DNA markers for identifying biotypes B and Q of Bemisia tabaci (Hemiptera: Aleyrodidae) and studying population dynamics

Natural habitat connectivity and organic management modulate pest dispersal, gene flow, and natural enemy communities

The simplification and fragmentation of agricultural landscapes generate effects on insects at multiple spatial scales. As each functional group perceives and uses the habitat differently, the response of pest insects and their associated natural enemies to environmental changes varies. Therefore, landscape structure may have consequences on gene flow among pest populations in space. This study aimed to evaluate the effects of local and landscape factors, at multiple scales, on the local infestation, gene flow and broad dispersion dynamics of the pest insect Bemisia tabaci (Genn.) Middle East-Asia Minor 1 (MEAM-1, former biotype B) (Hemiptera: Aleyrodidae) and its associated natural enemies in a tropical agroecosystem. We evaluated the abundance of B. tabaci populations and their natural enemy community in 20 tomato farms in Brazil and the gene flow between farms from 2019 to 2021. Landscapes dominated by agriculture resulted in larger B. tabaci populations and higher gene flow, especially in conventional farms. A higher density of native vegetation patches disfavored pest populations, regardless of the management system. The results revealed that whitefly responds to intermediate spatial scales and that landscape factors interact with management systems to modulate whitefly populations on focal farms. Conversely, whitefly natural enemies benefited from higher amounts of natural vegetation at small spatial scales, while the connectivity between natural habitat patches was beneficial for natural enemies regardless of the distance from the focal farm. The resulting dispersion model predicts that the movement of whiteflies between farms increases as the amount of natural vegetation decreases. Our findings demonstrate that landscape features, notably landscape configuration, can mediate infestation episodes, as they affect pest insects and natural enemies in opposite ways. We also showed that landscape features interact with farm traits, which highlights the need for management strategies at multiple spatial scales. In conclusion, we demonstrated the importance of the conservation of natural areas as a key strategy for area-wide ecological pest management and the relevance of organic farming to benefit natural enemy communities in tropical agroecosystems.

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.

A plant virus enhances odorant-binding protein 5 (OBP5) in the vector whitefly for more actively olfactory orientation to the host plant

BACKGROUND

The whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is a notorious agricultural pest and the effective vector of many plant viruses worldwide. Cucurbit chlorotic yellows virus (CCYV), exclusively transmitted by B. tabaci in a semipersistent manner, is a serious causal agent in cucurbit crops in many countries. Plant viruses can manipulate the behaviors of insect vectors to promote the spread of themselves, but underlying mechanisms are remaining unclear.

RESULTS

In this study, our observations indicated that B. tabaci, when carrying CCYV, oriented more actively to the host plant cucumber. Transcriptome analysis and quantitative polymerase chain reaction with reverse transcription analysis showed that the odorant-binding protein 5 (OBP5) was upregulated with viral acquisition. Sequence and phylogenetic analysis showed that BtabOBP5 was highly homologous with nine OBPs from other hemipteran insects. In addition, OBP5-silenced whiteflies significantly altered their orientation behavior towards cucumber plants and towards some typical volatile organic compounds released from cucumbers.

CONCLUSION

This study described a novel mechanism by which the olfactory system of vector insects could be regulated by a semipersistent plant virus, thereby affecting insect olfactory behavior and relationship with host plants. These results provided a basis for developing potential olfaction-based pest management strategies in the future. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Sulfoxaflor Alters Bemisia tabaci MED (Hemiptera: Aleyrodidae) Preference, Feeding, and TYLCV Transmission

Many damaging agricultural pests can, in addition to their direct feeding damage, acquire and transmit plant pathogens. Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) is considered a 'supervector' of disease-causing plant pathogens and viruses. One of the most damaging of these is Tomato yellow leaf curl virus (TYLCV), a circulatively transmitted begomovirus than can extensively damage field and greenhouse crops. Because sustained feeding periods are necessary to acquire and transmit circulatively transmitted viruses, pesticides that, in addition to their direct lethality, suppress feeding in surviving individuals may be particularly effective in decreasing viral transmission. We assessed the impact of sulfoxaflor, a sulfoximine insecticide, on the settling preference, feeding, and viral transmission of TYLCV-carrying B. tabaci on tomato. We found that viruliferous B. tabaci avoided both settling and feeding on sulfoxaflor-treated plants, and that sulfoxaflor virtually eliminated the transmission of TYLCV by B. tabaci. The antifeedant properties of sulfoxaflor have previously been reported in other pest systems; our results document similar effects on viruliferous B. tabaci and demonstrate that this pesticide can reduce TYLCV transmission by surviving individuals.

Lethal and Sublethal Effects of Flupyradifurone on Bemisia tabaci MED (Hemiptera: Aleyrodidae) Feeding Behavior and TYLCV Transmission in Tomato

Pesticides primarily affect target organisms via direct toxicity, but may also alter the feeding behaviors of surviving individuals in ways that alter their effect on host plants. The latter impact is especially important when pests can transmit plant pathogens. The Mediterranean (MED) population of the sweetpotato whitefly Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) transmits Tomato yellow leaf curl virus (TYLCV), a pathogen that can be economically devastating in field and greenhouse cropping systems. We first assessed the impact of sublethal (LC15) and label concentrations of flupyradifurone, a butenolide-derived insecticide, on the feeding behavior of TYLCV-infected MED on tomato. We next measured the effect of flupyradifurone on plant TYLCV load, vector transmission efficiency, and MED survival. Both the LC15 and label flupyradifurone concentrations dramatically altered MED feeding and caused the near cessation of both salivation and phloem ingestion (necessary for viral transmission and acquisition, respectively). Both concentrations also significantly reduced plant TYLCV load, and the label rate of flupyradifurone sharply decreased TYLCV transmission while killing >99% of MED. As the first report of pesticide-induced changes in the feeding behavior of viruliferous Bemisia, our findings highlight the potential importance of chemically driven feeding cessation in the control of TYLCV and other Bemisia-transmitted plant pathogens.

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.

Tomato Chlorosis Virus Infection Facilitates Bemisia tabaci MED Reproduction by Elevating Vitellogenin Expression

Simple Summary

The sweet potato whitefly, Bemisia tabaci, is a polyphagous, global invasive insect pest. It can damage vegetables and crops directly by feeding and indirectly by transmitting plant viruses. Previously, we showed that virus infection of host plants can promote B. tabaci MED (Q biotype) reproduction. Here, using a whitefly-tomato chlorosis virus (ToCV)-tomato system, we investigated how ToCV modulates B. tabaci reproduction to facilitate its spread. ToCV infection significantly increased whitefly fecundity and the relative expression of vitellogenin gene (Vg). Both ovarian development and fecundity of whitefly were suppressed when Vg expression was silenced with or without ToCV infection. These combined results reveal that ToCV infection increases B. tabaci MED fecundity via elevated vitellogenin gene expression.

Abstract

Transmission of plant pathogenic viruses mostly relies on insect vectors. Plant virus could enhance its transmission by modulating the vector. Previously, we showed that feeding on virus infected plants can promote the reproduction of the sweet potato whitefly, Bemisia tabaci MED (Q biotype). In this study, using a whitefly-Tomato chlorosis virus (ToCV)-tomato system, we investigated how ToCV modulates B. tabaci MED reproduction to facilitate its spread. Here, we hypothesized that ToCV-infected tomato plants would increase B. tabaci MED fecundity via elevated vitellogenin (Vg) gene expression. As a result, fecundity and the relative expression of B. tabaci MED Vg was measured on ToCV-infected and uninfected tomato plants on days 4, 8, 12, 16, 20 and 24. The role of Vg on B. tabaci MED reproduction was examined in the presence and absence of ToCV using dietary RNAi. ToCV infection significantly increased B. tabaci MED fecundity on days 12, 16 and 20, and elevated Vg expression on days 8, 12 and 16. Both ovarian development and fecundity of B. tabaci MED were suppressed when Vg was silenced with or without ToCV infection. These combined results suggest that ToCV infection increases B. tabaci MED fecundity via elevated Vg expression.

