Novel ability of diflubenzuron as an inhibitor of mitochondrial function.

Compounds classified as benzoylphenylurea (BPU), such as diflubenzuron (DFB), are used as insecticides. Although BPU disrupts molting by inhibiting chitin biosynthesis and exhibits insecticidal activity, their exact mode of action remains unknown. Since epidermal cells proliferate and morphologically change from squamous to columnar cells during the early stages of insect molting, we speculate that a transition similar to that from epithelium to mesenchyme occurs and that BPU may inhibit this transition. Here, we addressed this possibility. We found that DFB decreases actin expression in insect cells (the tissue cultures of insect integument). Detailed analysis in Schneider S2 cells reveals that DFB inhibits the expression of actin isoforms (Act5C and Act42A) and the Drosophila ortholog of myocardin-related transcription factor (Mrtf), leading to cell growth suppression. Proteomics identified the Drosophila ortholog of prohibitin (Phb1D and Phb2E) as one of the DFB-binding proteins. DFB inhibits the interaction between Phb1D and Phb2E and induces mitochondrial dysfunction. The knock-down of Phb2E suppresses the expression of Act5C, Act42A, and Mrtf, leading to cell growth inhibition. Thus, the disruption of Phb function is a possible novel target of DFB.

Effects of solid and aqueous dietary diflubenzuron ingestion on some biological parameters in synthetic pyrethroid-resistant German cockroach, Blattella germanica L. (Blattodea: Ectobiidae).

Cockroaches, widespread pests found in metropolitan areas, are known as vectors of various disease agents, including viruses, fungi and antibiotic-resistant bacteria, as well as causing allergies in humans. Insect growth regulators have been used in pest management for several decades. These insecticides disrupt insect development and reproduction. Chitin synthesis inhibitors interfere with chitin biosynthesis in insects, causing abortive moulting and mortality, as well as inhibiting egg fertility, and larval hatching in insects. In this research, we evaluated the various effects of diflubenzuron, a chitin synthesis inhibitor, on synthetic pyrethroid-resistant German cockroach (Blattella germanica L. Blattodea: Ectobiidae), including ootheca production, oothecal viability, ootheca incubation time, the number of nymphs emerging from the ootheca and survivorship of nymphs. The cockroaches were fed diets that contained diflubenzuron, which was added to solid bait (impregnated fish food) and ingestible aqueous bait (impregnated cotton). Three concentrations (0.5%, 1% and 2%) were used in the experiments. As a result, diflubenzuron treatment led to ootheca production ranging from 60% to 100%; statistically, no difference was found between the treatment and the control groups. The number of nymphs emerging from the first and second ootheca was reduced by 40%-100% in the diflubenzuron-treated groups compared with the control. Nymphs exposed to diflubenzuron-impregnated solid bait and ingestible aqueous bait experienced mortality exceeding 92.1% and 66.27% within 15 days, respectively. In conclusion, diflubenzuron is a potential insecticide for use in cockroach baits to control B. germanica, as it caused high nymphal and embryonic mortality in the synthetic pyrethroid-resistant population and decreased the number of nymphs emerging from the ootheca.

Co-application of entomopathogenic fungi with chemical insecticides against Culex pipiens.

