropy (MaxEnt) to predict the potential seasonal and year-round distribution areas of B. dorsalis. We also projected suitable habitat areas in 2040 and 2060 under global warming scenarios, such as SSP126 and SSP585. These models achieved AUC values of 0.860 and 0.956 for the seasonal and year-round scenarios, respectively, indicating their good prediction capabilities. The precipitation of the wettest month (Bio13) and the mean diurnal temperature range (Bio2) contributed 83.9% to the seasonal distribution prediction model. Bio2 and the minimum temperature of the coldest month (Bio6) provided important information related to the year-round distribution prediction. In future scenarios, the suitable area of B. dorsalis will increase and the range will expand northward. Four important temperate fruits, namely, apples, peaches, pears, and oranges, will be seriously threatened. The information from this study provides a useful reference for implementing improved population management strategies for B. dorsalis.

When Does the Prey/Predator Ratio Work for the Effective Biocontrol of Cotton Aphid on Cotton Seedlings?

The decision to delay or cancel spraying insecticides against pest aphids is dependent on the ratio of prey/predator, which reflects how well the predator can suppress the aphid population increase in the field. It is challenging to estimate the ratio of prey/predator due to the multiple factors involved in the interaction between prey and predator. Cotton aphid (Aphis gossypii Glover) is a serious pest, widely distributed in cotton-growing areas around the world. We combined different ratios of aphids with aphid oligophagous ladybird beetles (Coccinella septempunctata Linnaeus) under laboratory and garden conditions to investigate the critical threshold for prey/predator which effectively reduced the cotton aphid population increase. Two kinds of modeling were developed to understand the relationships between the ratio of prey/predator and the PGR (population growth rate), and with the effectiveness of biocontrol (EBC). We found the critical values of PGR should be less than -0.0806 (predators artificially released after 5 days) and then less than -0.075 (predators released after 10 days) if EBC is less than 50%. We recommend that the ratio of prey/predator should be less than 450 for the effective biocontrol of cotton aphids at the cotton seedling stage. These values can be reference indices for the management of aphids in mid-summer.

Relationships Among the Feeding Behaviors of a Mirid Bug on Cotton Leaves of Different Ages and Plant Biochemical Substances.

Apolygus lucorum (Meyer-Dür) (Hemiptera: Miridae) tends to feed on young plant tissues. To explore the relationship between stylet probing behaviors of adult A. lucorum and conditions of cotton leaves, we conducted an experiment using electropenetrography (EPG). Behaviors were recorded on four cotton varieties, in relation to thickness and biochemical traits of differently-aged leaves. Cotton leaf age had a significant effect on the probing behavior of A. lucorum but cotton variety did not. One-day-old leaves of A. lucorum received the highest mean number of stylet probes (penetrations) per insect, and longest mean durations per insect of combined stylet probing or its components, cell rupture and ingestion behaviors. All of the leaf traits (thickness and biochemical substances) were similar among these four cotton varieties. Leaf thickness had a significantly negative effect on the same four variables above. Gossypol and tannin also had a negative impact on combined probing duration. Redundancy analysis showed that the four EPG variables were closely related to nutrient substances (amino acids, sugar, and water) while they had the opposite relationship with plant defense substances (gossypol and tannin). On cotton in the seedling stages, A. lucorum fed more readily on the youngest, thinnest leaves in our no-choice EPG experiments. Nutrients and chemical resistance substances determined the probing duration of A. lucorum. Our findings can contribute to better understanding of patterns of feeding and host consumption by A. lucorum, ultimately improving cotton resistance to A. lucorum.

Comparison between sustained effects of spray and injection thiamethoxam on apple aphids and non-target insects in apple orchard.

Spray application is considered to be the most common method of insecticide use in apple orchard, while trunk injection has often be used in labor-intensive areas. Here, a comparison of both methods in aphid control efficiency and non-target effects was conducted. We evaluated the effects of thiamethoxam by either spray or injection on apple aphid Aphis citricola, and examined the temporal dynamic of thiamethoxam in leaves by using residue analysis. Results showed that thiamethoxam had a remarkable suppression effect on Aphis citricola, and both application methods had obvious control efficiency with the highest value above 90%. The control effect of spray method on Aphis citricola reached the maximum at 7 days after application, while that of injection method reached the maximum at 14 days after application. Moreover, the control effect of injection after 14 days and that of spray after 7 days were not significant, suggesting the spray method had a higher quick-acting effect than the injection method, and the two methods had a similar persistence effect. The population dynamics of non-target insects (ladybugs, parasitoid wasps and predatory bugs) showed basically the same as that of blank controlled. The control effect evaluation of thiamethoxam on Aphis citricola suggest that injection treatment was more effective in protecting natural enemies than spray treatment, and thiamethoxam didn't interfere with natural enemies to control Aphis citricola with both two application methods.

