Toxicity of spiromesifen to the developmental stages of Bemisia tabaci biotype B

Base-line susceptibility of Tribolium castaneum to various synthetic insecticides

The red flour beetle, Tribolium castaneum (Herbst) is a cosmopolitan insect pest that causes severe loss in broad range of stored commodities worldwide. Chemical control has been considered as major preventive measure to manage this dangerous pest. Therefore, we have evaluated 22 different insecticides against T. castaneum collected from five districts of southern Punjab, Pakistan. Bioassays were performed to assess the LC50 values of considered insecticides. Our results showed that emamectin benzoate was the most toxic insecticide among all the insecticides. There was non-significant difference in toxicity of deltamethrin and triazophos to all tested populations based on overlapping 95% confidence interval of LC50. The spinosad was more toxic as compared with the spinetoram. In case of ketoenoles, spirotetramat was more toxic as compared with the spiromesifen. In case of acetamaprid and nitenpyram, there is non- significant difference in LC50 values of the all tested strains of T. castaneum based on overlapping 95%CI. We used four insect growth regulators (IGRS) including methoxyfenozide, pyriproxyfen, lufenuron and cyromazine as larvicides. The lufenuron was more toxic as compared with the pyriproxyfen, methoxyfenozide and cyromazine in most of the studied populations. Methoxyfenozide was moderately toxic. These results will provide help in establishing an effective management program for T. castaneum.

Lipid Metabolism as a Target Site in Pest Control

Lipid metabolism is essential to insect life as insects use lipids for their development, reproduction, flight, diapause, and a wide range of other functions. The central organ for insect lipid metabolism is the fat body, which is analogous to mammalian adipose tissue and liver, albeit less structured. Various other systems including the midgut, brain, and neural organs also contribute functionally to insect lipid metabolism. Lipid metabolism is under the control of core lipogenic [e.g. acetyl-CoA-carboxylase (ACC), fatty acid synthase (FAS), perilipin 2 (LSD2)], and lipolytic (lipases, perilipin 1) enzymes that are primarily expressed in the fat body, as well as hormones [insulin-like peptides (ILP), adipokinetic hormone (AKH)], transcription factors (SREBPs, foxO, and CREB), secondary messengers (calcium) and post-translational modifications (phosphorylation). Essential roles of the fat body, together with the fact that proper coordination of lipid metabolism is critical for insects, render lipid metabolism an attractive target site in pest control. In the current chapter, we focus on pest control tactics that target insect lipid metabolism. Various classes of traditional chemical insecticides [e.g. organophosphates, pyrethroids, neonicotinoids, and chitin synthesis inhibitors (Sects. 2.1 and 2.2)] have been shown to interfere with lipid metabolism, albeit it is not their primary site of action. However, the discovery of "lipid biosynthesis inhibitors", tetronic and tetramic acid derivatives commonly known as ketoenols (Sect. 2.3), was a milestone in applied entomology as they directly target lipid biosynthesis, particularly in sucking pests. Spirodiclofen, spiromesifen, and spirotetramat targeting ACC act against various insect and mite pests, while spiropidion and spidoxamat have been introduced to the market only recently. Efforts have concentrated on the development of chemical alternatives, such as hormone agonists and antagonists (Sect. 2.4), dsRNA-based pesticides that depend on RNA interference, which have great potential in pest control (Sect. 2.5) and other eco-friendly alternatives (Sect. 2.6).

Repurposing Insecticides for Mosquito Control: Evaluating Spiromesifen, a Lipid Synthesis Inhibitor against Aedes aegypti (L.)

The growing resistance of Aedes aegypti (L.) to conventional insecticides presents a major challenge in arbovirus control, necessitating the exploration of alternative insecticidal chemistries. Spiromesifen, derived from spirocyclic tetronic acids, is widely used against agricultural pests and is crucial in resistance management due to its unique lipid synthesis inhibition. This study evaluates the insecticidal activity of spiromesifen against temephos-resistant Ae. aegypti populations, focusing on larval body weight, volume, biochemical composition, and adult female reproductive potential. Spiromesifen demonstrated effective larvicidal activity, significantly reducing adult emergence. Resistance to spiromesifen was not observed, with resistance ratios (RR50, RR90) ranging from 0.36- to 3.31-fold. Larvae exposed to LC50 showed significant reductions in body weight and volume, and reduced carbohydrate, lipid, and protein contents. Enhanced catalase activity and malondialdehyde levels indicated increased oxidative stress and lipid peroxidation, highlighting its effects on lipid metabolism. Spiromesifen also exhibited sterilizing effects, significantly reducing fecundity and fertility in adult females, thereby impacting Ae. aegypti reproductive capacity. These findings highlight the potential of spiromesifen as a component of integrated vector management strategies, especially in regions with prevalent insecticide resistance in Ae. aegypti, serving as an effective larvicide and impacting adult reproductive outcomes.