Tomato Yellow Leaf Curl Virus Infection Alters Bemisia tabaci MED (Hemiptera: Aleyrodidae) Vulnerability to Flupyradifurone

The whitefly, Bemisia tabaci Gennadius, is a major phloem-feeding pest of agricultural crops that is also an important vector of many plant diseases. The B. tabaci Mediterranean ('MED') biotype is a particularly effective vector of Tomato yellow leaf curl virus (TYLCV), a devastating plant pathogen. Although insecticides play an important role in the control of MED and TYLCV, little is known about how TYLCV infection affects MED susceptibility to insecticides. We conducted research addressing how MED susceptibility to flupyradifurone, the first commercially available systemic control agent derived from the butenolide class of insecticides, was affected by TYLCV infection. We first conducted bioassays determining the LC15 and LC50 for control and viruliferous MED feeding on either water- or insecticide-treated plants. We next measured several demographic parameters of control and viruliferous MED exposed to either insecticide- or water-treated plants. TYLCV infection increased MED tolerance of flupyradifurone: the LC15 and LC50 of viruliferous MED were double that of uninfected MED. Viral infection also altered MED demographic responses to flupyradifurone, but in an inconsistent manner. Although the ability of TYLCV and other persistently transmitted viruses to benefit Bemisia via manipulation of host plant defense is well known, this appears to be the first example of virally mediated changes in vector susceptibility to an insecticide.

A rapid insect species identification system using mini-barcode pyrosequencing

BACKGROUND

Rapid and accurate species identification is not only important for biodiversity studies and pest quarantine and management, but in some cases may also influence the results of international trade negotiations. In this study, we developed a rapid species identification system for insects.

RESULTS

A universal DNA mini-barcode primer pair was designed to target ∼ 120 bp of the mitochondrial 16S rDNA gene. This primer set can amplify the targeted region from all 300 species of 26 insect orders tested as well as other classes of Arthropoda. Although we found no within-species variation in this region, it provided enough information to separate closely related species or species complexes, in particular Thrips spp. and Bemisia spp. By combining a quick DNA extraction method with pyrosequencing, we were able to generate DNA sequences and complete species identification within 5 h.

CONCLUSION

Mini-barcode pyrosequencing of 16S rDNA coupled with the GenBank database provides a rapid, accurate, and efficient species identification system. This system is therefore useful for biodiversity discovery, forensic identification, and quarantine control and management. © 2019 Society of Chemical Industry.

Secondary endosymbiont diversity of Bemisia tabaci and its parasitoids

Cotton whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is one of the most important insect pests worldwide. It is known as a species complex consisting of at least 40 cryptic species. Although there are substantial data regarding species composition, parasitoids and endosymbionts of B. tabaci, data on relationship between the pest, parasitoids and endosymbionts are very restricted. Therefore, in this study, secondary endosymbionts in populations of B. tabaci and their parasitoids collected from Turkey and the USA were determined by PCR-based DNA analysis. Whitefly populations in Turkey represented both Mediterranean (MED) and Middle East-Asia Minor1 (MEAM1) genotypes from single or mixed populations of both genotypes. Arsenophonus, Rickettsia and Wolbachia were found in MED, while Hamiltonella and Rickettsia in MEAM1. Whitefly populations collected from Arizona were all MEAM1 and dually infected with Hamiltonella and Rickettsia. The aphelinid parasitoids Encarsia lutea and Eretmocerus mundus predominated in all Turkish populations. While almost all En. lutea populations were infected with Wolbachia, no endosymbionts were detected in any Er. mundus. Parasitoid species and the pattern of secondary endosymbiont infection in Arizona populations were different with Rickettsia detected only from Encarsia sophia while both Rickettsia and Wolbachia were found in Eretmocerus species. As a result, four secondary endosymbionts, namely, Rickettsia, Hamiltonella, Arsenophonus and Wolbachia, were detected from B.tabaci and its parasitoids. Among them only Wolbachia and Rickettsia were found in both the pest and parasitoids. It is conclude that further studies should be pursued to determine effect of these endosymbionts on biology of the parasitoids and success in biological control of B. tabaci.

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.

Changes in Bemisia tabaci feeding behaviors caused directly and indirectly by cucurbit chlorotic yellows virus

Background

Plant viruses can affect vector’s behaviors in order to enhance viral transmission. Cucurbit chlorotic yellows virus (CCYV) (genus Crinivirus) is an emergent RNA plant virus and is transmitted specifically by biotypes B and Q of tobacco whitefly, Bemisia tabaci (Gennadius), in a semipersistent manner.

Methods

We used the electrical penetration graph (EPG) to investigate the effect of CCYV on the feeding behaviors of B. tabaci biotypes B and Q.

Results

CCYV could affect, both directly and indirectly, the feeding behaviors of B. tabaci to various degrees, depending on biotypes and sexes of the insect. CCYV showed stronger direct effects on biotype Q than on biotype B in terms of increased non-phloem probing and phloem salivation. CCYV increased non-phloem probing and phloem salivation more on females than on males of biotype Q, and increased phloem salivation more on females than on males of biotype B. CCYV had stronger indirect effects, via virus-infested plants, on biotype B than on biotype Q by enhancing phloem sap ingestion and feeding bouts. CCYV increased non-phloem probing and feeding bouts more on males than on females of biotype B, and decreased phloem sap ingestion more on males than on females on biotype Q indirectly.

Conclusions

The results clearly indicated that CCYV affects the feeding behaviors of B. tabaci, which may lead to increased ability of the B. tabaci for CCYV transmission.

Isaria fumosorosea-based zero-valent iron nanoparticles affect the growth and survival of sweet potato whitefly, Bemisia tabaci (Gennadius)

BACKGROUND

Nanoparticles can be used for effective pest management as a combined formulation of metal and some other material that has proven efficacy against a given pest. This study reports the synthesis, characterization and efficacy of Isaria fumosorosea-based zero-valent iron (ZVI) nanoparticles against sweet potato whitefly Bemisia tabaci (Gennadius).

RESULTS

The I. fumosorosea-ZVI nanoparticles showed a characteristic surface plasmon absorption band at 470 nm during UV-visible spectroscopy. The scanning electron micrographs of nanoparticles showed spherical shaped nanoparticles with sizes ranging between 1.71 and 3.0 µm. The EDX analysis showed the characteristic peak of iron at 0.6 and 6.8 KeV. The XRD analysis showed characteristic peaks at 44.72°, 65.070°, 82.339° and 82.65°. The bioassay results indicated that the percentage of larval mortality of B. tabaci challenged with I. fumosorosea ZVI nanoparticles was both concentration and age dependent. Isaria fumosorosea ZVI nanoparticles showed high pathogenicity against second and third instar nymphs, and pupae with LC₅₀ values of 19.17, 26.10 and 37.71 ppm, respectively. The LT₅₀ was lowest for second instar nymphs (3.15 days) and highest for pupae (4.22 days) when inoculated with a concentration of 50 ppm.

