Biotype and insecticide resistance status of the whitefly Bemisia tabaci from China

Identification and Characterization of Hibiscus mutabilis Varieties Resistant to Bemisia tabaci and Their Resistance Mechanisms

Hibiscus mutabilis, the city flower of Chengdu, is culturally significant and has nutritional and medicinal benefits. However, frequent infestations of Bemisia tabaci have caused economic losses. This study aimed to identify insect-resistant H. mutabilis varieties. Over two years, varieties like Jinqiusong, Zuiyun, and Zuifurong showed moderate to high resistance based on reproductive indices. Assessments of antixenosis and developmental impacts revealed that adult B. tabaci exhibited low selectivity toward these resistant varieties, indicating a strong repellent effect. Gas chromatography-mass spectrometry analysis identified volatile organic compounds, such as alcohols, alkanes, and terpenes. Notably, 2-ethylhexanol and 6-methylheptanol exhibited repellent properties. Using nontargeted metabolomics, this study compared the metabolite profiles of the insect-resistant variety Jinqiusong (JQS), moderately resistant Bairihuacai (BRHC), and highly susceptible Chongbanbai (CBB) post B. tabaci infestation. Fifteen key metabolites were linked to resistance, emphasizing the phenylpropanoid biosynthesis pathway as crucial in defense. These findings offer a theoretical foundation for breeding insect-resistant H. mutabilis varieties and developing eco-friendly strategies against B. tabaci infestations.

A comprehensive framework for the delimitation of species within the Bemisia tabaci cryptic complex, a global pest-species group

Identifying cryptic species poses a substantial challenge to both biologists and naturalists due to morphological similarities. Bemisia tabaci is a cryptic species complex containing more than 44 putative species; several of which are currently among the world's most destructive crop pests. Interpreting and delimiting the evolution of this species complex has proved problematic. To develop a comprehensive framework for species delimitation and identification, we evaluated the performance of distinct data sources both individually and in combination among numerous samples of the B. tabaci species complex acquired worldwide. Distinct datasets include full mitogenomes, single-copy nuclear genes, restriction site-associated DNA sequencing, geographic range, host speciation, and reproductive compatibility datasets. Phylogenetically, our well-supported topologies generated from three dense molecular markers highlighted the evolutionary divergence of species of the B. tabaci complex and suggested that the nuclear markers serve as a more accurate representation of B. tabaci species diversity. Reproductive compatibility datasets facilitated the identification of at least 17 different cryptic species within our samples. Native geographic range information provides a complementary assessment of species recognition, while the host range datasets provide low rate of delimiting resolution. We further summarized different data performances in species classification when compared with reproductive compatibility, indicating that combination of mtCOI divergence, nuclear markers, geographic range provide a complementary assessment of species recognition. Finally, we represent a model for understanding and untangling the cryptic species complexes based on the evidence from this study and previously published articles.

Cytochrome P450 CYP6EM1 Underpins Dinotefuran Resistance in the Whitefly Bemisia tabaci

Being a destructive pest worldwide, the whitefly Bemisia tabaci has evolved resistance to neonicotinoid insecticides. The third-generation neonicotinoid dinotefuran has commonly been applied to the control of the whitefly, but its underlying mechanism is currently unknown. On the base of our transcriptome data, here we aim to investigate whether the cytochrome P450 CYP6EM1 underlies dinotefuran resistance in the whitefly. Compared to the susceptible strain, the CYP6EM1 gene was found to be highly expressed in both laboratory and field dinotefuran-resistant populations. Upon exposure to dinotefuran, the mRNA levels of CYP6EM1 were increased. These results demonstrate the involvement of this gene in dinotefuran resistance. Loss and gain of functional studies in vivo were conducted through RNAi and transgenic Drosophila melanogaster assays, confirming the role of CYP6EM1 in conferring such resistance. In a metabolism assay in vitro, the CYP6EM1 protein could metabolize 28.11% of dinotefuran with a possible dinotefuran-dm-NNO metabolite via UPLC-QTOF/MS. Docking of dinotefuran to the CYP6EM1 protein showed a good binding affinity, with an energy of less than -6.0 kcal/mol. Overall, these results provide compelling evidence that CYP6EM1 plays a crucial role in the metabolic resistance of B. tabaci to dinotefuran. Our work provides new insights into the mechanism underlying neonicotinoid resistance and applied knowledge that can contribute to sustainable control of a global pest such as whitefly.

Dynamic monitoring of the insecticide resistance status of Bemisia tabaci across China from 2019-2021

BACKGROUND

Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a major agricultural insect pest that causes severe economic losses worldwide. Several insecticides have been applied to effectively control this key pest. However, owing to the indiscriminate use of chemical insecticides, B. tabaci has developed resistance against these chemical compounds over the past several years.

RESULTS

From 2019 to 2021, 23 field samples of B. tabaci were collected across China. Twenty species were identified as the Mediterranean 'Q' type (MED) and three were identified as MED/ Middle East-Asia Minor 1 mixtures. Subsequently, resistance of the selected populations to different insecticides was evaluated. The results showed that 13 populations developed low levels of resistance to abamectin. An overall upward trend in B. tabaci resistance toward spirotetramat, cyantraniliprole and pyriproxyfen was observed. In addition, resistance to thiamethoxam remained low-to-moderate in the 23 field populations.

CONCLUSION

These findings suggest that the overall resistance of the field-collected B. tabaci populations has shown an upward trend over the years in China. We believe our study can provide basic data to support integrated pest management and insecticide resistance management of field B. tabaci in China. © 2023 Society of Chemical Industry.

Microbiome diversity and composition in Bemisia tabaci SSA1-SG1 whitefly are influenced by their host's life stage

Within the Bemisia tabaci group of cryptic whitefly species, many are damaging agricultural pests and plant-virus vectors, conferring upon this group the status of one of the world's top 100 most invasive and destructive species, affecting farmers' income and threatening their livelihoods. Studies on the microbiome of whitefly life stages are scarce, although their composition and diversity greatly influence whitefly fitness and development. We used high-throughput sequencing to understand microbiome diversity in different developmental stages of the B. tabaci sub-Saharan Africa 1 (SSA1-SG1) species of the whitefly from Uganda. Endosymbionts (Portiera, Arsenophonus, Wolbachia, and Hemipteriphilus were detected but excluded from further statistical analysis as they were not influenced by life stage using Permutational Multivariate Analysis of Variance Using Distance Matrices (ADONIS, p = 0.925 and Bray, p = 0.903). Our results showed significant differences in the meta microbiome composition in different life stages of SSA1-SG1. The diversity was significantly higher in eggs (Shannon, p = 0.024; Simpson, p = 0.047) than that in nymphs and pupae, while the number of microbial species observed by the amplicon sequence variant (ASV) was not significant (n(ASV), p = 0.094). At the phylum and genus levels, the dominant constituents in the microbiome changed significantly during various developmental stages, with Halomonas being present in eggs, whereas Bacillus and Caldalkalibacillus were consistently found across all life stages. These findings provide the first description of differing meta microbiome diversity in the life stage of whiteflies, suggesting their putative role in whitefly development.

A ferritin protein is involved in the development and reproduction of the whitefly, Bemisia tabaci

Ferritins are conserved iron-binding proteins that exist in most living organisms and play an essential role in the maintenance of cellular iron homeostasis. Although ferritin has been studied in many species, little is known about its role in the whitefly, Bemisia tabaci. In this study, we identified an iron-binding protein from B. tabaci and named it BtabFer1. The full-length cDNA of BtabFer1 is 1,043 bp and encodes a protein consisting of 224 amino acids with a deduced molecular weight of 25.26 kDa, and phylogenetic analysis shows that BtabFer1 is conserved among Hemiptera insects. The expression levels of BtabFer1 in different developmental stages and tissues were analyzed by real-time PCR, and results showed that BtabFer1 was ubiquitously expressed at all developmental stages and in all examined tissues. The RNAi-mediated knockdown of BtabFer1 caused a significant reduction in survival rate, egg production, and egg hatching rate of whiteflies. Knockdown of BtabFer1 also inhibited the transcription of genes in the juvenile hormone signal transduction pathway. Taken together, these results suggest that BtabFer1 plays a critical role in the development and reproduction of whiteflies. This study can broaden our understanding of ferritin in insect fecundity and development, as well as provide baseline data for future studies.