Whiteflies can excrete insecticide-tainted honeydew on tomatoes

Whiteflies are important insect pests in a wide variety of agricultural crops that are targeted with large quantities of insecticides on a global scale. Chemical control is the most common strategy to manage whiteflies, however, recent studies had reported that whiteflies and other hemipterans can excrete insecticides through their honeydew, which could have unanticipated, non-target effects. The objective of this study was to determine the concentration of imidacloprid in honeydew excreted by whiteflies feeding on tomato plants. Imidacloprid was applied at its labeled rate to soil at the base of whitefly-infested plants. Densities of whiteflies were assessed before insecticide treatment and 21 days after treatment (DAT). Honeydew was collected in Petri dishes from 1 to 4 DAT and from 5 to 8 DAT. The volume of the honeydew was calculated using stereo microscopy and then rinsed with ethanol. The rinsates were analyzed to determine imidacloprid concentration using liquid chromatography coupled to mass spectrometry. Honeydew production was further quantified by using water sensitive papers. Imidacloprid reduced densities of nymph and adult whiteflies by 81.5% and 76.0% compared to the control at 21DAT. The non-metabolized parent compound imidacloprid was detected from honeydew samples at both collection dates. At 1-4 DAT, imidacloprid concentrations were 180 ng/30 mL in a volume of 39 mm3 of honeydew. At 5-8 DAT, the imidacloprid concentration was 218 ng/30 mL in a volume of 25 mm3 of honeydew. Though the volume of honeydew decreased, the concentration of imidacloprid numerically increased. Last, whiteflies were still producing honeydew 22 DAT in both treatments. These results revealing significant imidacloprid concentrations in honeydew suggest a strong potential for negative secondary impacts on beneficial insects.

Negative Impact of Unstable Spiromesifen Resistance on Fitness of Tetranychus urticae (Acari: Tetranychidae)

Two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is a phytophagous haplodiploid mite and its control is largely based on the use of pesticides. But, the short life cycle and high reproductive rate allow them to develop resistance to many pesticides. To design a strategy for resistance management, a fitness cost study was conducted on different populations of T. urticae, i.e., spiromesifen selected (SPIRO-SEL), unselected (Unsel), and reciprocal crosses. After twelve rounds of selections, T. urticae developed high spiromesifen resistance (71.7-fold) compared to the Unsel strain. Results showed a fitness cost for SPIRO-SEL, Cross1 (Unsel ♀ × SPIRO-SEL ♂), and Cross2 (SPIRO-SEL ♀ × Unsel ♂) with a relative fitness values of 0.63, 0.86, and 0.70, respectively. There was a significant increase in the incubation period, quiescent larvae, and egg to adult male and female developmental period of the SPIRO-SEL compared with Unsel strain. Moreover, resistance to spiromesifen was unstable with a decline in resistance value of - 0.05. The presence of unstable spiromesifen resistance associated with fitness costs suggests that intermittent withdrawal of its usage could potentially preserve its effectiveness for management of T. urticae.