CONCLUSION

Isaria fumosorosea ZVI nanoparticles can be an eco-friendly tool for effective B. tabaci management. © 2019 Society of Chemical Industry.

Genetic Status and Endosymbionts Diversity of Bemisia tabaci (Gennadius) on Hosts Belonging to Family Malvaceae in India

A study was instigated to examine the genetic status and distribution of known endosymbionts namely Portiera, Rickettsia, Wolbachia, Cardinium, and Arsenophonus in the populations of Bemisia tabaci (Gennadius) from three host plants: cotton (Gossypium herbaceum), okra (Abelmoschus esculentus L.), and China rose (Hibiscus rosa-sinensis) belonging to the family Malvaceae. The presence of four secondary endosymbionts Rickettsia, Wolbachia, Cardinium, and Arsenophonus was checked in Bemisia tabaci populations. Phylogenetic analyses grounded on the mitochondrial cytochrome oxidase I gene (mtCO1) unveiled the presence of Asia 1, Asia II 1, and Asia II 7 genetic groups for Bemisia tabaci on abovementioned crops. Individuals were examined for symbiotic bacterial infection with specific primers amplifying the 16S rRNA gene for Portiera, Rickettsia, Cardinium, and Wolbachia, and the 23S rRNA gene for Arsenophonus. The results show that Portiera was present in all the Bemisia tabaci samples. However, variations were noted in the circulation frequencies of secondary endosymbionts among the Bemisia tabaci populations. A significant difference was noticed in the distribution frequency of Rickettsia between cotton and China rose or okra with their p values as 0.016 and 0.033 respectively. The uneven incidence of secondary endosymbionts ropes the assumption that each endosymbiotic bacterium not only has a role in the endurance but may contribute to the polyphagous nature of Bemisia tabaci. It also brings an uncomplicated evidence for progressive studies on control measures of this notorious insect pest.

Methyl benzoate exhibits insecticidal and repellent activities against Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae)

Methyl benzoate (MB) is a plant-derived volatile organic compound with insecticidal properties, but such activity has not been evaluated against the sweetpotato whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), a major crop pest. In this study, we tested methyl benzoate control efficacy on B. tabaci infecting tomato plants in a greenhouse, specifically measuring contact and fumigant toxicity, as well as repellent activity. For direct spray applications of 0% (control), 0.1%, 0.25%, 0.5%, 1%, 2% MB onto tomato leaves infested with adults of B. tabaci (< 5-d-old), 2% MB showed the highest corrected mortality (100%) at 24 h post-treatment. For residual toxicity in which the same MB solutions were sprayed onto tomato leaves and allowed to dry for 2 h before < 5-d-old adults were released, the 2% MB also showed the highest corrected mortality (100%) at 48 h post-treatment. The lethal median concentration (LC50) for eggs, fourth-instar nymphs, and adults were 0.3%, 0.2%, and 0.2%, respectively. In pot culture experiments, 1% MB concentration was found more effective at killing nymphs and preventing adult eclosion than all other concentrations, and gave 100 percent population reduction compared with the control. MB repelled adult whiteflies and caused 96.5% fumigant toxicity within 10 h post-treatment. Repellency and anti-oviposition rates against B. tabaci had median effective doses of 0.24% and 0.16%, respectively. Our results suggest that MB has strong potential as an environmentally friendly biopesticide for control of B. tabaci but field trials and further greenhouse studies are required to establish efficacy under more natural conditions.

Effects of high temperature on insecticide tolerance in whitefly Bemisia tabaci (Gennadius) Q biotype

Bemisia tabaci (Gennadius) Q biotype (BTQ) has spread to many tropical and subtropical regions over the past several decades. This may reflect an advantage biotype Q has over closely related forms in having greater thermal and/or insecticide resistance, although the effects of higher temperatures on insecticide tolerance of BTQ has, to date, been largely ignored. In this study, the effects of elevated temperatures on BTQ's tolerance to the insecticide thiamethoxam were investigated. The effect on the activities of detoxifying enzymes [carboxylesterase (CarE), glutathione S-transferase (GST), and cytochrome P450 monooxygenase (P450)] and expression profiling of eleven genes of detoxifying enzymes were also determined. In addition, RNA interference (RNAi) and bioassay methods were used to further identify the function of CYP6CM1 in tolerance to thiamethoxam following exposure to higher temperatures. The results showed that elevated temperatures were responsible for causing different outcomes in the tolerance of BTQ to thiamethoxam: Temperatures of 35 °C or higher decreased the tolerance of BTQ to thiamethoxam, while a moderately high temperature of 31 °C increased the tolerance. The high temperature influenced the tolerance of BTQ by affecting the activity of P450. Quantitative real-time PCR (qPCR) showed that CYP6CM1 was significantly up-regulated in most treatments at 31 °C, but was suppressed at 35 °C, which was closely associated with the mortality rates. Feeding on double-stranded RNA (dsRNA) of CYP6CM1 significantly reduced the mRNA levels of the target gene in the adults, and dramatically decreased tolerance to thiamethoxam induced by a temperature of 31 °C for 6 h. Our finding provides useful information to better understand the invasion mechanism of BTQ.

Targeting detoxification genes by phloem-mediated RNAi: A new approach for controlling phloem-feeding insect pests

Many phloem-feeding insects are considered severe pests of agriculture and are controlled mainly by chemical insecticides. Continued extensive use of these inputs is environmentally undesirable, and also leads to the development of insecticide resistance. Here, we used a plant-mediated RNA interference (RNAi) approach, to develop a new control strategy for phloem-feeding insects. The approach aims to silence "key" detoxification genes, involved in the insect's ability to neutralize defensive and toxic plant chemistry. We targeted a glutathione S-transferase (GST) gene, BtGSTs5, in the phloem-feeding whitefly Bemisia tabaci, a devastating global agricultural pest. We report three major findings. First, significant down regulation of the BtGSTs5 gene was obtained in the gut of B. tabaci when the insects were fed on Arabidopsis thaliana transgenic plants expressing dsRNA against BtGSTs5 under a phloem-specific promoter. This brings evidence that phloem-feeding insects can be efficiently targeted by plant-mediated RNAi. Second, in-silico and in-vitro analyses indicated that the BtGSTs5 enzyme can accept as substrates, hydrolyzed aliphatic- and indolic-glucosinolates, and produce their corresponding detoxified conjugates. Third, performance assays suggested that the BtGSTs5 gene silencing prolongs the developmental period of B. tabaci nymphs. Taken together, these findings suggest that BtGSTs5 is likely to play an important role in enabling B. tabaci to successfully feed on glucosinolate-producing plants. Targeting the gene by RNAi in Brassicaceae cropping systems, will likely not eliminate the pest populations from the fields but will significantly reduce their success over the growing season, support prominent activity of natural enemies, eventually allowing the establishment of stable and sustainable agroecosystem.