CYP6CX2 and CYP6CX3 mediate thiamethoxam resistance in field whitefly, Bemisia tabaci (Hemiptera:Aleyrodidae)

Cytochrome P450 monooxygenases (P450s) are well-known for their crucial roles in the detoxification of xenobiotics. However, whether CYP6CX2 and CYP6CX3, 2 genes from our Bemisia tabaci (B. tabaci) MED/Q genome data were associated with detoxification metabolism and confer resistance to thiamethoxam is unclear. In this study, we investigated the role of CYP6CX2 and CYP6CX3 in mediating whitefly thiamethoxam resistance. Our results showed that mRNA levels of CYP6CX2 and CYP6CX3 were up-regulated after exposure to thiamethoxam. Transcriptional levels of 2 genes were overexpressed in laboratory and field thiamethoxam resistant strains by RT-qPCR. These results indicate that the enhanced expression of CYP6CX2 and CYP6CX3 appears to confer thiamethoxam resistance in B. tabaci. Moreover, linear regression analysis showed that the expression levels of CYP6CX2 and CYP6CX3 were positively correlated with thiamethoxam resistance levels among populations. The susceptibility of whitefly adults was markedly increased after silencing 2 genes by RNA interference (RNAi) which further confirming their major role in thiamethoxam resistance. Our findings provide information to better understand the roles of P450s in resistance to neonicotinoids and suggest that these genes may be applied to develop target genes for sustainable management tactic of agricultural pests such as B. tabaci.

Cytochrome P450 CYP6DB3 was involved in thiamethoxam and imidacloprid resistance in Bemisia tabaci Q (Hemiptera: Aleyrodidae)

High level resistance for a variety of insecticides has emerged in Bemisia tabaci, a globally notorious insect. Neonicotinoid insecticides have been applied widely to control B. tabaci. Whether a differentially expressed gene CYP6DB3 discovered from transcriptome data of B. tabaci is involved in the resistance to neonicotinoid insecticides remains unclear. In the study, CYP6DB3 expression was significantly up-regulated in both thiamethoxam- and imidacloprid-resistant strains relative to the susceptive strains. We also found that CYP6DB3 expression was up-regulated after B. tabaci adults were exposed to thiamethoxam and imidacloprid. Moreover, knocking down CYP6DB3 expression via feeding corresponding dsRNA significantly reduced CYP6DB3 mRNA levels by 34.1%. Silencing CYP6DB3 expression increased the sensitivity of B. tabaci Q adults against both thiamethoxam and imidacloprid. Overexpression of CYP6DB3 gene reduced the toxicity of imidacloprid and thiamethoxam to transgenic D. melanogaster. In addition, metabolic studies showed that CYP6DB3 can metabolize 24.41% imidacloprid in vitro. Collectively, these results strongly support that CYP6DB3 plays an important role in the resistance of B. tabaci Q to imidacloprid and thiamethoxam. This work will facilitate a deeper insight into the part of cytochrome P450s in the evolution of insecticide resistance and provide a theoretical basis for the development of new integrated pest resistance management.

Insecticide Resistance and Its Management in Two Invasive Cryptic Species of Bemisia tabaci in China

The sweet potato whitefly Bemisia tabaci is a major agricultural pest with a wide host range throughout the world. The species designation for B. tabaci includes numerous distinct cryptic species or biotypes. Two invasive B. tabaci biotypes, MEAM1 (B) and MED (Q), were found in China at the end of the 20th century and at the beginning of the 21st century. MEAM1 (B) and MED (Q) show higher pesticide resistance levels than native strains, and the levels of resistance vary with changes in insecticide selection pressure. Recent studies have revealed metabolic resistance mechanisms and target site mutations in invasive B. tabaci strains that render them resistant to a range of insecticides and have uncovered the frequency of these resistance-related mutations in B. tabaci populations in China. Novel pest control agents, such as RNA-based pesticides and nano-pesticides, have achieved effective control effects in the laboratory and are expected to be applied for field control of B. tabaci in the future. In this review, we discuss the mechanisms of resistance developed by these invasive B. tabaci populations since their invasion into China. We also provide suggestions for ecologically sound and efficient B. tabaci control.

CYP6DW3 Metabolizes Imidacloprid to Imidacloprid-urea in Whitefly (Bemisia tabaci)

Bemisia tabaci has developed high resistance to many insecticides and causes substantial agricultural and economic losses annually. The insecticide resistance of whitefly has been widely reported in previous studies; however, the underlying mechanism remains little known. In this study, we cloned two P450 genes: CYP6DW3 and CYP6DW5v1; these genes were markedly overexpressed in imidacloprid-resistant whitefly populations compared with susceptible populations, and knockdown of these genes decreased the imidacloprid resistance of whitefly. Moreover, heterologous expression of whitefly P450 genes in SF9 cells and metabolic studies showed that the CYP6DW3 protein could metabolize 14.11% imidacloprid and produced imidacloprid-urea in vitro. Collectively, the expression levels of CYP6DW3 and CYP6DW5v1 are positively correlated with imidacloprid resistance in B. tabaci. Our study further reveals that cytochrome P450 enzymes affect the physiological activities related to resistance in insects, which helps scholars more deeply understand the resistance mechanism, and contributes to the development of integrated pest management framework.

Expression profile of CYP402C1 and its role in resistance to imidacloprid in the whitefly, Bemisia tabaci

Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a cosmopolitan insect pest causing serious damage to crop production. Cytochromes P450 (CYPs) of B. tabaci are widely known to be involved in the metabolic resistance to a variety of insecticides, continuously increasing the difficulty in controlling this pest. In this study, four P450 genes (CYP6CM1, CYP6CX1, CYP6CX3, and CYP402C1) in B. tabaci exhibited correlations with the resistance to imidacloprid. We have focused on trying to understand the function and metabolism capacity of CYP402C1. The expression profiles of CYP402C1 were examined by reverse transcription quantitative real-time PCR and fluorescence in situ hybridizations. Its role in resistance to imidacloprid was investigated by RNA interference, transgenic Drosophila melanogaster, and heterologous expression. The results showed that CYP402C1 was highly expressed in the active feeding stages of B. tabaci, such as nymphs and female adults. CYP402C1 was mainly expressed in midguts of nymphs and adults, especially in the filter chamber. Knockdown of CYP402C1 significantly decreased the resistance of B. tabaci to imidacloprid by 3.96-fold (50% lethal concentration: 186.46 versus 47.08 mg/L). Overexpression of CYP402C1 in a transgenic D. melanogaster line (Gal4 > UAS-CYP402C1) significantly increased the resistance to imidacloprid from 12.68- to 14.92-fold (129.01 and 151.80 mg/L versus 1925.14 mg/L). The heterologous expression of CYP402C1 showed a metabolism ability of imidacloprid (imidacloprid decreased by 12.51% within 2 h). This study provides new insights for CYP402C1 function in B. tabaci and will help develop new strategies in B. tabaci control and its insecticide resistance.

Resistance to dinotefuran in Bemisia tabaci in China: status and characteristics

BACKGROUND

Bemisia tabaci (Gennadius) is a serious agricultural pest worldwide. Neonicotinoids are the most important new class of synthetic insecticides used in the management of B. tabaci. However, B. tabaci populations have developed resistance to various active ingredients in neonicotinoids following long-term and widespread application.

RESULTS

Dinotefuran exhibited high toxicity against most B. tabaci field populations. One population (Din-R) with a high level of resistance to dinotefuran (255.6-fold) was first identified in the field. The Din-R population exhibited medium to high levels of resistance to all the tested neonicotinoid insecticides and a high level of resistance to spinetoram. Genetic inheritance analysis revealed that resistance to dinotefuran was incompletely recessive and polygenic. The synergist piperonyl butoxide significantly increased the toxicity of dinotefuran to Din-R. P450 activity in the Din-R population was 2.19-fold higher than in the susceptible population. RNA-sequencing analysis showed that 12 P450 genes were significantly upregulated in the Din-R population, of which CYP6DW5, CYP6JM1 and CYP306A1 were found to exhibit more than 3.00-fold higher expression in Din-R when using a reverse transcription quantitative real-time polymerase chain reaction. Expression of eight P450 genes was obviously induced by dinotefuran, and CYP6DW5 showed the highest expression level. After knockdown of CYP6DW5 in Din-R, the toxicity of dinotefuran increased significantly.

CONCLUSION

P450 had a crucial role in dinotefuran resistance in B. tabaci, and CYP6DW5 was involved in the resistance. These results provide important information for the management of resistance in B. tabaci and improve our understanding of the resistance mechanism of dinotefuran. © 2022 Society of Chemical Industry.