Residual characteristics and safety assessment of the insecticides spiromesifen and chromafenozide in lettuce and perilla

This study was performed to investigate the residual characteristics, safety assessment, and pre-harvest interval (PHI) of spiromesifen and chromafenozide in lettuce (Latuca sativa L.) and perilla (Perilla frutescens (L.) Britton) leaves. Samples were harvested periodically, extracted using QuEChERS method, and analyzed by LC-MS/MS. Average recoveries of spiromesifen and its metabolite BSN2060-enol and chromafenozide were ranged from 80.6 to 107.9%, with relative standard deviation < 10%. Spiromesifen and cromafenozide initial residues in lettuce were dissipated to 81.45 and 95.52% after 7 days, with half-lives of 2.89 and 1.69 days respectively. Values in perilla leaves were 76.68 and 61.27% after the same period, with half-lives of 4.25 and 6.30 days, respectively. Risk assessment results showed that %ADI (acceptable daily intake) of spiromesifen and chromafenozide was 6.83 and 0.56, in lettuce and 4.60 and 0.25% in perilla leaves, respectively. Theoretical maximum daily intakes of spiromesifen and chromafenozide were 67.49 and 3.43%, respectively, indicating that residues of both compounds pose no considerable health risks to consumers. This study provides data for setting maximum residue limits and PHIs for the safe use of spiromesifen and chromafenozide in lettuce and perilla.

The Effects of Spray Volume on the Management of Bemisia tabaci (Hemiptera: Aleyrodidae) in the Greenhouse

The sweetpotato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), is a major insect pest of poinsettias (Euphorbia pulcherrima Willd. ex Klotzsch; Family: Euphorbiaceae) in the greenhouse. Currently, neonicotinoids are widely used for B.tabaci management in the greenhouse, which is less favored by the consumers because of the potential nontarget effects of these insecticides on beneficial insects. Little is known on how the high spray volumes of spinetoram (20%) + sulfoxaflor (20%) (XXpire®) affect the B.tabaci population in the greenhouse. The objective of the study was to determine the efficacy of spinetoram + sulfoxaflor and dinotefuran (Zylam®) applied as foliar-spray volumes (high, referred to as spench, and low, referred to as foliar) and soil drench against B.tabaci. The high foliar-spray volume application (spench) of both insecticides reduced the B.tabaci immature densities, compared with low foliar-spray volume (foliar) and soil drench applications. The soil drench application did not provide adequate B.tabaci control regardless of insecticide type. Spinetoram + sulfoxaflor applied as a high-spray volume treatment was moderately effective in controlling B.tabaci nymphs relative to nontreated control.

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.

Pre-imaginal exposure to Oberon® disrupts fatty acid composition, cuticular hydrocarbon profile and sexual behavior in Drosophila melanogaster adults

Oberon® is a commercial formulation of spiromesifen, a pesticide inhibitor of lipid biosynthesis via acetyl CoA carboxylase, widely used in agricultural crop protection. However, its mode of action requires further analysis. We currently examined the effect of this product on Drosophila melanogaster as a non-target and model organism. Different concentrations of spiromesifen were administered by ingestion (and contact) during pre-imaginal development, and we evaluated its delayed action on adults. Our results suggest that spiromesifen induced insecticidal activity on D. melanogaster. Moreover, spiromesifen treatment significantly increased the duration of larval and pupal development at all tested concentrations while it shortened longevity in exposed males as compared to control males. Also, pre-imaginal exposure to spiromesifen quantitatively affected fatty acids supporting its primary mode of action on lipid synthesis. In addition, this product was found to modify cuticular hydrocarbon profiles in exposed female and male flies as well as their sexual behavior and reproductive capacity.

Distribution and phylogenetics of whiteflies and their endosymbiont relationships after the Mediterranean species invasion in Brazil

The Bemisia tabaci is a polyphagous insect and a successful vector of plant viruses. B. tabaci is a species complex and in Brazil native species from the New World (NW) group, as well as the invasive species, Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) were reported. For better understanding the distribution of the different species four years after the Mediterranean species invasion in Brazil, whiteflies were collected from 237 locations throughout the country between the years of 2013 and 2017, species were identified and the facultative endosymbionts detected. The survey revealed that MEAM1 was the prevalent species found on major crops across Brazil. It is the only species present in North, Northwestern and Central Brazil and was associated with virus-infected plants. MED was found in five States from Southeast to South regions, infesting mainly ornamental plants and was not associated with virus-infected plants. The prevalent endosymbionts identified in MEAM1 were Hamiltonella and Rickettsia; and the mtCOI analysis revealed low genetic diversity for MEAM1. In contrast, several different endosymbionts were identified in MED including Hamiltonella, Rickettsia, Wolbachia and Arsenophonus; and two distinct genetic groups were found based on the mtCOI analysis. Monitoring the distribution of the whiteflies species in Brazil is essential for proper management of this pest.