Abundance of Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) and its parasitoids on vegetables and cassava plants in Burkina Faso (West Africa)

The whitefly Bemisia tabaci is a pest of many agricultural and ornamental crops worldwide and particularly in Africa. It is a complex of cryptic species, which is extremely polyphagous with hundreds of host plants identified around the world. Previous surveys in western Africa indicated the presence of two biotypes of the invasive MED species (MED-Q1 and MED-Q3) living in sympatry with the African species SSA and ASL. This situation constitutes one of the rare cases of local coexistence of various genetic entities within the B. tabaci complex. In order to study the dynamics of the distribution and abundance of genetic entities within this community and to identify potential factors that could contribute to coexistence, we sampled B. tabaci populations in Burkina Faso in 2015 and 2016 on various plants, and also their parasitoids. All four genetic entities were still recorded, indicating no exclusion of local species by the MED species. While B. tabaci individuals were found on 55 plant species belonging to eighteen (18) families showing the high polyphagy of this pest, some species/biotypes exhibited higher specificity. Two parasitoid species (Eretmocerus mundus and Encarsia vandrieschei) were also recorded with E. mundus being predominant in most localities and on most plants. Our data indicated that whitefly abundance, diversity, and rate of parasitism varied according to areas, plants, and years, but that parasitism rate was globally highly correlated with whitefly abundance suggesting density dependence. Our results also suggest dynamic variation in the local diversity of B. tabaci species/biotypes from 1 year to the other, specifically with MED-Q1 and ASL species. This work provides relevant information on the nature of plant-B. tabaci-parasitoid interactions in West Africa and identifies that coexistence might be stabilized by niche differentiation for some genetic entities. However, MED-Q1 and ASL show extensive niche overlap, which could ultimately lead to competitive exclusion.

Assessment of bacterial endosymbionts and the host, Bemisia tabaci (Hemiptera: Aleyrodidae), using rRNA and mitochondrial cytochrome oxidase I gene sequences

Endosymbionts are vital factor for arthropod ecology. The whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is a cryptic species complex composed of more than 34 putative species. Moreover to the primary endosymbiont Portiera aleyrodidarum, six secondary endosymbionts Cardinium, Arsenophonus, Rickettsia, Wolbachia, Hamiltonella and Fritschea are known in B. tabaci. Here, we tested four of the six secondary endosymbiont lineages (excluding Fritschea and Hamiltonella) from 180 whitely individuals collected from six host plants belonging to families Solanaceae (Brinjal, Tomato and Potato) and Fabaceae (Soyabean, Mungbean and Subabool). Phylogenetic studies grounded on the mitochondrial cytochrome I gene revealed the presence of Asia 1, Asia II 1 and Asia II 7 genetic groups for B. tabaci. Specific primers targeting 16S rRNA and 23S rRNA gene were used for estimating the bacterial endosymbionts. As a primary endosymbiont Portiera aleyrodidarum was present in all the studied samples; whereas, an uneven distribution of secondary endosymbionts were recorded. Overall our finding exposes the variation and diversity of endosymbionts within the B. tabaci collected from different host plants and outlines the genetic groups of the insect pest. The study delivers a significant information concerning the circulation of secondary endosymbionts with host preferences of B. tabaci and provides suggestion for progressive studies on targeting the specific endosymbionts with respect to host for the control measures.

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.

Analysis of Species, Subgroups, and Endosymbionts of Bemisia tabaci (Hemiptera: Aleyrodidae) From Southwestern Cotton Fields in Turkey

Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is one of the most important insect pests worldwide including Turkey. Although there are substantial data regarding species composition of Turkish B. tabaci populations, the situation is still not clear and further investigations are needed. Therefore, in this study, species and subgroups of B. tabaci collected from cotton fields in southwestern part of Turkey (Antalya, Aydın, Denizli, and Muğla) were determined using microsatellite analysis, AluI-based mtCOI polymerase chain reaction-random length polymorphism, and sequencing. Secondary endosymbionts were also determined using diagnostic species-specific PCR. Middle East Asia Minor 1 (MEAM1), Mediterranean (MED) Q1, and MED Q2 were the species and subgroups found in this study. The MED species (85.3%) were found to be more dominant than MEAM1. Species status of B. tabaci varied depending on the location. Although all samples collected from Aydın were found to be Q1, three species and subgroups were found in Muğla. Secondary endosymbionts varied according to species and subgroups. Arsenophonus was found only from Q2, while Hamiltonella was detected in MEAM1 and Q1. In addition, high Rickettsia and low Wolbachia infections were detected in MEAM1 and Q1 populations, respectively. In conclusion, for the first time, we report the presence and symbiotic communities of Q1 from Turkey. We also found that the symbiont complement of the Q1 is more congruent with Q1 from Greece than other regions of the world, which may have some interesting implications for movement of this invasive subgroup.

Toxicological and biochemical basis of synergism between the entomopathogenic fungus Lecanicillium muscarium and the insecticide matrine against Bemisia tabaci (Gennadius)

The sweetpotato whitefly Bemisia tabaci (Gennadius) was challenged with different combinations of matrine (insecticide) and Lecanicillium muscarium (entomopathogenic fungus). Our results revealed a synergistic relationship between matrine and L. muscarium on mortality and enzyme activities of B. tabaci. To illustrate the biochemical mechanisms involved in detoxification and immune responses of B. tabaci against both control agents, activities of different detoxifying and antioxidant enzymes were quantified. After combined application of matrine and L. muscarium, activities of carboxylestrease (CarE), glutathione-s-transferase (GSTs) and chitinase (CHI) decreased during the initial infection period. Acetylcholinestrase (AChE) activities increased during the entire experimental period, whereas those of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) decreased during the later infection period. The increased mortality and suppression of enzymatic response of B. tabaci following matrine and L. muscarium application suggests a strong synergistic effect between both agents. The strong synergistic effect is possibly related to the disturbance of acetylcholine balance and changes in AchE activities of the whitefly as both matrine and L. muscarium target insect acetylcholine (Ach) receptors which in turn effects AchE production. Therefore, our results have revealed the complex biochemical processes involved in the synergistic action of matrine and L. muscarium against B. tabaci.

Dynamics of Bemisia tabaci biotypes and insecticide resistance in Fujian province in China during 2005-2014

The whitefly Bemisia tabaci (Gennadius) is an important agricultural insect pest worldwide. The B and Q biotypes are the two most predominant and devastating biotypes prevalent across China. However, there are few studies regarding the occurrence of the Q biotype in Fujian Province, China, where high insecticide resistance has been reported in the B biotype. Differences in some biological characteristics between the B and Q biotypes, especially insecticide resistance, are considered to affect the outcome of their competition. Extensive surveys in Fujian revealed that the B biotype was predominant during 2005–2014, whereas the Q biotype was first detected in some locations in 2013 and widely detected throughout the province in 2014. Resistance to neonicotinoids (that have been used for more than 10 years) exhibited fluctuations in open fields, but showed a continual increasing trend in protected areas. Resistance to lambda-cyhalothrin, chlorpyrifos, and abamectin exhibited a declining trend. Resistance to novel insecticides, such as nitenpyram, pymetrozine, sulfoxaflor, and cyantraniliprole, in 2014 was generally below a moderate level. A decline in insecticide resistance in the B biotype and the rapid buildup of protected crops under global temperature increase may have promoted the establishment of the Q biotype in Fujian.

A Semipersistent Plant Virus Differentially Manipulates Feeding Behaviors of Different Sexes and Biotypes of Its Whitefly Vector

It is known that plant viruses can change the performance of their vectors. However, there have been no reports on whether or how a semipersistent plant virus manipulates the feeding behaviors of its whitefly vectors. Cucurbit chlorotic yellows virus (CCYV) (genus Crinivirus, family Closteroviridae) is an emergent plant virus in many Asian countries and is transmitted specifically by B and Q biotypes of tobacco whitefly, Bemisia tabaci (Gennadius), in a semipersistent manner. In the present study, we used electrical penetration graph (EPG) technique to investigate the effect of CCYV on the feeding behaviors of B. tabaci. The results showed that CCYV altered feeding behaviors of both biotypes and sexes of B. tabaci with different degrees. CCYV had stronger effects on feeding behaviors of Q biotype than those of B biotype, by increasing duration of phloem salivation and sap ingestion, and could differentially manipulate feeding behaviors of males and females in both biotype whiteflies, with more phloem ingestion in Q biotype males and more non-phloem probing in B biotype males than their respective females. With regard to feeding behaviors related to virus transmission, these results indicated that, when carrying CCYV, B. tabaci Q biotype plays more roles than B biotype, and males make greater contribution than females.