Culex pipiens (Diptera: Culicidae) is a vector of many human and animal diseases. Its control is regarded as a preventative approach that is focused on effectively managing such diseases. In this context, dose response assays of two insecticides, bendiocarb and diflubenzuron were performed with two entomopathogenic fungi, Beauveria bassiana and Metarhizium anisopliae against 3rd instar C. pipiens larvae. The most effective agents, combination experiments as well as enzymatic activities of phenoloxidase (PO) and chitinase (CHI) were also assessed. The results showed that diflubenzuron was more effective at low concentrations (LC50: 0.001 ppm) than bendiocarb (LC50: 0.174 ppm), whereas M. anisopliae was more effective (LC50: 5.2 × 105 conidia/mL) than B. bassiana (LC50: 7.5 × 107 conidia/mL). Synergistic interactions were observed when diflubenzuron was applied at 2- and 4-days post- exposure to M. anisopliae, with the highest degree of synergism observed when diflubenzuron was applied 2 days post-fungal exposure (χ2 = 5.77). In contrast, additive interactions were recorded with all other insecticide-fungal combinations. PO activities significantly (p ≤ 0.05) increased during 24 h after a single diflubenzuron treatment as well as when diflubenzuron was applied prior to M. anisopliae, whereas suppressed after 24 h when M. anisopliae applied prior to diflubenzuron as well as after 48 h from single and combined treatments. CHI activity increased 24 h after both single and combined treatments, the activity remained elevated 48 h after a single diflubenzuron treatment and when diflubenzuron was applied after M. anisopliae. Histological study of the cuticle by transmission electron microscopy revealed abnormalities following single and combined treatments. Germination of the conidia and production of the mycelium that colonizes the lysing cuticle was obvious when diflubenzuron was applied 48 h after M. anisopliae exposure. Overall, these results demonstrate that M. anisopliae is compatible with diflubenzuron at lower concentrations and that combined applications can improve C. pipiens management.

The impact of insect growth regulators on adult emergence inhibition and the fitness of Aedes aegypti field populations in Thailand.

BACKGROUND: The frequent use of insecticides in vector control causes the development of insecticide resistance. Insect growth regulators (IGRs), which effect insect development, are used as a promising alternative to control resistant insect vectors. This study aimed to develop novel effective tools for Aedes aegypti control by evaluating the efficacy of different IGRs on larval development, blood feeding capacity, fecundity, and fertility in females and sperm productivity in males across geographical regions of Thailand. METHODS: The efficacy of 16 technical grade IGRs were evaluated against laboratory strain Ae. aegypti larvae in order to determine their emergence inhibition (EI) at 50% and 95% under laboratory conditions. Six IGRs were selected for fecundity, fertility, and sperm productivity studies using feed-through treatments at EI95 concentration levels against adult Ae. aegypti field strains. RESULTS: The results from larval bioassay tests indicate that juvenile hormone mimics (EI50 = 0.010-0.229 ppb; EI95 = 0.066-1.118 ppb) and chitin synthesis inhibitors affecting CHS1 (EI50 = 0.240-2.412 ppb; EI95 = 0.444-4.040 ppb) groups effectively inhibited adult Ae. aegypti emergence. Methoprene and fenoxycarb significantly reduced blood feeding capacity. Egg production was comparable among strains while methoprene, pyriproxyfen and diflubenzuron induced egg production. Egg retention was detected in females fed on diflubenzuron. Methoprene, fenoxycarb, diflubenzuron, and teflubenzuron reduced egg hatching rates in mosquito field strains compared to laboratory strain. Male mosquitoes fed on fenoxycarb showed significantly lower sperm production compared to other treatments. CONCLUSION: Juvenile hormone analogues and chitin synthesis inhibitors affecting CHS1 groups showed excellent results in adult emergence inhibition in this study. They also disrupted reproductive systems in both adult males and females. This study suggested that they can be used as an alternative larvicide in mosquito control programs.

Risk assessment of resistance to diflubenzuron in Musca domestica: Realized heritability and cross-resistance to fourteen insecticides from different classes.

The Musca domestica L. is a well-known vector for a number of livestock and human diseases. One major challenge for maintaining effective control of this pest is its propensity to develop resistance to insecticides. This study utilized laboratory selection and realized heritability methods to examine the risk of resistance development to diflubenzuron in Musca domestica L. Cross-resistance (CR) to fourteen other insecticides was measured in diflubenzuron-selected (Diflu-SEL) strain which was selected for 20 generations. The resistance ratio (RR) of Diflu-SEL larvae to diflubenzuron increased from 30.33 in generation five (G5) to 182.33 in G24 compared with the susceptible strain, while realized heritability (h2) was 0.08. The number of needed generations (G) for a tenfold increase in the median lethal concentration (LC50) for diflubenzuron ranged from 4 to 45 at h2 values of 0.08, 0.18, and 0.28, at a slope of 1.51. At h2 = 0.08 and slopes of 1.51, 2.51, and 3.51, the number of needed G for a tenfold increase in the LC50 ranged from 9 to 104. The level of CR shown by the Diflu-SEL strain to all other fourteen tested insecticides (insect growth regulators, organophosphates, and pyrethroids) was either absent or very low compared to the field population. The value of h2 and the absent or low CR indicate potential successful management of resistance to diflubenzuron and recommend the use of the tested insecticides in rotation with diflubenzuron to control M. domestica.