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.

Comparative transcriptome analysis reveals genetic diversity in the endosymbiont Hamiltonella between native and exotic populations of Bemisia tabaci from Brazil

The whitefly, Bemisia tabaci, is a species complex of more than 40 cryptic species and a major agricultural pest. It causes extensive damage to plants mainly by transmitting plant viruses. There is still a lack of genomic data available for the different whitefly species found in Brazil and their bacterial endosymbionts. Understanding the genetic and transcriptomic composition of these insect pests, the viruses they transmit and the microbiota is crucial to sustainable solutions for farmers to control whiteflies. Illumina RNA-Seq was used to obtain the transcriptome of individual whiteflies from 10 different populations from Brazil including Middle East-Asia Minor 1 (MEAM1), Mediterranean (MED) and New World 2 (NW2). Raw reads were assembled using CLC Genomics Workbench and subsequently mapped to reference genomes. We obtained whitefly complete mitochondrial genomes and draft genomes from the facultative bacterial endosymbiont Hamiltonella for further phylogenetic analyses. In addition, nucleotide sequences of the GroEL chaperonin gene from Hamiltonella from different populations were obtained and analysed. There was concordance in the species clustering using the whitefly complete mitogenome and the mtCOI gene tree. On the other hand, the phylogenetic analysis using the 12 ORF's of Hamiltonella clustered the native species NW2 apart from the exotics MEAM1 and MED. In addition, the amino acid analysis of GroEL chaperonin revealed a deletion only in Hamiltonella infecting NW2 among whiteflies populations analysed which was further confirmed by PCR and Sanger sequencing. The genomic data obtained in this study will aid understanding the functions that Hamiltonella may have in whitefly biology and serve as a reference for further studies regarding whiteflies in Brazil.

Ligand-binding characterization of simulated β-adrenergic-like octopamine receptor in Schistocerca gregaria via progressive structure simulation

It is important to design insecticides having both low drug resistance and less undesirable toxicity for desert locust control. Specific GPCRs of Schistocerca gregaria, especially β-adrenergic-like octopamine receptor (SgOctβR), can be considered as its potential effective insecticide targets. However, either the unavailability of SgOctβR's structure or the inadequate capability of its sequence lead the development of insecticide for Schistocerca gregaria meets its plateau. To relax this difficulty, this paper develops a promising progressive structure simulation from SgOctβR's sequence, to its predicted structure of SgOctβR in vacuum, to its conformation as well as its complex with endogenous ligand octopamine in a solvent-membrane system. The combined approach of multiple sequence alignment, static structural characterization, and dynamic process of conformational change during binding octopamine reveal three important aspects. The first one is the characterization of SgOctβR's active pocket, including the attending secondary structure elements, its hydrophobic residues and nonpolar surface. The second one is the interaction with octopamine, especially the involved hydrogen bonds and an aromatic stacking of pi-pi interactions. The third one is the potential binding sites, including six highly conserved residues and one highly variable residue for locust insecticide design. This work is definitely helpful for the further structure-based drug design for efficient and eco-friendly insecticides, as well as site-directed mutagenesis biochemical research of SgOctβR.

Lethal and Inhibitory Activities of Plant-Derived Essential Oils Against Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) Biotype B in Tomato

The silverleaf whitefly Bemisia tabaci (Genn.) biotype B (Hemiptera: Aleyrodidae) is one of the most severe tomato pests in the world. The damage caused by this insect may compromise up to 100% of crop production, and management of this pest has relied on spraying of synthetic insecticides. However, due to the environmental issues associated with this practice, alternative methods such as the use of botanical pesticides are now used as a strategy of integrated pest management (IPM). We evaluated the effects of essential oils of five plant species on B. tabaci biotype B in tomato and demonstrate that the essential oils (0.5%) of Piper callosum (PC-EO), Adenocalymma alliaceum (AA-EO), Pelargonium graveolens (PG-EO), and Plectranthus neochilus (PN-EO) inhibit the settlement and oviposition of B. tabaci biotype B adults in tomato plants. In fumigation tests, A. alliaceum (AA-EO) at 0.4 μL/L of air after 72 h and 0.1 μL/L of air after 6 h was the most effective against nymphs and adults of B. tabaci biotype B, respectively. The major chemical constituents of PC-EO were identified as being safrole (29.3%), α-pinene (19.2%), and β-pinene (14.3%), whereas diallyl trisulfide (66.9%) and diallyl disulfide (23.3%) were the major compounds identified in AA-EO. This is the first report on the reduction of oviposition by the use of P. callosum (PC-EO) and A. alliaceum (AA-EO). In addition, the fumigant effect of A. alliaceum (AA-EO) on nymphs and adults has also been reported here for the first time.