Development of an antibody-based diagnostic method for the identification of Bemisia tabaci biotype B

The whitefly Bemisia tabaci is a very destructive pest. B. tabaci is composed of various morphologically undistinguishable biotypes, among which biotypes B and Q, in particular, draw attention because of their wide distribution in Korea and differential potentials for insecticide resistance development. To develop a biotype-specific protein marker that can readily distinguishes biotypes B from other biotypes in the field, we established an ELISA protocol based on carboxylesterase 2 (COE2), which is more abundantly expressed in biotypes B compared with Q. Recombinant COE2 was expressed, purified and used for antibody construction. Polyclonal antibodies specific to B. tabaci COE2 [anti-COE2 pAb and deglycosylated anti-COE2 pAb (DG anti-COE2 pAb)] revealed a 3-9-fold higher reactivity to biotype B COE2 than biotype Q COE2 by Western blot and ELISA analyses. DG anti-COE2 pAb exhibited low non-specific activity, demonstrating its compatibility in diagnosing biotypes. Western blot and ELISA analyses determined that one of the 11 field populations examined was biotype B and the others were biotype Q, suggesting the saturation of biotype Q in Korea. DG anti-COE2 pAb discriminates B. tabaci biotypes B and Q with high specificity and accuracy and could be useful for the development of a B. tabaci biotype diagnosis kit for on-site field applications.

Only a minority of broad-range detoxification genes respond to a variety of phytotoxins in generalist Bemisia tabaci species

Generalist insect can utilize two different modes for regulating their detoxification genes, the constitutive mode and the induced mode. Here, we used the Bemisia tabaci sibling species MEAM1 and MED, as a model system for studying constitutive and induced detoxification resistance and their associated tradeoffs. B. tabaci adults were allowed to feed through membranes for 24 h on diet containing only sucrose or sucrose with various phytotoxins. Quantitative real-time PCR analyses of 18 detoxification genes, indicated that relatively few transcripts were changed in both the MEAM1 and MED species, in response to the addition of phytotoxins to the diet. Induced transcription of detoxification genes only in the MED species, in response to the presence of indole-3-carbinol in the insect’s diet, was correlated with maintenance of reproductive performance in comparison to significant reduction in performance of the MEAM1 species. Three genes, COE2, CYP6-like 5 and BtGST2, responded to more than one compound and were highly transcribed in the insect gut. Furthermore, functional assays showed that the BtGST2 gene encodes a protein capable of interacting with both flavonoids and glucosinolates. In conclusion, several detoxification genes were identified that could potentially be involved in the adaptation of B. tabaci to its host plants.

Sequence variation of Bemisia tabaci Chemosensory Protein 2 in cryptic species B and Q: New DNA markers for whitefly recognition

Bemisia tabaci Gennadius biotypes B and Q are two of the most important worldwide agricultural insect pests. Genomic sequences of Type-2 B. tabaci chemosensory protein (BtabCSP2) were cloned and sequenced in B and Q biotypes, revealing key biotype-specific variations in the intron sequence. A Q260 sequence was found specifically in Q-BtabCSP2 and Cucumis melo LN692399, suggesting ancestral horizontal transfer of gene between the insect and the plant through bacteria. A cleaved amplified polymorphic sequences (CAPS) method was then developed to differentiate B and Q based on the sequence variation in exon of BtabCSP2 gene. The performances of CSP2-based CAPS for whitefly recognition were assessed using B. tabaci field collections from Shandong Province (P.R. China). Our SacII based CAPS method led to the same result compared to mitochondrial cytochrome oxidase-based CAPS method in the field collections. We therefore propose an explanation for CSP origin and a new rapid simple molecular method based on genomic DNA and chemosensory gene to differentiate accurately the B and Q whiteflies of the Bemisia complex around the world.

Detection of genetically isolated entities within the Mediterranean species of Bemisia tabaci: new insights into the systematics of this worldwide pest

BACKGROUND

The taxonomy of the species complex Bemisia tabaci, a serious agricultural pest worldwide, is not well resolved yet, even though species delimitation is critical for designing effective control strategies. Based on a threshold of 3.5% mitochondrial (mtCOI) sequence divergence, recent studies have identified 28 putative species. Among them, mitochondrial variability associated with particular symbiotic compositions (=cytotypes) can be observed, as in MED, which raises the question of whether it is a single or a complex of biological species.

RESULTS

Using microsatellites, an investigation was made of the genetic relatedness of Q1 and ASL cytotypes that belong to MED. Samples of the two cytotypes were collected in West Africa where they live in sympatry on the same hosts. Genotyping revealed a high level of differentiation, without evidence of gene flow. Moreover, they differed highly in frequencies of resistance alleles to insecticides, which were much higher in Q1 than in ASL.

CONCLUSION

Q1 and ASL are sufficiently reproductively isolated for the introgression of neutral alleles to be prevented, suggesting that they are actually different species. This indicates that nuclear genetic differentiation must be investigated within groups with less than 3.5% mtCOI divergence in order to elucidate the taxonomy of B. tabaci at a finer level. Overall, these data provide important information for pest management.

Genome evolution in the primary endosymbiont of whiteflies sheds light on their divergence

Whiteflies are important agricultural insect pests, whose evolutionary success is related to a long-term association with a bacterial endosymbiont, Candidatus Portiera aleyrodidarum. To completely characterize this endosymbiont clade, we sequenced the genomes of three new Portiera strains covering the two extant whitefly subfamilies. Using endosymbiont and mitochondrial sequences we estimated the divergence dates in the clade and used these values to understand the molecular evolution of the endosymbiont coding sequences. Portiera genomes were maintained almost completely stable in gene order and gene content during more than 125 Myr of evolution, except in the Bemisia tabaci lineage. The ancestor had already lost the genetic information transfer autonomy but was able to participate in the synthesis of all essential amino acids and carotenoids. The time of divergence of the B. tabaci complex was much more recent than previous estimations. The recent divergence of biotypes B (MEAM1 species) and Q (MED species) suggests that they still could be considered strains of the same species. We have estimated the rates of evolution of Portiera genes, synonymous and nonsynonymous, and have detected significant differences among-lineages, with most Portiera lineages evolving very slowly. Although the nonsynonymous rates were much smaller than the synonymous, the genomic dN/dS ratios were similar, discarding selection as the driver of among-lineage variation. We suggest variation in mutation rate and generation time as the responsible factors. In conclusion, the slow evolutionary rates of Portiera may have contributed to its long-term association with whiteflies, avoiding its replacement by a novel and more efficient endosymbiont.

Invasion of the Q2 mitochondrial variant of Mediterranean Bemisia tabaci in southern Italy: possible role of bacterial endosymbionts

BACKGROUND

The whitefly Bemisia tabaci (Gennadius) is a complex of cryptic species, some of which, namely the Mediterranean (MED) and the Middle East-Asia Minor 1 (MEAM1), are highly invasive and injurious crop pests worldwide and able to displace local genotypes. Invasiveness of B. tabaci may depend on the phenotype of inherited bacterial endosymbionts. Here, the B. tabaci genetic diversity variation that has occurred in recent years in southern Italy was examined. Whitefly was genotyped by restriction fragment length polymorphism analysis of polymerase-chain-reaction-amplified fragments (PCR-RFLP) of the COI gene and molecular identification of endosymbionts. Possible factors leading to the observed genetic diversity were examined.