Unraveling the Basis of Neonicotinoid Resistance in Whitefly Species Complex: Role of Endosymbiotic Bacteria and Insecticide Resistance Genes

Bemisia tabaci (whitefly) is one of the most detrimental agricultural insect pests and vectors of many plant viruses distributed worldwide. Knowledge of the distribution patterns and insecticide resistance of this cryptic species is crucial for its management. In this study, genetic variation of mitochondrial cytochrome oxidase subunit 1 (MtCoI) gene of B. tabaci was analyzed followed by a study of the infection profile of various endosymbionts in 26 whitefly populations collected from West Bengal, India. Phylogenetic analysis revealed Asia I as the major cryptic species (65.38%), followed by Asia II 5, China 3, and Asia II 7, which were diversified into 20 different haplotypes. In addition to the primary endosymbiont (C. poriera), each of the four whitefly species showed a variable population of three secondary endosymbionts, majorly Arsenophonus with the highest infection rate (73.07%), followed by Wolbachia and Rickettsia. Further phylogenetic analyses revealed the presence of two subgroups of Arsenophonus, viz., A1 and A2, and one each in Wolbachia (W1) and Rickettsia (R3). Resistance to thiamethoxam, imidacloprid, and acetamiprid insecticides was analyzed for a clear picture of pesticide resistance status. The highest susceptibility was noted toward thiamethoxam (LC₅₀ = 5.36 mg/L), followed by imidacloprid and acetamiprid. The whitefly population from Purulia and Hooghly districts bearing Asia II 7 and Asia II 5 cryptic species, respectively, shows maximum resistance. The differences in mean relative titer of four symbiotic bacteria among field populations varied considerably; however, a significant positive linear correlation was observed between the resistance level and relative titer of Arsenophonus and Wolbachia in the case of imidacloprid and thiamethoxam, while only Wolbachia was found in case of acetamiprid. Expression analysis demonstrated differential upregulation of insecticide resistance genes with Purulia and Hooghly populations showing maximally upregulated P450 genes. Moreover, thiamethoxam and imidacloprid resistance ratio (RR) showed a significant correlation with CYP6CM1, CYP6DZ7, and CYP4C64 genes, while acetamiprid RR correlated with CYP6CX1, CYP6DW2, CYP6DZ7, and CYP4C64 genes. Taken together, these findings suggested that P450 mono-oxygenase and symbiotic bacteria together affected whitefly resistance to neonicotinoids. Hence, a symbiont-oriented management programme could be a better alternative to control or delay resistance development in whitefly and can be used for pesticide clean-up in an agricultural field.

Metabolic Resistance in Abamectin-Resistant Bemisia tabaci Mediterranean from Northern China

Abamectin, produced by the soil-dwelling actinomycete Streptomyces avermitilis, belongs to the macrocyclic lactones class of pesticides, has nematocidal, acaricidal, and insecticidal activity, and is highly effective when used against targeted species. Bemisia tabaci, the tobacco whitefly, is a highly destructive insect to agricultural production worldwide, and various insecticide-resistant strains have been identified in China. Here, we monitored levels of resistance to abamectin in twelve field-collected B. tabaci populations from northern China, and confirmed that, compared with the lab reference strain, six field populations exhibited strong abamectin resistance, while the other six exhibited low-to-medium resistance. Among these, the Xinzheng (XZ) population displayed about a 40-fold increased resistance to abamectin, and experienced significant cross-resistance to chlorpyrifos and imidacloprid. The abamectin resistance of XZ was found to be autosomal and incompletely dominant. Metabolic enzyme and synergism tests were conducted, and two metabolic enzymes, glutathione S-transferase and P450 monooxygenase, were found to be conducive to the field-developed abamectin resistance of the XZ population. The above results provide valuable information that can be used in identifying new pest control strategies and delaying the evolution of resistance to abamectin in field populations of whiteflies.

Transcriptional analysis of Bemisia tabaci MEAM1 cryptic species under the selection pressure of neonicotinoids imidacloprid, acetamiprid and thiamethoxam

BACKGROUND

Neonicotinoids are widely applied in the control of the destructive agricultural pest Bemisia tabaci, and resistance against these chemicals has become a common, severe problem in the control of whiteflies. To investigate the molecular mechanism underlying resistance against nenonicotinoids in whiteflies, RNA-seq technology was applied, and the variation in the transcriptomic profiles of susceptible whiteflies and whiteflies selected by imidacloprid, acetamiprid and thiamethoxam treatment was characterized.

RESULTS

A total of 90.86 GB of clean sequence data were obtained from the 4 transcriptomes. Among the 16,069 assembled genes, 584, 110 and 147 genes were upregulated in the imidacloprid-selected strain (IMI), acetamiprid-selected strain (ACE), and thiamethoxam (THI)-selected strain, respectively, relative to the susceptible strain. Detoxification-related genes including P450s, cuticle protein genes, GSTs, UGTs and molecular chaperone HSP70s were overexpressed in the selected resistant strains, especially in the IMI strain. Five genes were downregulated in all three selected resistant strains, including 2 UDP-glucuronosyltransferase 2B18-like genes (LOC 109030370 and LOC 109032577).

CONCLUSIONS

Ten generations of selection with the three neonicotinoids induced different resistance levels and gene expression profiles, mainly involving cuticle protein and P450 genes, in the three selected resistant whitefly strains. The results provide a reference for research on resistance and cross-resistance against neonicotinoids in B. tabaci.

Up-regulation of calmodulin involved in the stress response to cyantraniliprole in the whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae)

Cyantraniliprole is the first diamide insecticide to have cross-spectrum activity against a broad range of insect orders. The insecticide, like other diamides, selectively acts on ryanodine receptor, destroys Ca²⁺ homeostasis, and ultimately causes insect death. Although expression regulations of genes associated with calcium signaling pathways are known to be involved in the response to diamides, little is known regarding the function of calmodulin (CaM), a typical Ca²⁺ sensor central in regulating Ca²⁺ homeostasis, in the stress response of insects to the insecticide. In this study, we cloned and identified the full-length complementary DNA of CaM in the whitefly, Bemisia tabaci (Gennadius), named BtCaM. Quantitative real-time reverse transcription polymerase chain reaction-based analyses showed that the messenger RNA level of BtCaM was rapidly induced from 1.51- to 2.43-fold by cyantraniliprole during 24 h. Knockdown of BtCaM by RNA interference increased the toxicity of cyantraniliprole in whiteflies by 42.85%. In contrast, BtCaM expression in Sf9 cells significantly increased the cells' tolerance to cyantraniliprole as much as 2.91-fold. In addition, the expression of BtCaM in Sf9 cells suppressed the rapid increase of intracellular Ca²⁺ after exposure to cyantraniliprole, and the maximum amplitude in the Sf9-BtCaM cells was only 34.9% of that in control cells (Sf9-PIZ/V5). These results demonstrate that overexpression of BtCaM is involved in the stress response of B. tabaci to cyantraniliprole through regulation of Ca²⁺ concentration. As CaM is one of the most evolutionarily conserved Ca²⁺ sensors in insects, outcomes of this study may provide the first details of a universal insect response to diamide insecticides.

Evaluation of Resistance Development in Bemisia tabaci Genn. (Homoptera: Aleyrodidae) in Cotton against Different Insecticides

Simple Summary

In the tropical and sub-tropical regions of Asia, Africa, and America, the Bemisia tabaci (cotton whitefly) has attained a major pest status of cotton. It produces injury to the plant by feeding, excreting honeydews, and by transmitting viruses on many crops. The heavy application of insecticides for controlling the insect pest is one of the main reasons for the outbreaks of whitefly. Due to several reports of control failure of the whitefly, the present study was conducted to evaluate the resistance development in B. tabaci. Therefore, the field population of B. tabaci was collected, and the resistance development was evaluated against the commonly used insecticides. For evaluating the development of resistance, the B. tabaci was selected with the insecticides under the controlled laboratory conditions. The data of mortality was calculated at each generation, and the overall development of resistance up to five generations was evaluated. Results showed that the field collected population was susceptible to the selected insecticides at G1, indicating their effectiveness. However, a continuous selection for only five generations resulted in a significant increase in the resistance development. The present study provided very valuable information on the resistance development in B. tabaci.

Abstract

Cotton is a major crop of Pakistan, and Bemisia tabaci (Homoptera: Aleyrodidae) is a major pest of cotton. Due to the unwise and indiscriminate use of insecticides, resistance develops more readily in the whitefly. The present study was conducted to evaluate the resistance development in the whitefly against the different insecticides that are still in use. For this purpose, the whitefly population was selected with five concentrations of each insecticide, for five generations. At G1, compared with the laboratory susceptible population, a very low level of resistance was observed against bifenthrin, cypermethrin, acetamiprid, imidacloprid, thiamethoxam, nitenpyram, chlorfenapyr, and buprofezin with a resistance ratio of 3-fold, 2-fold, 1-fold, 4-fold, 3-fold, 3-fold, 3-fold, and 3-fold, respectively. However, the selection for five generations increased the resistance to a very high level against buprofezin (127-fold), and to a high level against imidacloprid (86-fold) compared with the laboratory susceptible population. While, a moderate level of resistance was observed against cypermethrin (34-fold), thiamethoxam (34-fold), nitenpyram (30-fold), chlorfenapyr (29-fold), and acetamiprid (21-fold). On the other hand, the resistance was low against bifenthrin (18-fold) after selection for five generations. A very low level of resistance against the field population of B. tabaci, at G1, showed that these insecticides are still effective, and thus can be used under the field conditions for the management of B. tabaci. However, the proper rotation of insecticides among different groups can help to reduce the development of resistance against insecticides.