A Primary Screening and Applying of Plant Volatiles as Repellents to Control Whitefly Bemisia tabaci (Gennadius) on Tomato

With the goal of finding a new way to reduce population densities of Bemisia tabaci biotype Q in greenhouses, seven repellent volatile chemicals and their combinations were screened. The mixture of DLCO (D-limonene, citral and olive oil (63:7:30)) had a better cost performance(SC50 = 22.59 mg/ml)to repel whiteflies from settling than the other mixtures or single chemicals. In the greenhouse, in both the choice test and the no-choice tests, the number of adult whiteflies that settled on 1% DLCO-treated tomato plants was significantly lower than those settling on the control plants for the different exposure periods (P < 0.01). In the choice test, the egg amount on the treated tomato plants was significantly lower (P < 0.01) than that on the control plants, but there was no significant difference (P > 0.05) between the number of eggs on treated and control plants in the no-choice test. Compared with the controls, 1% DLCO did not cause significantly statistic mortality rates (P > 0.05) out of different living stages of B. tabaci. The tests for evaluating the repellent efficacy, showed that a slow-releasing bottle containing the mixture had a period of efficacy of 29 days, and the application of this mixture plus a yellow board used as a push-pull strategy in the greenhouse was also effective.

Chemical control of the Asian citrus psyllid and of huanglongbing disease in citrus

By 2014, huanglongbing (HLB), the most destructive disease of citrus, and its insect vector, the Asian citrus psyllid (ACP), Diaphorina citri (Kuwayama), became established in all major citrus-growing regions of the world, including the United States, with the exception of California. At present, application of insecticides is the most widely followed option for reducing ACP populations, while application of antibiotics for suppressing HLB disease/symptoms is being practiced in some citrus-growing regions. Application of insecticides during the dormant winter season, along with cultivation of HLB-free seedlings and early detection and removal of symptomatic and asymptomatic trees, has been very effective in managing ACP. Area-wide management of ACP by application of insecticides at low volume in large areas of citrus cultivation has been shown to be effective in managing HLB and reducing management costs. As insecticide resistance is a major problem in sustainable management of ACP, rotation/alternation of insecticides with different chemistries and modes of action needs to be followed. Besides control of the insect vector, use of antibiotics has temporarily suppressed the symptoms of HLB in diseased trees. Recent efforts to discover and screen existing as well as new compounds for their antibiotic and antimicrobial activities have identified some promising molecules for HLB control. There is an urgent need to find a sustainable solution to the HLB menace through chemical control of ACP populations and within HLB-infected trees through the judicious use of labeled insecticides (existing and novel chemistries) and antibiotics in area-wide management programs with due consideration to the insecticide resistance problem.

Is there a role for symbiotic bacteria in plant virus transmission by insects?

During the process of circulative plant virus transmission by insect vectors, viruses interact with different insect vector tissues prior to transmission to a new host plant. An area of intense debate in the field is whether bacterial symbionts of insect vectors are involved in the virus transmission process. We critically review the literature in this area and present a simple model that can be used to quantitatively settle the debate. The simple model determines whether the symbiont is involved in virus transmission and determines what fraction of the pathogen transmission phenotype is contributed by the symbiont. The model is general and can be applied to any vector-pathogen-symbiont interactions.

Chemical class rotations for control of Bemisia tabaci (Hemiptera: Aleyrodidae) on poinsettia and their effect on cryptic species population composition

BACKGROUND

Bemisia tabaci, a polyphagous insect with over 900 host plants, is an effective vector of more than 100 plant viruses. Being highly fecund, B. tabaci has the potential to develop insecticide resistance rapidly, as demonstrated by reports of use failures with MEAM1 and MED cryptic species (commonly known as biotypes B and Q respectively). Insecticide resistance management is a key component of pest management practices. The research herein studied season-long rotational management programs on poinsettia and their impact on the ratio of MEAM1:MED cryptic species in the surviving treated populations.