RESULTS

Q1 and Q2 mitochondrial types of MED, the only species found, coexisted in the field, while MEAM1 disappeared. A large spreading of Q2 (70% of individuals) was observed for the first time in Italy. Q2 showed a significant female-biased sex ratio and largely outnumbered Q1 on solanaceous hosts, in greenhouses and on insecticide-treated plants. Q1, with an even sex ratio, slightly prevailed on non-solanaceous hosts, especially on wild and untreated plants. Endosymbiont composition was associated with the mitochondrial type. Hamiltonella and Rickettsia were found at near fixation in Q1 and Q2 respectively; Arsenophonus, Cardinium and Wolbachia were found in both types, although at different frequencies.

CONCLUSIONS

Q2 invasion seems to have been favoured by the agroecological conditions of southern Italy and by the female-biased sex ratio. Endosymbionts may have a role in Q2 invasiveness, acting as sex-ratio manipulators (e.g. Rickettsia) and possibly by benefiting the host fitness.

Genetic structure of Bemisia tabaci Med populations from home-range countries, inferred by nuclear and cytoplasmic markers: impact on the distribution of the insecticide resistance genes

BACKGROUND

Insecticide resistance management in Bemisia tabaci is one of the main issues facing agricultural production today. An extensive survey was undertaken in five Mediterranean countries to examine the resistance status of Med B. tabaci species in its range of geographic origin and the relationship between population genetic structure and the distribution of resistance genes. The investigation combined molecular diagnostic tests, sequence and microsatellite polymorphism studies and monitoring of endosymbionts.

RESULTS

High frequencies of pyrethroid (L925I and T929V, VGSC gene) and organophosphate (F331W, ace1 gene) resistance mutations were found in France, Spain and Greece, but not in Morocco or Tunisia. Sequence analyses of the COI gene delineated two closely related mitochondrial groups (Q1 and Q2), which were found either sympatrically (Spain) or separately (France). Only Q1 was observed in Greece, Morocco and Tunisia. Bayesian analyses based on microsatellite loci revealed three geographically delineated genetic groups (France, Spain, Morocco/Greece/Tunisia) and high levels of genetic differentiation even between neighbouring samples. Evidence was also found for hybridisation and asymmetrical gene flow between Q1 and Q2.

CONCLUSIONS

Med B. tabaci is more diverse and structured than reported so far. On a large geographic scale, resistance is affected by population genetic structure, whereas on a local scale, agricultural practices appear to play a major role.

Dynamics of biotypes B and Q of the whitefly Bemisia tabaci and its impact on insecticide resistance

BACKGROUND

The whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a key pest in many agricultural crops, including vegetables, ornamentals and field crops. B. tabaci is known for its genetic diversity, which is expressed in a complex of biotypes or, as recently suggested, a complex of distinct cryptic species. The biotypes are largely differentiated on the basis of biochemical or molecular polymorphism and differ in characteristics such as host plant range, attraction by natural enemies, secondary symbionts and expression of insecticide resistance. An extensive survey of B. tabaci biotypes and their impact on insecticide resistance was conducted from 2003 to 2012 in cotton fields and other crops from several locations in Israel.

RESULTS

Two biotypes of B. tabaci, B and Q, were identified, and some differences in the biotype dynamics were recorded from different areas. In northern Israel from 2003 to 2007, a higher proportion of the B biotype was consistently found in early season. However, by the end of the season a definite rise of the Q biotype was sampled, ranging from 60 to 100%, along with high resistance to the insect growth regulator (IGR) pyriproxyfen and to a lesser extent to the neonicotinoid insecticides. In fields located in the central part of Israel, the Q biotype was predominant throughout the seasons, with high resistance to pyriproxyfen. Since 2009, a significant shift in the biotype ratios has been observed: the B biotype has come to predominate over the Q biotype ranging up to 90% or more in most fields. At the same time, resistance to the IGR pyriproxyfen was reduced considerably.

CONCLUSION

The possible reasons for the change in the dynamics of B. tabaci biotypes, and its implications for resistance management, are discussed. Strong B. tabaci resistance to pyriproxyfen in Israel has been associated with the Q rather than with the B biotype. The B biotype is more competitive than the Q biotype under untreated conditions. Reduction in the acreage of cotton fields during recent years, along with a decrease in insecticide use, especially pyriproxyfen, has resulted in the expansion of the B biotype.

Nuclear and cytoplasmic differentiation among Mediterranean populations of Bemisia tabaci: testing the biological relevance of cytotypes

BACKGROUND

The taxonomy of the species complex Bemisia tabaci is still an unresolved issue. Recently, phylogenetic analysis based on mtCOI identified 31 cryptic species. However, mitochondrial diversity is observed within these species, associated with distinct symbiotic bacterial communities forming associations, which here are called cytotypes. The authors investigated the biological significance of two cytotypes (Q1 and Q2) belonging to the Mediterranean species, which have only been found in allopatry in the Western Mediterranean to date. Sampling was done over a few years in Western Europe, and sympatric situations were found that allowed their reproductive compatibility to be tested in the field with the use of microsatellites.

RESULTS

The field survey indicated that, in spite of its recent introduction, Q2 is well established in France and Spain, where it coexists with Q1. Microsatellite data showed that, in allopatry, Q1 and Q2 are highly differentiated, while there is little or no genetic differentiation when they coexist in sympatry, suggesting a high rate of hybridisation. Crossing experiments in the lab confirmed their interfertility.

CONCLUSION

Q1 and Q2 hybridise, which confirms that they belong to the same species, in spite of the high degree of genetic differentiation at both the cytoplasmic and nuclear levels, and also suggests that their symbiotic bacteria do not prevent hybridisation.

Bemisia tabaci females from the Mediterranean (Q) species detect and avoid laying eggs in the presence of pyriproxyfen, a juvenile hormone analogue

BACKGROUND

Pyriproxyfen, a juvenile hormone analogue, disrupts embryogenesis, metamorphosis and adult formation in Bemisia tabaci, but does not directly affect adult females. The effect of pyriproxyfen on egg-laying preference and performance of B. tabaci females and the influence of resistance to pyriproxyfen on these reproductive behaviours were studied.

RESULTS

Choice experiments utilising cotton plants treated and not treated with pyriproxyfen revealed a significant preference for egg laying on non-treated plants both by resistant and susceptible females. No-choice assays indicated a reduction of ∼60% in the number of eggs laid on pyriproxyfen-treated plants by both resistant and susceptible females. The reduction in oviposition on treated plants was not accompanied with reduced expression of the vitellogenin gene or a delay in oocyte maturation, but significant accumulation of mature oocytes in the ovaries was observed, and could be reversed by transferring the females to non-treated plants.

CONCLUSION

Pyriproxyfen caused reduced oviposition and enhanced mature oocyte accumulation in pyriproxyfen-resistant and pyriproxyfen-susceptible females. These findings can be explained by two alternative mechanisms: pyriproxyfen-regulated physiological arrest of oviposition, involving hormonal regulation of myotrophic factors, or the hierarchy-threshold behavioural theory of host choice, in which pyriproxyfen-treated plants are defined as low-quality hosts. Aspects of application are discussed.