Sulfoxaflor - A sulfoximine insecticide: Review and analysis of mode of action, resistance and cross-resistance

The sulfoximines, as exemplified by sulfoxaflor (Isoclast™active), are a relatively new class of nicotinic acetylcholine receptor (nAChR) competitive modulator (Insecticide Resistance Action Committee [IRAC] Group 4C) insecticides that provide control of a wide range of sap-feeding insect pests. The sulfoximine chemistry and sulfoxaflor exhibits distinct interactions with metabolic enzymes and nAChRs compared to other IRAC Group 4 insecticides such as the neonicotinoids (Group 4A). These distinctions translate to notable differences in the frequency and degree of cross-resistance between sulfoxaflor and other insecticides. Most insect strains exhibiting resistance to a variety of insecticides, including neonicotinoids, exhibited little to no cross-resistance to sulfoxaflor. To date, only two laboratory-based studies involving four strains (Koo et al. 2014, Chen et al. 2017) have observed substantial cross-resistance (>100 fold) to sulfoxaflor in neonicotinoid resistant insects. Where higher levels of cross-resistance to sulfoxaflor are observed the magnitude of that resistance is far less than that of the selecting neonicotinoid. Importantly, there is no correlation between presence of resistance to neonicotinoids (i.e., imidacloprid, acetamiprid) and cross-resistance to sulfoxaflor. This phenomenon is consistent with and can be attributed to the unique and differentiated chemical class represented by sulfoxalfor. Recent studies have demonstrated that high levels of resistance (resistance ratio = 124-366) to sulfoxaflor can be selected for in the laboratory which thus far appear to be associated with enhanced metabolism by specific cytochrome P450s, although other resistance mechanisms have not yet been excluded. One hypothesis is that sulfoxaflor selects for and is susceptible to a subset of P450s with different substrate specificity. A range of chemoinformatic, molecular modeling, metabolism and target-site studies have been published. These studies point to distinctions in the chemistry of sulfoxaflor, and its metabolism by enzymes associated with resistance to other insecticides, as well as its interaction with insect nicotinic acetylcholine receptors, further supporting the subgrouping of sulfoxaflor (Group 4C) separate from that of other Group 4 insecticides. Herein is an expansion of an earlier review (Sparks et al. 2013), providing an update that considers prior and current studies focused on the mode of action of sulfoxaflor, along with an analysis of the presently available resistance / cross-resistance studies, and implications and recommendations regarding resistance management.

Evidence of Spread of Bemisia tabaci (Hemiptera: Aleyrodidae) Mediated by Internal Transportation of Ornamental Plants in Brazil

Two Bemisia tabaci (Gennadius) species, Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED), are major pests that are dispersed throughout the world. While MEAM1 was introduced in Brazil in the 1990s, MED was reported recently with limited spread. Here, a survey was performed to examine whether MED whiteflies are widely present in the Federal District region, in central Brazil. Whiteflies were collected in various locations in the Federal District and surroundings between 2018 and 2020, including garden centers and small- and large-scale farms. The species were identified using RFLPand sequencing of the mitochondrial cytochrome c oxidase I subunit gene region. Out of 108 whitefly batches, 63.89% were composed exclusively by MEAM1, followed by 16.67% presenting only MED, and another 7.40% containing unidentified whitefly species (NI). Plant varieties serving as hosts for more than one whitefly species were observed in 12.04% of the samples, either by MEAM1/MED, MEAM1/NI, or MED/NI. This study highlights the still limited presence of MED in the Federal District and surroundings, predominantly in garden centers and in the green belt of Brasília, closer to urban areas. In contrast, only MEAM1 was identified in large-scale cultivated areas.

Effect of Neonicotinoids on Bacterial Symbionts and Insecticide-Resistant Gene in Whitefly, Bemisia tabaci

The silverleaf whitefly, Bemisia tabaci (Gennadius, Hemiptera: Aleyrodidae), is a major threat to field and horticultural crops worldwide. Persistent use of insecticides for the management of this pest is a lingering problem. In the present study, the status of sensitivity of B. tabaci to two neonicotinoids, imidacloprid and thiamethoxam, was evaluated. The expression pattern of two cytochrome P450 (cyp) genes and changes in the relative amount of symbionts in insecticide-treated B. tabaci were also assessed. Quantitative PCR (qPCR) studies indicate that the CYP6CM1 and CYP6CX1 genes were always expressed higher in imidacloprid-treated whitefly, suggesting a correlation between gene expression and the insect's ability to detoxify toxic compounds such as insecticides. In addition, the thiamethoxam-treated population harbored higher Portiera and lower Rickettsia titers, whereas the imidacloprid-treated population harbored more Rickettsia at different time intervals. Interestingly, we also examined that an increase in exposure to both the insecticides resulted in a reduction in the mutualistic partners from their insect host. These differential responses of endosymbionts to insecticide exposure imply the complex interactions among the symbionts inside the host insect. The results also provide a deeper understanding of the molecular mechanism of resistance development that might be useful for formulating effective management strategies to control B. tabaci by manipulating symbionts and detoxifying genes.

Insecticide resistance trait may contribute to genetic cluster change in Bemisia tabaci MED (Hemiptera: Aleyrodidae) as a potential driving force

BACKGROUND

Previously, we reported that the majority of the Bemisia tabaci Mediterranean (MED) populations converged from two dominant genetic clusters (cluster 1 and 2) to one (cluster 2) during 1 year in greenhouse tomatoes in Korea. To find possible mechanisms for this phenomenon, we investigated the concurrent changes in resistance traits of the two clusters for three insecticide classes (organophosphate, pyrethroid, and neonicotinoid).

RESULTS

Since the resistance mutation frequencies in regional samples were either high (i.e. the voltage-sensitive sodium channel L925I/T929V mutations and the F392 acetylcholinesterase 1 mutation) or zero (the nicotinic acetylcholine receptor R81T mutation), no meaningful correlation between the resistance allele frequency and genetic cluster was deduced. However, the actual resistance levels to all three insecticide classes were significantly higher in cluster 2 than in cluster 1, suggesting that cluster 2 has a higher resistance potential. Furthermore, thiamethoxam treatment to the mixed population of clusters 1 and 2 over three generations exhibited a strong tendency of population change from cluster 1 to cluster 2.

CONCLUSION

Our results demonstrated that the insecticide resistance trait is one of the driving forces for rapid genetic cluster change in B. tabaci MED populations. © 2021 Society of Chemical Industry.

Annual analysis of field-evolved insecticide resistance in Bemisia tabaci across China

BACKGROUND

Over recent decades, many efficacious insecticides have been applied for control of Bemisia tabaci, one of the most notorious insect pests worldwide. Field-evolved insecticide resistance in B. tabaci has developed globally, but remains poorly understood in China.

RESULTS

In this study, a total of 30 field samples of the whitefly Bemisia tabaci from eight provinces of China were collected in 2015 to 2018. Twenty-four of the populations were identified as Mediterranean, 'Q' type (MED), three were Middle East-Asia Minor 1, 'B' type (MEAM1), and three were mixtures of MED/ MEAM1. After identifying whether they belong to MED or MEAM1, the selected individuals were used in bioassays assessing insecticide resistance to abamectin, thiamethoxam, spirotetramat, cyantraniliprole, and pyriproxyfen. Our results showed that all populations in the eight regions had little or no resistance to abamectin; abamectin resistance was highest in the Hunan (Changsha) and Hubei (Wuhan) regions and was lowest in the island region of Hainan (Sanya). The resistance of B. tabaci to spirotetramat, cyantraniliprole, and pyriproxyfen increased each year. The resistance to thiamethoxam remained low because of the high LC₅₀ value for the laboratory strain.

CONCLUSION

These findings suggest that a rotation system using efficacious B. tabaci insecticides with differing mode of actions ought to be implemented for sustainable control to reduce the potential of resistance development. This study provides important data to support the integrated pest management and insecticide resistance management of B. tabaci in China. © 2021 Society of Chemical Industry.

Amplicon sequencing detects mutations associated with pyrethroid resistance in Bemisia tabaci (Hemiptera: Aleyrodidae)

BACKGROUND

Bemisia tabaci (Gennadius) is a major damaging agricultural pest that exhibits high resistance to pyrethroid insecticides. L925I (TTA to ATA) and T929V (ACT to GTT) mutations in the para-type voltage-gated sodium channel (VGSC) are associated with resistance of B. tabaci to pyrethroids. Amplicon sequencing is a reliable and highly efficient method to detect the frequency of mutations linked with insecticide resistance.