RESULTS

In all four experiments, only three of the treatments completely eliminated the adult or immature whiteflies, but all significantly reduced the populations. Out of 18 active ingredients tested, dinotefuran (applied as a soil drench) was the most efficacious against both MEAM1 and MED cryptic species compared with the other chemical or biorational insecticides evaluated. Reduced susceptibility of MED was reported against a variety of treatment regimes.

CONCLUSION

Rotations can be used to manage MEAM1 and MED cryptic species and maintain a very low population level or completely eliminate Bemisia on poinsettia. It is imperative to continue to emphasize the importance of rotating among different modes of action in pest management programs in order to retain effective chemistries for as long as possible in the market place.

Biology and management of Bemisia whitefly vectors of cassava virus pandemics in Africa

Cassava mosaic disease and cassava brown streak disease are caused by viruses transmitted by Bemisia tabaci and affect approximately half of all cassava plants in Africa, resulting in annual production losses of more than $US 1 billion. A historical and current bias towards virus rather than vector control means that these diseases continue to spread, and high Bemisia populations threaten future virus spread even if the extant strains and species are controlled. Progress has been made in parts of Africa in replicating some of the successes of integrated Bemisia control programmes in the south-western United States. However, these management efforts, which utilise chemical insecticides that conserve the Bemisia natural enemy fauna, are only suitable for commercial agriculture, which presently excludes most cassava cultivation in Africa. Initiatives to strengthen the control of B. tabaci on cassava in Africa need to be aware of this limitation, and to focus primarily on control methods that are cheap, effective, sustainable and readily disseminated, such as host-plant resistance and biological control. A framework based on the application of force multipliers is proposed as a means of prioritising elements of future Bemisia control strategies for cassava in Africa.

A review of the mechanisms and components that determine the transmission efficiency of Tomato yellow leaf curl virus (Geminiviridae; Begomovirus) by its whitefly vector

Begomoviruses are a group of icosahedral single stranded DNA viruses exclusively transmitted by the sweet potato whitefly Bemisia tabaci in a persistent, circulative manner. In this mode of transmission, begomoviruses are acquired by their insect vector as intact virions from the plant phloem, move along the food canal, foregut and esophagus and reach the midgut where they are absorbed into the hemolymph via the filter chamber. The filter chamber is the site where most of the ingested food is filtered, and the first site where the majority of begomoviruses appear to be translocated into the hemolymph via unknown proteins or receptors. Transport from the filter chamber to the hemolymph is aided by a Heat Shock Protein 70. Virus particles not translocated across the filter chamber circulate in the midgut loop but it is not known whether absorption into the hemolymph occurs along this loop. Localization studies have confirmed that begomoviruses are not associated with the hindgut and absorption of virions in this organ is unlikely. In the hemolymph, virions have been shown to interact with a GroEL chaperone produced by the whitefly's endosymbiontic bacteria for ensuring their safe journey to the salivary glands. Virions penetrate the primary salivary glands via unknown proteins or receptors and are transported and secreted outside the whitefly to the plant with salivary secretions. Several recent studies have demonstrated the implications of insect and endosymbiont proteins such as the heat shock protein 70 and the bacterial GroEL protein, in the transmission of begomoviruses by B. tabaci. Additional studies attempting to identify other proteins that aid or interact with begomoviruses along their circulation pathway in the whitefly are reviewed in this paper.

Molecular analysis of resistance to acaricidal spirocyclic tetronic acids in Tetranychus urticae: CYP392E10 metabolizes spirodiclofen, but not its corresponding enol