Competitive ability and fitness differences between two introduced populations of the invasive whitefly Bemisia tabaci Q in China

Background

Our long-term field survey revealed that the Cardinium infection rate in Bemisia tabaci Q (also known as biotype Q) population was low in Shandong, China over the past few years. We hypothesize that (1) the Cardinium-infected (C⁺) B. tabaci Q population cannot efficiently compete with the Cardinium-uninfected (C⁻) B. tabaci Q population; (2) no reproductive isolation may have occurred between C⁺ and C⁻; and (3) the C⁻ population has higher fitness than the C⁺ population.

Methodology and Results

To reveal the differences in competitive ability and fitness between the two introduced populations (C⁺ and C⁻), competition between C⁺ and C⁻ was examined over several generations. Subsequently, the reproductive isolation between C⁺ and C⁻ was studied by crossing C⁺ with C⁻ individuals, and the fitnesses of C⁺ and C⁻ populations were compared using a two-sex life table method. Our results demonstrate that the competitive ability of the C⁺ whiteflies was weaker than that of C⁻. There is that no reproductive isolation occurred between the two populations and the C⁻ population had higher fitness than the C⁺ population.

Conclusion

The competitive ability and fitness differences of two populations may explain why C⁻ whitefly populations have been dominant during the past few years in Shandong, China. However, the potential role Cardinium plays in whitefly should be further explored.

Evidence for rapid spatiotemporal changes in genetic structure of an alien whitefly during initial invasion

The sweetpotato whitefly Bemisia tabaci Q species is a recent invader and important pest of agricultural crops in China. This research tested the hypothesis that the Q populations that establish in agricultural fields in northern China each year are derived from multiple secondary introductions and/or local populations that overwinter in greenhouses (the pest cannot survive winters in the field in northern China). Here, we report the evidence that the Q populations in agricultural fields mainly derive from multiple secondary introductions. In addition, the common use of greenhouses during the winter in certain locations in northern China helps increase the genetic diversity and the genetic structure of the pest. The genetic structure information generated from this long-term and large-scale field analysis increases our understanding of B. tabaci Q as an invasive pest and has important implications for B. tabaci Q management.

Arabidopsis thaliana plants with different levels of aliphatic- and indolyl-glucosinolates affect host selection and performance of Bemisia tabaci

Generalist insects show reduced selectivity when subjected to similar, but not identical, host plant chemical signatures. Here, we produced transgenic Arabidopsis thaliana plants that over-express genes regulating the aliphatic- and indolyl- glucosinolates biosynthetic pathways with either a constitutive (CaMV 35S) or a phloem-specific promoter (AtSUC2). This allowed us to examine how exposure to high levels of aliphatic- or indolyl-glucosinolates in homogenous habitats (leaf cage apparatus containing two wild-type or two transgenic leaves) and heterogeneous habitats (leaf cage apparatus containing one wild-type and one transgenic leaf) affects host selection and performance of Bemsia tabaci, a generalist phloem-feeding insect. Data from homogenous habitats indicated that exposure to A. thaliana plants accumulating high levels of aliphatic- or indolyl-glucosinolates negatively affected the performance of both adult females and nymphs of B. tabaci. Data from heterogeneous habitats indicated that B. tabaci adult females selected for oviposition plants on which their offspring perform better (preference-performance relationship). However, the combinations of wild-type and transgenic plants in heterogeneous habitats increased the period of time until the first choice was made and led to increased movement rate on transgenic plants, and reduced fecundity on wild-type plants. Overall, our findings are consistent with the view that both performance and selectivity of B. tabaci decrease in heterogeneous habitats that contain plants with closely-related chemical signatures.

Insecticide resistance in Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) and Anopheles gambiae Giles (Diptera: Culicidae) could compromise the sustainability of malaria vector control strategies in West Africa

Insecticides from the organophosphate (OP) and pyrethroid (PY) chemical families, have respectively, been in use for 50 and 30 years in West Africa, mainly against agricultural pests, but also against vectors of human disease. The selection pressure, with practically the same molecules year after year (mainly on cotton), has caused insecticide resistance in pest populations such as Bemisia tabaci, vector of harmful phytoviruses on vegetables. The evolution toward insecticide resistance in malaria vectors such as Anopheles gambiae sensus lato (s.l.) is probably related to the current use of these insecticides in agriculture. Thus, successful pest and vector control in West Africa requires an investigation of insect susceptibility, in relation to the identification of species and sub species, such as molecular forms or biotypes. Identification of knock down resistance (kdr) and acetylcholinesterase gene (Ace1) mutations modifying insecticide targets in individual insects and measure of enzymes activity typically involved in insecticide metabolism (oxidase, esterase and glutathion-S-transferase) are indispensable in understanding the mechanisms of resistance. Insecticide resistance is a good example in which genotype-phenotype links have been made successfully. Insecticides used in agriculture continue to select new resistant populations of B. tabaci that could be from different biotype vectors of plant viruses. As well, the evolution of insecticide resistance in An. gambiae threatens the management of malaria vectors in West Africa. It raises the question of priority in the use of insecticides in health and/or agriculture, and more generally, the question of sustainability of crop protection and vector control strategies in the region. Here, we review the susceptibility tests, biochemical and molecular assays data for B. tabaci, a major pest in cotton and vegetable crops, and An. gambiae, main vector of malaria. The data reviewed was collected in Benin and Burkina Faso between 2008 and 2010 under the Corus 6015 research program. This review aims to show: (i) the insecticide resistance in B. tabaci as well as in An. gambiae; and (ii) due to this, the impact of selection of resistant populations on malaria vector control strategies. Some measures that could be beneficial for crop protection and vector control strategies in West Africa are proposed.

Inoculation of tomato plants with rhizobacteria enhances the performance of the phloem-feeding insect Bemisia tabaci

In their natural environment, plants experience multiple biotic interactions and respond to this complexity in an integrated manner. Therefore, plant responses to herbivory are flexible and depend on the context and complexity in which they occur. For example, plant growth promoting rhizobacteria (PGPR) can enhance plant growth and induce resistance against microbial pathogens and herbivorous insects by a phenomenon termed induced systemic resistance (ISR). In the present study, we investigated the effect of tomato (Solanum lycopersicum) pre-inoculation with the PGPR Pseudomonas fluorescens WCS417r, on the performance of the generalist phloem-feeding insect Bemisia tabaci. Based on the ability of P. fluorescens WCS417r to prime for ISR against generalists chewing insects and necrotrophic pathogens, we hypothesized that pre-inoculated plants will strongly resist B. tabaci infestation. In contrast, we discovered that the pre-inoculation treatment increased the tomato plant suitability for B. tabaci which was emphasized both by faster developmental rate and higher survivability of nymph stages on pre-inoculated plants. Our molecular and chemical analyses suggested that the phenomenon is likely to be related to: (I) the ability of the bacteria to reduce the activity of the plant induced defense systems; (II) a possible manipulation by P. fluorescens of the plant quality (in terms of suitability for B. tabaci) through an indirect effect on the rhizosphere bacterial community. The contribution of our study to the pattern proposed for other belowground rhizobacteria and mycorrhizal fungi and aboveground generalist phloem-feeders is discussed.