RESULTS

Similar frequencies of L925I and T929V mutations were obtained by amplicon sequencing and Sanger sequencing (L925I: 0.3548 vs 0.3619; T929V: 0.6140 vs 0.6381) with overlap of 95% confidence interval in the SX population of B. tabaci. In five populations of B. tabaci from China, the maximum and minimum frequencies of the two mutations were found in the LN (L925I: 0.1126; T929V: 0.8834) and JS (L925I: 0.8776; T929V: 0.1166) populations by amplicon sequencing. However, there was no significant difference in frequencies between the L925I and T929V mutations. The sum frequency of L925I and T929V exceeded 0.9688 in all populations. In addition, a combining mutation, L925 + T929V (L925I and T929V located in same allele), was found in five populations by amplicon sequencing even though its highest frequency was only 0.0157.

CONCLUSION

We established an efficient approach for detecting frequency of mutation by amplicon sequencing. The frequencies of L925I and T929V in VGSC associated with pyrethroid resistance were detected in this study, which could provide foundational data for resistance management of B. tabaci. © 2021 Society of Chemical Industry.

Genome-Wide Identification and Analysis of Chitinase-Like Gene Family in Bemisia tabaci (Hemiptera: Aleyrodidae)

Chitinases are of great importance in chitin degradation and remodeling in insects. However, the genome-wide distribution of chitinase-like gene family in Bemsia tabaci, a destructive pest worldwide, is still elusive. With the help of bioinformatics, we annotated 14 genes that encode putative chitinase-like proteins, including ten chitinases (Cht), three imaginal disk growth factors (IDGF), and one endo-β-N-acetylglucosaminidase (ENGase) in the genome of the whitefly, B. tabaci. These genes were phylogenetically grouped into eight clades, among which 13 genes were classified in the glycoside hydrolase family 18 groups and one in the ENGase group. Afterwards, developmental expression analysis suggested that BtCht10, BtCht5, and BtCht7 were highly expressed in nymphal stages and exhibit similar expression patterns, implying their underlying role in nymph ecdysis. Notably, nymphs exhibited a lower rate of survival when challenged by dsRNA targeting these three genes via a nanomaterial-promoted RNAi method. In addition, silencing of BtCht10 significantly resulted in a longer duration of development compared to control nymphs. These results indicate a key role of BtCht10, BtCht5, and BtCht7 in B. tabaci nymph molting. Our research depicts the differences of chitinase-like family genes in structure and function and identified potential targets for RNAi-based whitefly management.

CAP Analysis of the Distribution of the Introduced Bemisia tabaci (Hemiptera: Aleyrodidae) Species Complex in Xinjiang, China and the Southerly Expansion of the Mediterranean Species

Bemisia tabaci (Gennadius) cryptic complex has invaded Xinjiang, China, since 1998. The distribution of Mediterranean (MED) and Middle East-Asia Minor 1 (MEAM1) B. tabaci substrains has been gradually identified due to the development of molecular technology. In this study, the distribution of MED and MEAM1 in Xinjiang was determined by cleaved amplified polymorphic sequence (CAPs). Results showed that MED dominated in northern Xinjiang (84%), whereas MEAM1 was dominant in southern Xinjiang (72%). Five pairs of simple sequence repeat (SSR) primers were used to analyze the genetic diversity of B. tabaci among 36 geographic populations. The genetic diversity of MED and MEAM1was low and varied little among populations in Xinjiang (0.09 ± 0.14 and 0.09 ± 0.13, respectively). Based on ∆K statistic, 13 populations of MEAM1 could be classified into two subgroups at K = 2, whereas the 23 populations of MED could be classified into four subgroups at K = 4. However, Mantel t-test demonstrated no correlation between geographical and genetic distances among B. tabaci complex (R = 0.42, P = 1.00). Neighbor-joining and principal coordinate analysis showed that geographical isolation and interspecific differences were the main causes of the genetic variation. Gene flow predicted that MEAM1 was most likely introduced from Urumqi to the southern Xinjiang. Meanwhile, a large proportion of MED in Kashi region came from Changji and Yining. To block ongoing dispersal, strict detection and flower quarantine regulations need to be enforced.

Fitness cost of imidacloprid resistance in the cotton-staining bug, Dysdercus koenigii

Dysdercus koenigii, a serious cotton-staining insect pest in many countries, has shown high resistance to imidacloprid, a systemic neonicotinoid insecticide used to control sap-sucking pests. With the aim of creating an effective management strategy, the biological traits of susceptible (SS) and imidacloprid-resistant (Imida-RS) D. koenigii, as well as their reciprocal crosses (CR1 and CR2), were investigated here using a life table established on age, stage, and two-sex patterns. Compared with SS D. koenigii, Imida-RS and CR1 strains had lower relative fitness (0.80 and 0.47, respectively) and fecundity (eggs per female); prolonged egg duration and a prolonged adult preoviposition period; shorter nymphal duration, male/female total longevity, and oviposition days, and a shorter total preoviposition period. However, there were no differences among strains in nymphal survival rates and female ratio. The CR2 D. koenigii had similar relative fitness value (1.09), suggested no fitness cost in most of the parameters. Demographic parameters, including net reproductive rate, were lower in the Imida-RS strain than in SS and CR2 D. koenigii. Similarly, the Imida-RS and CR1 strains had shorter generation time and doubling time, lower reproductive value and life expectancy relative to the SS and CR2 D. koenigii. In addition, age-specific fecundity was negatively affected in the CR1 strain compared with the other strains. These findings could help facilitate the development of rational D. koenigii control strategies.

Susceptibility of Four Species of Aphids in Wheat to Seven Insecticides and Its Relationship to Detoxifying Enzymes

Sitobion avenae (Fabricius), Rhopalosiphum padi (Linnaeus), Schizaphis graminum (Rondani), and Metopolophium dirhodum (Walker) (Hemiptera: Aphididae) are important pests of wheat and other cereals worldwide. In this study, the susceptibilities of four wheat aphid species to seven insecticides were assessed. Furthermore, the activities of carboxylesterase (CarE), glutathione S-transferase (GSTs), and cytochrome P450 monooxygenase (P450s) were determined in imidacloprid treated and untreated aphids. The results showed that the susceptibilities of four wheat aphid species to tested insecticides are different and M. dirhodum has shown higher tolerance to most insecticides. Relatively higher CarE and GST activities were observed in M. dirhodum, and P450s activities increased significantly in response to imidacloprid treatment. Moreover, susceptibility to imidacloprid were increased by the oxidase inhibitor piperonyl butoxide in M. dirhodum (20-fold). The results we have obtained imply that P450s may play an important role in imidacloprid metabolic process in M. dirhodum. We suggest that a highly species-specific approach is essential for managing M. dirhodum.

Low Genetic Variability in Bemisia tabaci MEAM1 Populations within Farmscapes of Georgia, USA

Simple Summary

Sweetpotato whitefly, Bemisia tabaci Gennadius, is a serious pest of many agricultural crops worldwide. Numerous studies have examined the genetic structure of whitefly populations separated by geographical barriers; however, very few have assessed the population structure of B. tabaci at a farmscape level. A farmscape in this study is defined as heterogenous habitat with crop and non-crop areas spanning approximately 8 square kilometers. To assess the roles of farmscapes as drivers of B. tabaci genetic variation, thirty-five populations of the sweetpotato whitefly were collected from crop and non-crop plant species from fifteen farmscapes. Using mitochondrial COI gene sequences (mtCOI) and six nuclear microsatellite markers, the genetic diversity and genetic differentiation among collected B. tabaci MEAM1 populations were examined. Haplotype analysis using mtCOI sequences revealed the presence of a single B. tabaci MEAM1 haplotype across farmscapes of Georgia. Results from microsatellite markers further showed no significant genetic structuring among populations that corresponded to plant species or farmscapes from which they were collected. Annual whitefly population explosions and subsequent dispersal might have facilitated the persistence of a single panmictic B. tabaci population over all sampled farmscapes in this region.

Abstract

Bemisia tabaci is a whitefly species complex comprising important phloem feeding insect pests and plant virus vectors of many agricultural crops. Middle East–Asia Minor 1 (MEAM1) and Mediterranean (MED) are the two most invasive members of the B. tabaci species complex worldwide. The diversity of agroecosystems invaded by B. tabaci could potentially influence their population structure, but this has not been assessed at a farmscape level. A farmscape in this study is defined as heterogenous habitat with crop and non-crop areas spanning ~8 square kilometers. In this study, mitochondrial COI gene (mtCOI) sequences and six microsatellite markers were used to examine the population structure of B. tabaci MEAM1 colonizing different plant species at a farmscape level in Georgia, United States. Thirty-five populations of adult whiteflies on row and vegetable crops and weeds across major agricultural regions of Georgia were collected from fifteen farmscapes. Based on morphological features and mtCOI sequences, five species/cryptic species of whiteflies (B. tabaci MEAM1, B. tabaci MED, Dialeurodes citri, Trialeurodes abutiloneus, T. vaporariorum) were found. Analysis of 102 mtCOI sequences revealed the presence of a single B. tabaci MEAM1 haplotype across farmscapes in Georgia. Population genetics analyses (AMOVA, PCA and STRUCTURE) of B. tabaci MEAM1 (microsatellite data) revealed only minimal genetic differences among collected populations within and among farmscapes. Overall, our results suggest that there is a high level of gene flow among B. tabaci MEAM1 populations among farmscapes in Georgia. Frequent whitefly population explosions driven by a single or a few major whitefly-suitable hosts planted on a wide spatial scale may be the key factor behind the persistence of a single panmictic population over Georgia’s farmscapes. These population structuring effects are useful for delineating the spatial scale at which whiteflies must be managed and predicting the speed at which alleles associated with insecticide resistance might spread.