Landscape genetic analyses reveal host association of mitochondrial haplotypes in the Asian corn borer, Ostrinia furnacalis.

Crop expansion often leads to high pest pressure. These pests may have fitness trade-offs related to host use, and some host-associated genotypes may benefit and increase in frequency. However, evidence concerning the effect of host availability on spatial distribution and frequency of mitochondrial haplotypes is scarce. We studied genetic variation of the Asian corn borer, Ostrinia furnacalis (Guenée), across a large area during 2 years (2016 and 2017). Mitochondrial sequence data were obtained from 530 individuals collected from 79 locations in Shandong Province, China. In total, 155 haplotypes were found based on the combined cytochrome oxidase subunit 1 (COI) and COII genes. Three haplotypes (H2, H12, and H23) were dominant, whereas most of the other haplotypes occurred in low frequency. A haplotype network showed that the 155 haplotypes can be grouped into three clusters. Haplotype clusters seemed to be randomly distributed. The frequency of H12 (in Cluster 1) was positively correlated with maize crop proportion, but negatively correlated with other crops (primarily vegetables, oilseed crops, and cotton) at all spatial scales (1-, 3-, and 5-km radius). Cluster 2 had haplotype H23, and this cluster was negatively correlated with semi-natural habitats. Cluster 3 had no dominant haplotype and was not affected by landscape factors. We conclude that H12 may be a maize-associated haplotype. Further study is needed to verify the possibility that the carriers of this haplotype may possess some fitness trade-offs. Our study highlights the importance of host availability in O. furnacalis haplotype distribution and frequency.

Migration of Sogatella furcifera between the Greater Mekong Subregion and northern China revealed by mtDNA and SNP.

BACKGROUND: The white-backed planthopper (WBPH), Sogatella furcifera (Horváth) (Hemiptera, Delphacidae), is a migratory pest of rice in Asia. Shandong Province, in northern China, is located on the migration pathway of WBPH between southern and northeast China. The potential sources of WBPH in northern China are poorly understood. We studied the sources of WBPH in Shandong Province by determining the population genetic structure of WBPH in 18 sites distributed in Shandong and in six regions of the Greater Mekong Subregion (GMS). We used mitochondrial gene and single-nucleotide polymorphism (SNP) markers for analysis. RESULTS: All of the WBPH populations studied in the seven regions had low genetic diversity. Pairwise FST values based on mtDNA ranged from - 0.061 to 0.285, while FST based on SNP data ranged from - 0.007 to 0.009. These two molecular markers revealed that 4.40% (mtDNA) and 0.19% (SNP) genetic variation could be explained by the interpopulation variation, while the rest came from intrapopulation variation. The populations in the seven geographic regions comprised four hypothetical genetic clusters (K = 4) not associated with geographic location. Eighty-four of 129 individuals distributed across the given area were designated as recent migrants or of admixed ancestry. Although the substantial migration presented, a weak but significant correlation between genetic and geographic distances was found (r = 0.083, P = 0.004). CONCLUSION: The Greater Mekong Subregion was the main genetic source of WBPH in Shandong, while other source populations may also exist. The genetic structure of WBPH is shaped by both migration and geographic barriers. These results help clarify the migration route and the source of WBPH in northern China.

Landscape agricultural simplification correlates positively with the spatial distribution of a specialist yet negatively with a generalist pest.

Agricultural expansion at the cost of natural or semi-natural habitats is simplifying human-dominated landscapes. As croplands provide a large resource of food to herbivores, pest damage may increase, but such large-scale patterns across regions are little known. Here, we used two years of maize field data from 102 counties (each 1318 km2 on average) across Shandong Province in China to study the spatial distribution of two major co-occurring maize pests: the putative habitat specialist the Asian Corn Borer (Ostrinia furnacalis) (ACB) and the generalist Yellow Peach Moth (Conogethes punctiferalis) (YPM). We used Spatial Analysis by Distance Indices (SADIE) to assess the spatial distribution patterns of these pests and their relation to landscape factors. In both 2016 and 2017, the aggregation and abundance of the ACB was positively correlated with the proportion of maize on the county level, whereas the YPM exhibited the opposite pattern, i.e., a negative correlation with maize proportion. The ACB abundance was below the economic threshold level when maize was <31% in 2017, whereas the YPM abundance was below the threshold when maize was >27% (in 2016) or 23% (in 2017). Maize plant presence was the main determinant of the abundance of the ACB, while the YPM appeared to benefit from further resources in non-crop habitats. These contrasting distribution patterns suggest that the two pests are driven by their different resource requirements. In more diversified landscapes, pest control may need to focus primarily on the generalist consumer, the YPM, whereas in maize-dominated landscapes, the specialist consumer, the ACB, is dominant and needs attention.