Competitive displacement between two invasive whiteflies: insecticide application and host plant effects

The cryptic species Middle East-Asia Minor 1 (MEAM1), formerly referred to as 'B biotype', of the whitefly Bemisia tabaci complex entered China in the mid 1990s, and the Mediterranean (MED) cryptic species, formerly referred to as 'Q biotype', of the same whitefly complex entered China around 2003. Field surveys in China after 2003 indicate that in many regions MED has been replacing the earlier invader MEAM1. The factors underlying this displacement are unclear. We conducted laboratory experiments and field sampling to examine the effects of insecticide application on the competitive interactions between MEAM1 and MED. In the laboratory, on cotton, a plant showing similar levels of suitability to both whitefly species, MEAM1 displaced MED in five generations when initial populations of the two species were equal and no insecticide was applied. In contrast, MED displaced MEAM1 in seven and two generations, respectively, when 12.5 and 50.0 mg l⁻¹ imidacloprid was applied to the plants via soil drench. Field sampling indicated that in a single season MED displaced MEAM1 on crops heavily sprayed with neonicotinoid insecticides but the relative abundance of the two species changed little on crops without insecticide spray. We also examined the effects of host plants on the competitive interactions between the two species in the laboratory. When cohorts with equal abundance of MEAM1 and MED were set up on different host plants, MEAM1 displaced MED on cabbage and tomato in five and seven generations, respectively, but MED displaced MEAM1 on pepper in two generations. As field populations of MED have lower susceptibility than those of MEAM1 to nearly all commonly used insecticides including imidacloprid, insecticide application seems to have played a major role in shifting the species competitive interaction effects in favour of MED in the field across China. Host plants may also shape competition between the two species depending on the relative levels of plant suitability.

Characterization and comparative profiling of miRNAs in invasive Bemisia tabaci (Gennadius) B and Q

BACKGROUND

MicroRNAs (miRNAs) are small, conserved, non-coding RNAs that post-transcriptionally regulate gene expression. Bemisia tabaci (Gennadius) B and Q are two invasive and dominant whiteflies, and B. tabaci Q has been displacing B in China. Differences in biological traits (fecundity, host range, resistance to insecticides, etc.) as affected by miRNAs might be involved in the displacement. In this study, we performed high-throughput sequencing to identify miRNAs in B. tabaci B and Q.

RESULTS

We identified 170 conserved miRNAs and 15 novel candidates, and found significant differences in the expression of miRNAs between B. tabaci B and Q.

CONCLUSION

Expression levels of miRNAs differ in B. tabaci B vs. Q. Additional research is needed to determine whether these differences are related to differences in the biology of B. tabaci B and Q, and whether these differences help explain why B. tabaci Q is displacing B in China.

Complete genome sequence of "Candidatus Portiera aleyrodidarum" BT-QVLC, an obligate symbiont that supplies amino acids and carotenoids to Bemisia tabaci

The genome of "Candidatus Portiera aleyrodidarum," the primary endosymbiont of the whitefly Bemisia tabaci (Mediterranean species), is reported. It presents a reduced genome (357 kb) encoding the capability to synthetize, or participate in the synthesis of, several amino acids and carotenoids, being the first insect endosymbiont capable of supplying carotenoids.

Loop-mediated isothermal amplification for rapid identification of biotypes B and Q of the globally invasive pest Bemisia tabaci, and studying population dynamics

BACKGROUND

Bemisia tabaci, the sweetpotato whitefly, is a globally invasive pest that causes serious agricultural damage by transmitting plant viruses. This pest forms a cryptic species complex that displays morphologically indistinguishable biotypes. Among them, the B and Q biotypes are the most important pests worldwide. Because they have different levels of insecticide resistance, these biotypes must be identified in order to achieve proper pest control. Therefore, a convenient, rapid and specific detection method for identifying the two biotypes is necessary.

RESULTS

Loop-mediated isothermal amplification (LAMP) was employed for rapid identification of B. tabaci B and Q biotypes. By combining a quick DNA extraction method, identification of the two biotypes was achieved within 1 h of detection time. The LAMP assay was applied to study the dynamics of B. tabaci biotypes both in the field and in greenhouses. It was found that, while temperature may be important for population dynamics of the whitefly in the field, population dynamics in greenhouse conditions may be influenced by the types of insecticide.

CONCLUSION

The newly designed LAMP assay is a simple, rapid and accurate method for identifying the B and Q biotypes. It can be conducted by non-specialists and can contribute to pest management.

Distribution of Bemisia tabaci (Hemiptera: Aleyrodidae) biotypes in North America after the Q invasion

After the 2004 discovery of the Bemisia tabaci (Gennadius) (Hemiptera Aleyrodidae) Q biotype in the United States, there was a vital need to determine the geographical and host distribution as well as its interaction with the resident B biotype because of its innate ability to rapidly develop high-level insecticide resistance that persists in the absence of exposure. As part of a coordinated country-wide effort, an extensive survey of B. tabaci biotypes was conducted in North America, with the cooperation of growers, industry, local, state, and federal agencies, to monitor the introduction and distribution of the Q biotype. The biotype status of submitted B. tabaci samples was determined either by polymerase chain reaction amplification and sequencing of a mitochondrial cytochrome oxidase I small subunit gene fragment and characterization of two biotype discriminating nuclear microsatellite markers or esterase zymogram analysis. Two hundred and eighty collections were sampled from the United States, Bermuda, Canada, and Mexico during January 2005 through December 2011. Host plants were split between ornamental plant and culinary herb (67%) and vegetable and field crop (33%) commodities. The New World biotype was detected on field-grown tomatoes (Solanum lycopersicum L.) in Mexico (two) and in commercial greenhouses in Texas (three) and represented 100% of these five collections. To our knowledge, the latter identification represents the first report of the New World biotype in the United States since its rapid displacement in the late 1980s after the introduction of biotype B. Seventy-one percent of all collections contained at least one biotype B individual, and 53% of all collections contained only biotype B whiteflies. Biotype Q was detected in 23 states in the United States, Canada (British Columbia and Ontario territories), Bermuda, and Mexico. Forty-five percent of all collections were found to contain biotype Q in samples from ornamentals, herbs and a single collection from tomato transplants located in protected commercial horticultural greenhouses, but there were no Q detections in outdoor agriculture (vegetable or field crops). Ten of the 15 collections (67%) from Canada and a single collection from Bermuda contained biotype Q, representing the first reports of biotype Q for both countries. Three distinct mitochondrial haplotypes of B. tabaci biotype Q whiteflies were detected in North America Our data are consistent with the inference of independent invasions from at least three different locations. Of the 4,641 individuals analyzed from 517 collections that include data from our previous work, only 16 individuals contained genetic or zymogram evidence of possible hybridization of the Q and B biotypes, and there was no evidence that rare hybrid B-Q marker co-occurrences persisted in any populations.

Difference in feeding behaviors of two invasive whiteflies on host plants with different suitability: implication for competitive displacement

In China, Bemisia tabaci Q (commonly known as biotype Q) has rapidly displaced B (commonly known as biotype B) in the past 6 years. The mechanisms underlying such phenomenon have been studied extensively in recent years; however, we have not come to a definitive conclusion yet. In the present study, the differences in host suitability between B and Q whitefly adults to five host plants (cabbage, cotton, cucumber, poinsettia, and tomato) were evaluated based on their respective feeding behaviors using a direct-current electrical penetration graph (DC-EPG) system. Pair-wise comparisons of B. tabaci B and Q feeding on each of the five host plants clearly indicate that Q feeds better than B on tomato, cotton and poinsettia, while B feeds better than Q on cabbage and cucumber. The EPG parameters related to both phloem and non-phloem phases confirm that cabbage and cucumber are best suited to B, while tomato, cotton, and poinsettia are best suited to Q. Our present results support the contention that host suitability and adult feeding behavior contribute to the competitive displacement of biotype B by biotype Q. The discrepancy between field (previous studies) and laboratory results (this study), however, suggests that 1) whitefly displacement is apparently contributed by multiple factors; and 2) factor(s) other than the host plant suitability may play a vital role in dictating the whitefly biotypes in the field.