Transcriptome analysis and comparison reveal divergence between the Mediterranean and the greenhouse whiteflies

Both the Mediterranean (MED) species of the Bemisia tabaci whitefly complex and the greenhouse whitefly (Trialeurodes vaporariorum, TV) are important agricultural pests. The two species of whiteflies differ in many aspects such as morphology, geographical distribution, host plant range, plant virus transmission, and resistance to insecticides. However, the molecular basis underlying their differences remains largely unknown. In this study, we analyzed the genetic divergences between the transcriptomes of MED and TV. In total, 2,944 pairs of orthologous genes were identified. The average identity of amino acid sequences between the two species is 93.6%. The average nonsynonymous (Ka) and synonymous (Ks) substitution rates and the ratio of Ka/Ks of the orthologous genes are 0.0389, 2.23 and 0.0204, respectively. The low average Ka/Ks ratio indicates that orthologous genes tend to be under strong purified selection. The most divergent gene classes are related to the metabolisms of xenobiotics, cofactors, vitamins and amino acids, and this divergence may underlie the different biological characteristics between the two species of whiteflies. Genes of differential expression between the two species are enriched in carbohydrate metabolism and regulation of autophagy. These findings provide molecular clues to uncover the biological and molecular differences between the two species of whiteflies.

A Chemosensory Protein BtabCSP11 Mediates Reproduction in Bemisia tabaci

The olfactory system serves a vital role in the evolution and survival of insects, being involved in behaviors such as host seeking, foraging, mating, and oviposition. Odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) are involved in the olfactory recognition process. In this study, BtabCSP11, a CSP11 gene from the whitefly Bemisia tabaci, was cloned and characterized. The open reading frame of BtabCSP11 encodes 136 amino acids, with four highly conserved cysteine residues. The temporal and spatial expression profiles showed that BtabCSP11 was highly expressed in the abdomens of B. tabaci females. Dietary RNA interference (RNAi)-based functional analysis showed substantially reduced fecundity in parthenogenetically reproduced females, suggesting a potential role of BtabCSP11 in B. tabaci reproduction. These combined results expand the function of CSPs beyond chemosensation.

Bemisia tabaci (Hemiptera: Aleyrodidae) Insecticide Resistance in Shandong Province, China

The Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) complex comprises important pests and virus vectors in agricultural crops worldwide. In China, B. tabaci has spread to more than 20 provinces and caused severe losses of vegetables, fruits, and ornamental plants. However, B. tabaci has developed resistance to many insecticidal classes in Shandong Province, eastern China. In this study, we investigated the cryptic species, insecticide resistance and detoxifying enzymes of B. tabaci from six representative locations exhibiting severe damage in Shandong. At four of the six locations, B. tabaci Mediterranean (MED) comprised 100% of the samples collected. In a further two locations, species composition was predominantly (>94%) MED with B. tabaci Middle East-Asia Minor 1 (MEAM1), comprising a low proportion (<6%) of the samples collected. For all field populations, avermectin was the most effective insecticide against adult B. tabaci, pyriproxyfen had a significant effect on B. tabaci eggs and field populations were susceptible to pymetrozine. Six field populations of B. tabaci have developed low-to-moderate resistance to neonicotinoids. The detoxifying enzyme activity of carboxylesterase, glutathione S-transferase, and multifunctional oxidase were quantified. Multifunctional oxidase and glutathione S-transferase activity were positively correlated with insecticide resistance in several B. tabaci populations.

Monitoring insecticide resistance and diagnostics of resistance mechanisms in Bemisia tabaci Mediterranean (Q biotype) in China

Bemisia tabaci is one of notorious agricultural insect pests in China, and the strategies of management largely depend on application of insecticides. In order to assess levels of resistance in field populations of B. tabaci to six insecticides including abamectin, cyantraniliprole, pymetrozine, imidacloprid, chlorpyrifos and bifenthrin, we monitored the susceptibility to all tested insecticides in five field populations across China and the results indicated that field populations of B. tabaci have developed various levels of resistance to each chemical agent. Furthermore, para-type voltage gated sodium channel mutations (L925I and T929V) and acetylcholinesterase ace1 mutation (F331W) were confirmed, and expression levels of CYP6CM1, CYP4C64, GSTd7 and ABCG3 were detected for investigating mechanisms of imidacloprid resistance in the five field-collected populations. The results showed that, in all tested populations, frequencies of F331W were 100%, and the frequencies of the L925I and T929V were in the range of 28.5 to 47.0% and 11.0 to 53.5%, respectively. Moreover, CYP6CM1 and CYP4C64 were significantly overexpressed in two tested populations, respectively, and GSTd7 was significantly overexpressed in one population. No overexpression of ABCG3 was observed in all the populations. Above results provided valuable insight into the current status of insecticide resistance and could be contributed to design strategies of management for B. tabaci.

Role of several cytochrome P450s in the resistance and cross-resistance against imidacloprid and acetamiprid of Bemisia tabaci (Hemiptera: Aleyrodidae) MEAM1 cryptic species in Xinjiang, China

Neonicotinoids are commonly used for the control of the whitefly Bemisia tabaci in cotton field. Laboratory test and field experiments have found that whitefly has a high risk of developing resistance and cross-resistance to the pesticide. Over expression of cytochrome P450 is one of the main mechanism that controls pesticide resistance in many insects. In this study we use MEAM1 whitefly, the dominant cryptic species of B. tabaci in Xinjiang cotton field, to investigate the possible resistance and cross-resistance mechanism controlled by cytochrome P450 enzymes. The P450 enzyme activity was higher in both selected strains of imidacloprid and acetamipird than that of susceptible strain. Synergism test showed that piperonyl butoxide (PBO) distinctly increased the control efficiency of imidacloprid and acetamiprid to the two resistance selected strains. Four out of 13 cytochrome genes, CYP4CS3, CYP6CX5, CYP6DW2 and CYP6CM1 were significantly up-regulated in the two selected strains based on real-time fluorescence quantitative PCR results. Other 3 genes, CYP6CX2, CYP6CX4 and CYP6DW3 were only highly expressed in the acetamiprid selected strain instead of the susceptible strain and imidacloprid selected strain. CYP6CM1 showed the highest expression level among all the 13 tested genes. No functional mutation of CYP6CM1 was found by sequence analysis. The possible role of these genes involving the resistance and cross-resistance of the whitefly MEAM1 cryptic species against neonicotinoids was discussed.

Genetic Diversity of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) Colonizing Sweet Potato and Cassava in South Sudan

Bemisia tabaci (Gennadius) is a polyphagous, highly destructive pest that is capable of vectoring viruses in most agricultural crops. Currently, information regarding the distribution and genetic diversity of B. tabaci in South Sudan is not available. The objectives of this study were to investigate the genetic variability of B. tabaci infesting sweet potato and cassava in South Sudan. Field surveys were conducted between August 2017 and July and August 2018 in 10 locations in Juba County, Central Equatoria State, South Sudan. The sequences of mitochondrial DNA cytochrome oxidase I (mtCOI) were used to determine the phylogenetic relationships between sampled B. tabaci. Six distinct genetic groups of B. tabaci were identified, including three non-cassava haplotypes (Mediterranean (MED), Indian Ocean (IO), and Uganda) and three cassava haplotypes (Sub-Saharan Africa 1 sub-group 1 (SSA1-SG1), SSA1-SG3, and SSA2). MED predominated on sweet potato and SSA2 on cassava in all of the sampled locations. The Uganda haplotype was also widespread, occurring in five of the sampled locations. This study provides important information on the diversity of B. tabaci species in South Sudan. A comprehensive assessment of the genetic diversity, geographical distribution, population dynamics, and host range of B. tabaci species in South Sudan is vital for its effective management.