Food web structure of parasitoids in greenhouses is affected by surrounding landscape at different spatial scales.

Landscape management affects species interactions and can have notable effects on food web structure. Local parasitoid populations in greenhouses usually migrate from outside crops; biological control of greenhouse aphids may be thus highly dependent on the composition of surrounding landscape. However, it is less clear how surrounding landscape composition affects primary-hyperparasitoid food webs and pest control services in greenhouses. We investigated the food web of parasitoids on melon-cotton aphid (Aphis gossypii Glover) in watermelon greenhouses in two suburban Beijing counties over two years. We used the quantitative food web metrics (generality, vulnerability, link density, and interaction evenness) to assess the effects of landscape composition on primary-hyperparasitoid food web structure. We found that landscape with more cropland within 1-3 km tended to have more primary parasitoids per hyperparasitoid species (generality). Higher proportions of woodland at the 0.5 km scale were negatively correlated with the mean numbers of hyperparasitoid per primary parasitoid species (vulnerability), as well as with hyperparasitism rate and hyperparasitoid richness. Link density, interaction evenness and aphid mortality caused by parasitoids (parasitism rate) were not affected by landscape factors. However, active primary parasitism (biocontrol potential) increased with the proportion of woodland. This suggested that the bottom-up effect induced by primary parasitoids might benefit hyperparasitoids, thus exerting little influence of primary parasitoids on pest control. The top-down effect of hyperparasitoids may reduce with increasing woodland proportion. To enhance the effects of primary parasitoids, landscape management programs should also target, and thus limit the impact of hyperparasitoids.

Genetic diversity of melon aphids Aphis gossypii associated with landscape features.

Despite increasing evidence that landscape features strongly influence the abundance and dispersal of insect populations, landscape composition has seldom been explicitly linked to genetic structure. We conducted a genetic study of the melon aphid, Aphis gossypii, in two counties of Beijing, China during spring migration using samples from watermelon. We performed aphid genetic analysis using restriction site associated DNA sequencing (2b-RAD) and investigated the relationship between land cover and the genetic diversity. The percentage area of land cover (cropland, vegetable, orchard, grassland, woodland) was quantified in each particular scale (ranging from 0.5 km to 3 km) and was used as a predictor variable in our generalized linear models. We found a moderate level of genetic differentiation among nine sampled populations. Geographic distance and genetic distance were not significantly associated, indicating that geographic location was not a barrier to migration. These nine populations could be clustered depending on their level of genetic diversity (high and low). The genetic diversity (Shannon's information index) was positively correlated with grassland at the spatial scales of 1 and 2 km and negatively with orchard and vegetable at 0.5 and 1 km. Genetic diversity was best predicted by the grassland + orchard + vegetable model at a spatial scale of 1 km. Based on the method of relative weights, orchard land had the greatest relative importance, followed by grassland and vegetable land, in that order. This study contributes to our understanding of the genetic variation of aphids in agricultural landscapes.

Responses of cereal aphids and their parasitic wasps to landscape complexity.

The intensification of agriculture has caused a decline in the complexity of agricultural landscapes because of the expansion of arable lands and the removal of natural habitats. These landscape changes, which have substantial effects on natural enemies (e.g., parasitoids) and on biological control services, have received considerable attention recently. In the current study, we analyzed the effects of landscape complexity on cereal aphids and their parasitic wasps in 24 sites during a period of 3 yr. In total, 11 primary parasitoid species and 6 hyperparasitoid species, comprising 5,220 individuals, were collected in our experiments. With the exception of two primary parasitic wasps (Trioxys asiaticus Telenga and Toxares sp.) and one hyperparasitic wasp (Dendrocerus carpenteri [Curtis]), most species were sensitive to landscape complexity after > or = 1 yr. Species diversity, primary parasitism, and hyperparasitism increased with increasing landscape complexity. However, the relationship between the population density of active primary parasitoids (effective primary parasitoids) and landscape complexity was indicated by a quadratic function, not a linear function. The effective population density of primary parasitoids was maximal (2.04 individuals per 100 wheat stems) if the percentage of noncrop habitat was 38%. The hypothesis that landscape complexity may enhance the activity or higher diversity of primary parasitoids and hyperparasitoids was well-supported by our study. However, the hyperparasitoids had a more sensitive response to landscape complexity than the primary parasitoids. Further studies should aim to enhance the biological pest control of primary parasitoids and suppress hyperparasitoids by habitat manipulation. This technique could be used effectively for pest management in mosaic landscapes through habitat rearrangement and reorganization.