Spirodiclofen is one of the most recently developed acaricides and belongs to the new family of spirocyclic tetronic acids (ketoenols). This new acaricidal family is an important chemical tool in resistance management strategies providing sustainable control of spider mites such as Tetranychus urticae. Spirodiclofen targets lipid biosynthesis mediated by direct inhibition of acetyl coenzyme A carboxylase (ACCase). In this study, we investigated two genetically distant spider mite strains with high resistance to spirodiclofen. Despite the strong resistance levels to spirodiclofen (up to 680-fold), only limited cross-resistance with other members of this group such as spiromesifen and spirotetramat could be detected. Amplification and sequencing of the ACCase gene from resistant and susceptible strains did not reveal common non-synonymous mutations, and expression levels of ACCase were similar in both resistant and susceptible strains, indicating the absence of target-site resistance. Furthermore, we collected genome-wide expression data of susceptible and resistant T. urticae strains using microarray technology. Analysis of differentially expressed genes revealed a broad response, but within the overlap of two resistant strains, several cytochrome P450s were prominent. Quantitative PCR confirmed the constitutive over-expression of CYP392E7 and CYP392E10 in resistant strains, and CYP392E10 expression was highly induced by spirodiclofen. Furthermore, stage specific expression profiling revealed that expression levels were not significantly different between developing stages, but very low in eggs, matching the age-dependent resistance pattern previously observed. Functional expression of CYP392E7 and CYP392E10 confirmed that CYP392E10 (but not CYP392E7) metabolizes spirodiclofen by hydroxylation as identified by LC-MS/MS, and revealed cooperative substrate binding and a Km of 43 μM spirodiclofen. CYP392E10 also metabolizes spiromesifen, but not spirotetramat. Surprisingly, no metabolism of the hydrolyzed spirodiclofen-enol metabolite could be detected. These findings are discussed in the light of a likely resistance mechanism.

Resistance to spiromesifen in Trialeurodes vaporariorum is associated with a single amino acid replacement in its target enzyme acetyl-coenzyme A carboxylase

Spiromesifen is a novel insecticide and is classed as a tetronic acid derivative. It targets the insects' acetyl-coenzyme A carboxylase (ACCase) enzyme, causing a reduction in lipid biosynthesis. At the time of this publication, there are no reports of resistance to this class of insecticides in insects although resistance has been observed in several mite species. The greenhouse whitefly Trialeurodes vaporariorum (Westwood) is a serious pest of protected vegetable and ornamental crops in temperate regions of the world and spiromesifen is widely used in its control. Mortality rates of UK and European populations of T. vaporariorum to spiromesifen were calculated and up to 26-fold resistance was found. We therefore sought to examine the molecular mechanism underlying spiromesifen resistance in this important pest. Pre-treatment with piperonyl butoxide did not synergize spiromesifen, suggesting a target-site resistance mechanism. The full length ACCase gene was sequenced for a range of T. vaporariorum strains and a strong association was found between spiromesifen resistance and a glutamic acid substitution with lysine in position 645 (E645K) of this gene. A TaqMan allelic discrimination assay confirmed these findings. Although this resistance is not considered sufficient to compromise the field performance of spiromesifen, this association of E645K with resistance is the first report of a potential target site mechanism affecting an ACCase inhibitor in an arthropod species.

Synthesis and bioactivity evaluation of novel spiromesifen derivatives

BACKGROUND

The development of environmentally friendly and novel structural pesticides is now an area of intense research in the agriculture field. Spirocyclic tetronic acids such as spiromesifen are typical compounds of this kind. In order to discover novel compounds with improved and broader-spectrum insecticidal activities, a series of spiromesifen derivatives were synthesised and bioassayed.

RESULTS

The derivatives were identified by (1) H NMR and MS. Preliminary bioassays demonstrated that some bioactivities of compounds 5a to 5u were better and had a broader spectrum than the lead compound spiromesifen. Moreover, these compounds showed better insecticidal activities against Mythimna sepatara and Aphis fabae than acaricidal activities against Tetranychus cinnabari. Furthermore, LC(50) of 5s against Aphis fabae reached 1.09 mg L(-1) . At the same time, compounds 5g, 5i, 5k and 5r also warrant further study because of their superior bioactivities to spiromesifen. What is more, suitable carbon chain length in the 4-position ester and the log P value of these spiromesifen derivatives dramatically influenced their insecticidal activities. Butyric or pentanoic ester and a log P value of 4.0-6.0 may be preferred.

CONCLUSION

The present work demonstrates that some spiromesifen derivatives can be used as potential lead compounds for developing novel insecticides and acaricides.

Insecticide resistance in field populations of Asian citrus psyllid in Florida

BACKGROUND

Asian citrus psyllid (ACP), Diaphorina citri, is a major pest of citrus because it vectors the putative causal agent of huanglongbing disease. Insecticides are currently the basis of psyllid management programs, and the number of annual insecticide applications has increased significantly. In this paper, a series of investigations of insecticide resistance among field populations of adult and immature ACP in Florida is described.