Competitive Displacement between Bemisia tabaci MEAM1 and MED and Evidence for Multiple Invasions of MED

Despite the severe ecological damage and economic loss caused by invasive species, the factors contributing to successful invasion or displacement remain elusive. The whitefly, Bemisia tabaci (Gennadius), is an important invasive agricultural pest worldwide, causing severe damage to numerous crops by feeding or transmitting plant viruses. In this study, we monitored the dynamics of two invasive whitefly cryptic species, Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED), in Jiangsu, China, from 2005-2016. We found that B. tabaci MED quickly established and asserted dominance over MEAM1, resulting in their population displacement in Jiangsu in only three years (from 2005 to 2008). We further investigated the possible mechanisms underlying the successful invasion and competitive displacement from a genetic perspective. Based on sequencing of mitochondrial gene sequences from large numbers of whitefly samples, multiple invasion events of MED were validated by our genetic analyses. MED invaded Jiangsu starting from multiple introduction sites with secondary and/or subsequent invasive events. This may favor their invasion and displacement of MEAM1. This study advances our understanding of the mechanisms that enabled the successful invasion of MED.

Cloning and Functional Analysis of Two Ca2+-Binding Proteins (CaBPs) in Response to Cyantraniliprole Exposure in Bemisia tabaci (Hemiptera: Aleyrodidae)

Ca²⁺-binding proteins (CaBPs) are widely distributed as Ca²⁺ sensor relay proteins that regulate various cellular processes, including Ca²⁺ homeostasis. Diamide insecticides such as cyantraniliprole kill insects by disrupting the Ca²⁺ homeostasis in muscle cells. However, less attention has been paid to the roles of CaBPs in response to insecticides. In this study, two CaBP genes (BtCaBP1 and BtCaBP2) were identified in the whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), and their functions in response to cyantraniliprole were investigated. After expression of BtCaBP1 and BtCaBP2 in vitro, the results of Ca²⁺ imaging and cytotoxicity assay revealed that the overexpression of each of the BtCaBPs stabilized Ca²⁺ concentration in the cytoplasm after exposure to cyantraniliprole and decreased the toxicity of cyantraniliprole against Sf9 cells. However, the knockdown of BtCaBP1 or BtCaBP2 in vivo significantly increased the toxicity of cyantraniliprole to B. tabaci. Taken together, these results provide evidence that BtCaBP1 and BtCaBP2 play a role in response to cyantraniliprole exposure through stabilization of Ca²⁺ concentration in whiteflies.

Genome-Wide Analysis of Carboxylesterases (COEs) in the Whitefly, Bemisia tabaci (Gennadius)

The whitefly (Bemisia tabaci), an important invasive pest that causes severe damage to crops worldwide, has developed resistance to a variety of insecticides. Carboxylesterases (COEs) are important multifunctional enzymes involved in the growth, development, and xenobiotic metabolism of insects. However, systematic studies on the COEs of B. tabaci are scarce. Here, 42 putative COEs in different functional categories were identified in the Mediterranean species of B. tabaci (B. tabaci MED) based on a genome database and neighbor-joining phylogeny. The expression patterns of the COEs were affected by the development of B. tabaci. The expression levels of six COEs were positively correlated with the concentration of imidacloprid to which B. tabaci adults were exposed. The mortality of B. tabaci MED adults fed dsBTbe5 (67.5%) and dsBTjhe2 (58.4%) was significantly higher than the adults fed dsEGFP (41.1%) when treated with imidacloprid. Our results provide a basis for functional research on COEs in B. tabaci and provide new insight into the imidacloprid resistance of B. tabaci.

Discrimination of Oviposition Deterrent Volatile β-Ionone by Odorant-Binding Proteins 1 and 4 in the Whitefly Bemisia tabaci

: The whitefly, Bemisia tabaci, is an important invasive economic pest of agricultural crops worldwide. β-ionone has a significant oviposition repellent effect against B. tabaci, but the olfactory molecular mechanism of this insect for recognizing β-ionone is unclear. To clarify the binding properties of odorant-binding proteins (OBPs) with β-ionone, we performed gene cloning, evolution analysis, bacterial expression, fluorescence competitive binding assay, and molecular docking to study the binding function of OBP1 and OBP4 on β-ionone. The results showed that after the OBP1 and OBP4 proteins were recombined, the compound β-ionone exhibited a reduction in the fluorescence binding affinity to <50%, with a dissociation constant of 5.15 and 3.62 μM for OBP1 and OBP4, respectively. Our data indicate that β-ionone has high affinity for OBP1 and OBP4, which play a crucial role in the identification of oviposition sites in B. tabaci. The findings of this study suggest that whiteflies employ β-ionone compound in the selection of the suitable egg-laying sites on host plants during the oviposition behavior.

Differential metabolism of imidacloprid and dinotefuran by Bemisia tabaci CYP6CM1 variants

Imidacloprid has been used to control one of most serious pests, Bemisia tabaci. However, B. tabaci has developed imidacloprid resistance mainly by over-expressing CYP6CM1. It was reported that imidacloprid-resistant B. tabaci showed no or low level of cross-resistance against dinotefuran. Here, we expressed CYP6CM1 variants using Sf9/baculovirus and/or Drosophila S2 cells and showed that CYP6CM1 variants metabolized imidacloprid but not dinotefuran. In addition, we demonstrated that imidacloprid and pymetrozine competed for a CYP6CM1 variant more efficiently than dinotefuran, using a luminescent substrate competition assay. These results suggest that lack of metabolic activity of CYP6CM1 variants against dinotefuran caused no or low level of cross-resistance.

The repellency and toxicity effects of essential oils from the Libyan plants Salvadora persica and Rosmarinus officinalis against nymphs of Ixodes ricinus

Essential oils extracted from the leaves of Libyan Rosemary (Rosmarinus officinalis L.), and Miswak (Salvadora persica L.) were evaluated for their acaricidal and repellent effects on Ixodes ricinus L. nymphs (Acari: Ixodidae) using a bioassay based on an 'open filter paper method'. Rosmarinus officinalis leaf essential oil diluted to 0.5 and 1 µl/cm² in acetone exhibited, respectively, 20 and 100% tick mortality after about 5 h of exposure. A total of 50 and 95% of I. ricinus nymphs were killed by direct contact with the oil when exposed to lethal concentrations (LC) of 0.7 µl/cm² (LC₅₀) and 0.95 µl/cm² (LC₉₅), respectively. The LC₅₀ (0.5 µl/cm²) was reached before the end of the first 24 h of exposure time (ET), as tick mortality at 24 h was 60%. Salvadora persica leaf essential oil at 1 µl/cm² showed a significant repellency effect against I. ricinus nymphs at 1.5 h ET. A 95% repellency was observed at a repellent concentration (RC₉₅) of 1 µl/cm² of S. persica, but no significant mortality was recorded at this dose of S. persica oil. Gas chromatography-mass spectrometry analyses showed that the main monoterpenes in both oils were 1,8-cineol, α-pinene, and β-pinene, although in markedly different proportions. These results suggest that essential oils have substantial potential as alternative approaches for I. ricinus tick control.

Monitoring, Cross-Resistance, Inheritance, and Synergism of Plutella xylostella (Lepidoptera: Plutellidae) Resistance to Pyridalyl in China

Pyridalyl is an insecticide that shows significant efficacy against Plutella xylostella, a notorious pest insect worldwide. In this study, we monitored resistance of P. xylostella to pyridalyl in China from 2016 to 2017, determined cross-resistance, inheritance, and synergism of pyridalyl resistance in two pyridalyl-resistant populations, one field-evolved resistant population (ZL-PR) and one laboratory-selected resistant population (XY-PR). We found that variation in susceptibility among 15 field populations in China from 2016 to 2017 was high, with mean LC50 values ranging from 1.839 to 1,652 mg/liter. The laboratory-selected XY-PR strain showed 31.3-fold resistance to pyridalyl and moderate cross-resistance to fipronil. The ZL-PR displayed 1,050.2-fold resistance to pyridalyl and high resistance to all tested insecticides. Genetic analysis illustrated that pyridalyl resistance in ZL-PR was autosomally inherited and incompletely recessive. However, pyridalyl resistance in the XY-PR strain was autosomally inherited but incompletely dominant. Moreover, piperonyl butoxide significantly inhibited pyridalyl resistance in the XY-PR strain. In conclusion, P. xylostella field populations from South China have high levels of resistance to pyridalyl and different modes of inheritance of resistance were found in XY-PR and ZL-PR. Moreover, enhanced oxidative metabolism is possibly involved in resistance of the XY-PR strain but not in the ZL-PR strain.

Susceptibility of cotton whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) to diverse pesticides in Pakistan

The cotton whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a serious pest of several summer crops in hot and dry climates. Its field populations (Asia II-1 biotype) were assessed for their susceptibility to diverse pesticides by using leaf-dip bioassay. There was no or a very low resistance to amitraz, hexythiazox, and pyridaben during 1992-2015. B. tabaci also exhibited no resistance to endosulfan during 1992-1997 and a very low resistance during 1998-2010, which then rose to a low level during 2011-2015. Chlorfenapyr resistance was very low during 1997-2008 and it reached to a high level during 2009-2011 and to a very high level in 2013 and 2015. Among avermectins, abamectin showed a very low resistance up to 2013 but a high resistance in 2015. Emamectin benzoate also demonstrated a very low resistance up to 2010, but a moderate-to-high resistance during 2011-2015. It may be concluded that the diverse chemistries, having novel modes of action and showing no, very low or low levels of resistance, can be substituted in rotation in the wake of resistance development to conventional insecticides.