RESULTS

In 2009, the highest level of resistance for adult ACP, as compared with the laboratory susceptible (LS) population, was found with imidacloprid with an LD(50) resistance ratio (RR(50) ) of 35 in one population. This was followed by chlorpyriphos (RR(50) = 17.9, 13.3, 11.8 and 6.9), thiamethoxam (RR(50) = 15 and 13), malathion (RR(50) = 5.4 and 5.0) and fenpropathrin (RR(50) = 4.8). In 2010, mortality of adults from all five sites sampled was lower than with the LS population at three diagnostic concentrations of each insecticide tested. Among nymph populations, indications of resistance were observed with carbaryl (RR(50) = 2.9), chlorpyriphos (RR(50) = 3.2), imidacloprid (RR(50) = 2.3 and 3.9) and spinetoram (RR(50) = 4.8 and 5.9). General esterase, glutathione S-transferase and monooxygenase levels were also elevated in field-collected adult and nymph ACP as compared with the LS population.

CONCLUSION

The present results suggest that varying levels of insecticide susceptibility exist in ACP populations across the citrus-growing areas of Florida. Increased levels of detoxifying enzymes in these populations may partially explain these differences. The present results indicate that insecticide resistance may become an emerging problem for ACP control if effective resistance management is not practiced.

Flufenerim, a novel insecticide acting on diverse insect pests: biological mode of action and biochemical aspects

A new chemical compound was tested for its insecticidal activity against several major insect pests. The compound, called "flufenerim", has a core pyrimidine structure and an unknown mode of action and showed potent activity against the sweet potato whitefly Bemisia tabaci (Gennadius), the green peach aphid Myzus persicae (Sulzer), and the African cotton leafworm Spodoptera littoralis (Boisduval); however, it did not show any activity against two thrips species: western flower thrips Frankliniella occidentalis (Pergande) and tobacco thrips Thrips tabaci (Lindeman). The compound was relatively potent against the three tested pests and caused mortality rates that reached up to 100% at concentrations under 10 mg of active ingredient (ai) L(-1). The action of the compound was very fast, and mortality was observed within 48 h after exposure of the insects to treated leaves. A unique characteristic of this compound is its very short residual activity, which approximates to 4 days after application under laboratory conditions and to 2 days under outdoor conditions for both B. tabaci and S. littoralis. Although this new compound's mode of action is yet unknown, its rapid and potent action against sap-sucking pests suggests that it acts on a very important target site in the insect body and possibly could be applied very close to harvesting.

The effects of spiromesifen on life history traits and population growth of two-spotted spider mite (Acari: Tetranychidae)

Laboratory bioassays were conducted to evaluate the effects of spiromesifen on gross fecundity, gross fertility, net fertility and population growth of two-spotted spider mite (Tetranychus urticae Koch) after treatments with four acaricide concentrations: 180 mg/l, i.e. maximum recommended concentration for use in glasshouses against spider mites, 18, 1.8, and 0.18 mg/l, i.e. concentration discriminative for eggs and immatures in preliminary studies which produced 100% mortality of these stages. Quiescent female deutonymphs were treated in the first assay, and young pre-ovipositing females in the second and third, in which exposure lasted 6 h and 20 h, respectively. In the first assay, the 180, 18, and 1.8 mg/l concentrations significantly reduced gross fecundity (61-85%), gross fertility (64-87%) and net fertility (85-94%) of the surviving females. In the second one, only the highest concentration achieved a significant statistical reduction in gross fecundity (52%), gross fertility (67%) and net fertility (84%). In the third assay, fecundity and fertility reduction under the two highest concentrations was 98-99% and 93-98%, whereas it was 50-74% under the 1.8 mg/l concentration, and statistically different from control values. In all three trials, treatments with 180, 18, and 1.8 mg/l concentrations significantly reduced the instantaneous rate of increase. In the third assay, treatments with the two highest concentrations caused population decline. Sublethal activity of the 0.18 mg/l concentration was not found in any assay to be statistically significant. Sublethal effects of spiromesifen and its impact on T. urticae management are discussed.