Toxicity of seven insecticides to different developmental stages of the whitefly Bemisia tabaci MED (Hemiptera: Aleyrodidae) in multiple field populations of China

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 LC₉₅ 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.

Expression profile changes of cytochrome P450 genes between thiamethoxam susceptible and resistant strains of Aphis gossypii Glover

Cytochrome P450 monooxygenases represent a key detoxification mechanism in neonicotinoids resistance in Aphis gossypii Glover. Synergism analysis has indicates that P450s are involved in thiamethoxam resistance. In this study, expression changes in the transcripts of P450 genes were determined in thiamethoxam-susceptible and thiamethoxam-resistant strains. Nine P450 genes in CYP3 clade were significantly overexpressed in the resistant strain (especially CYP6CY14, which was increased 17.67-fold) compared with the susceptible strain. Transcripts of ecdysone synthesis-related P450 genes, including CYP302A1, CYP306A1, CYP307A1 and CYP315A1, were up-regulated in the resistant strain, which may accelerate molting hormone production. The ecdysone response genes (ecdysone receptor (EcR), ultra-spiracle (USP) and Broad-complex protein (Br-C)) were overexpressed in the resistant strain. RNA interference (RNAi) targeting CYP6CY14 significantly increased the sensitivity of the resistant aphid to thiamethoxam. The results of the present study indicate the possible involvement of these P450 genes in thiamethoxam resistance. Our findings may facilitate further work to validate the roles of these P450s in thiamethoxam resistance based on heterologous expression, and show that screening the expression changes in P450 genes can reveal the impact of thiamethoxam on ecdysone synthesis-related P450 genes. These results are useful for understanding the mechanism of thiamethoxam resistance and will contribute to the management of insecticide-resistant cotton aphids in China.

Acaricide-Mediated Competition Between the Sibling Species Tetranychus cinnabarinus and Tetranychus urticae

The carmine spider mite (Tetranychus cinnabarinus [Acarifonnes: Tetranychidae]) and the two-spotted spider mite (Tetranychus urticae [Acarifonnes: Tetranychidae]) are two notorious pests of agricultural crops. Control of these pests has been dependent upon using different kinds of acaricides. The purpose of this study was to determine the differential responses of these two pest species collected from crops in the same field to acaricide treatments. Field trials have shown that without spraying acaricides, T. cinnabarinus will displace T. urticae. However, the application of abamectin has the potential to change the composition of spider mite complexes and facilitate the interspecific competition of T. urticae against T. cinnabarinus when both are fed on cowpeas and eggplants. Moreover, T. urticae is more prone to develop resistance than T. cinnabarinus when selected in the laboratory using cyflumetofen or fenpropathrin. After 20 generations of acaricide selection, the activities of detoxifying enzymes were considerably higher in T. urticae with more detoxifying enzymes upregulated after selection in this species. The results of this study demonstrate that differential responses to acaricide treatments have made it possible for T. urticae to overcome the competitive advantage present in T. cinnabarinus during the absence of acaricide application.

RNA interference of glutamate-gated chloride channel decreases abamectin susceptibility in Bemisia tabaci

The Bemisia tabaci (Gennadius) cryptic species complex comprises very destructive insect pests of agricultural crops worldwide and has been found to be resistant to various insecticides in China. Abamectin is one of the most widely used insecticides for insect pest control and the glutamate-gated chloride channel (GluCl) in insects was presumed to be the main target site of abamectin. In this study, a 1353bp full-length cDNA encoding GluCl (named BtGluCl, GenBank ID: MF673854) was cloned and characterized from B. tabaci. BtGluCl encodes 450 amino acids, which shares 71-81% identity with other insect GluCl isoforms. Spatial and temporal expression revealed BtGluCl was highly expressed in the 4th nymphal instar and adult head, and the least expressed in the 1st nymphal instar and adult leg. Dietary ingestion of dsBtGluCl significantly reduced the mRNA level of BtGluCl in the treated adults by 62.9% and greatly decreased abamectin-induced mortality. Thus, our results could be conducive to further understanding the mechanisms of resistance to abamectin in arthropods.

The invasive MED/Q Bemisia tabaci genome: a tale of gene loss and gene gain

BACKGROUND

Sweetpotato whitefly, Bemisia tabaci MED/Q and MEAM1/B, are two economically important invasive species that cause considerable damages to agriculture crops through direct feeding and indirect vectoring of plant pathogens. Recently, a draft genome of B. tabaci MED/Q has been assembled. In this study, we focus on the genomic comparison between MED/Q and MEAM1/B, with a special interest in MED/Q's genomic signatures that may contribute to the highly invasive nature of this emerging insect pest.

RESULTS

The genomes of both species share similarity in syntenic blocks, but have significant divergence in the gene coding sequence. Expansion of cytochrome P450 monooxygenases and UDP glycosyltransferases in MED/Q and MEAM1/B genome is functionally validated for mediating insecticide resistance in MED/Q using in vivo RNAi. The amino acid biosynthesis pathways in MED/Q genome are partitioned among the host and endosymbiont genomes in a manner distinct from other hemipterans. Evidence of horizontal gene transfer to the host genome may explain their obligate relationship. Putative loss-of-function in the immune deficiency-signaling pathway due to the gene loss is a shared ancestral trait among hemipteran insects.

CONCLUSIONS

The expansion of detoxification genes families, such as P450s, may contribute to the development of insecticide resistance traits and a broad host range in MED/Q and MEAM1/B, and facilitate species' invasions into intensively managed cropping systems. Numerical and compositional changes in multiple gene families (gene loss and gene gain) in the MED/Q genome sets a foundation for future hypothesis testing that will advance our understanding of adaptation, viral transmission, symbiosis, and plant-insect-pathogen tritrophic interactions.

Silence of inositol 1,4,5-trisphosphate receptor expression decreases cyantraniliprole susceptibility in Bemisia tabaci

Cyantraniliprole is the second active ingredient of anthranilic diamide insecticide, and the first to control a cross-spectrum of chewing and sucking pests such as sweetpotato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). The inositol 1,4,5-trisphosphate receptor (IP3R) and ryanodine receptor (RyR) are two families of Ca²⁺ release channels to raise the cytoplasmic free calcium concentration when it is activated by various extracellular stimuli. Previous study proved the over-expression of ryanodine receptor (RyR) was associated with the resistance to diamide insecticides, while the roles of IP3R in diamide resistance remain unknown. In this study, a full-length cDNA sequence of IP3R was cloned from B. tabaci through RT-PCR and rapid amplification of cDNA ends (RACE). The gene (named BtIP3R) is 9922bps long, with an open reading frame (ORF) of 8202bps, encoding a predicted IP3R of 2733 amino acids. The BtIP3R shares 47-78% identity with other insect IP3Rs. Quantitative real-time PCR (qRT-PCR) analysis showed that the BtIP3R was highly expressed in larva, pseudopupa, and female adult, while lowly expressed in egg and male adult. RNA interference (RNAi) by dietary introduction of double-stranded RNA (dsRNA) of BtIP3R significantly reduced the mRNA levels of the target gene in the adult, and dramatically decreased the susceptibility of adult B. tabaci to cyantraniliprole. The results shed light on further understanding of cyantraniliprole resistance mechanisms in B. tabaci as well as in other insects.

RNAi-mediated resistance to whitefly (Bemisia tabaci) in genetically engineered lettuce (Lactuca sativa)

RNA interference (RNAi)-based transgenic technologies have evolved as potent biochemical tools for silencing specific genes of plant pathogens and pests. The approach has been demonstrated to be useful in silencing genes in insect species. Here, we report on the successful construction of RNAi-based plasmid containing an interfering cassette designed to generate dsRNAs that target a novel v-ATPase transcript in whitefly (Bemisia tabaci), an important agricultural pest in tropical and sub-tropical regions. The presence of the transgene was confirmed in T0 and T1 generations of transgenic lettuce lines, segregating in a Mendelian fashion. Seven lines were infested with whiteflies and monitored over a period of 32 days. Analysis of mortality showed that within five days of feeding, insects on transgenic plants showed a mortality rate of 83.8-98.1%. In addition, a reduced number of eggs (95 fold less) was observed in flies feeding on transgenic lettuce plants than insects on control lines. Quantitative reverse transcription PCR showed decreased expression level of endogenous v-ATPase gene in whiteflies feeding on transgenic plants. This technology is a foundation for the production of whitefly-resistant commercial crops, improving agricultural sustainability and food security, reducing the use of more environmentally aggressive methods of pest control.