Mosquito population structure, pathogen surveillance and insecticide resistance monitoring in urban regions of Crete, Greece.

BACKGROUND: In Greece vector borne diseases (VBD) and foremost West Nile virus (WNV) pose an important threat to public health and the tourist industry, the primary sector of contribution to the national economy. The island of Crete, is one of Greece's major tourist destinations receiving annually over 5 million tourists making regional VBD control both a public health and economic priority. METHODOLOGY: Under the auspices of the Region of Crete, a systematic integrative surveillance network targeting mosquitoes and associated pathogens was established in Crete for the years 2018-2020. Using conventional and molecular diagnostic tools we investigated the mosquito species composition and population dynamics, pathogen infection occurrences in vector populations and in sentinel chickens, and the insecticide resistance status of the major vector species. PRINCIPAL FINDINGS: Important disease vectors were recorded across the island including Culex pipiens, Aedes albopictus, and Anopheles superpictus. Over 75% of the sampled specimens were collected in the western prefectures potentially attributed to the local precipitation patterns, with Cx. pipiens being the most dominant species. Although no pathogens (flaviviruses) were detected in the analysed mosquito specimens, chicken blood serum analyses recorded a 1.7% WNV antibody detection rate in the 2018 samples. Notably detection of the first WNV positive chicken preceded human WNV occurrence in the same region by approximately two weeks. The chitin synthase mutation I1043F (associated with high diflubenzuron resistance) was recorded at an 8% allelic frequency in Lasithi prefecture Cx. pipiens mosquitoes (sampled in 2020) for the first time in Greece. Markedly, Cx. pipiens populations in all four prefectures were found harboring the kdr mutations L1014F/C/S (associated with pyrethroid resistance) at a close to fixation rate, with mutation L1014C being the most commonly found allele (≥74% representation). Voltage gated sodium channel analyses in Ae. albopictus revealed the presence of the kdr mutations F1534C and I1532T (associated with putative mild pyrethroid resistance phenotypes) yet absence of V1016G. Allele F1534C was recorded in all prefectures (at an allelic frequency range of 25-46.6%) while I1532T was detected in populations from Chania, Rethymnon and Heraklion (at frequencies below 7.1%). Finally, no kdr mutations were detected in the Anopheles specimens included in the analyses. CONCLUSIONS/SIGNIFICANCE: The findings of our study are of major concern for VBD control in Crete, highlighting (i) the necessity for establishing seasonal integrated entomological/pathogen surveillance programs, supporting the design of targeted vector control responses and; ii) the need for establishing appropriate insecticide resistance management programs ensuring the efficacy and sustainable use of DFB and pyrethroid based products in vector control.

Statewide Efficacy Assessment of Insect Growth Regulators Against Aedes albopictus (Diptera: Culicidae) in Sabah, Malaysia: An Alternative Control Strategy?

The efficacy of three groups of insect growth regulators, namely juvenile hormone mimics (methoprene and pyriproxyfen), chitin synthesis inhibitors (diflubenzuron and novaluron), and molting disruptor (cyromazine) was evaluated for the first time, against Aedes albopictus Skuse (Diptera: Culicidae) larvae from 14 districts in Sabah, Malaysia. The results showed that all field populations of Ae. albopictus were susceptible towards methoprene, pyriproxyfen, diflubenzuron, novaluron, and cyromazine, with resistance ratio values ranging from 0.50-0.90, 0.60-1.00, 0.67-1.17, 0.71-1.29, and 0.74-1.07, respectively. Overall, the efficacy assessment of insect growth regulators in this study showed promising outcomes and they could be further explored as an alternative to conventional insecticides.

Effect of diflubenzuron on the chitin biosynthesis pathway in Conopomorpha sinensis eggs.

Conopomorpha sinensis is the dominant borer pest of Litchi chinensis (litchi) and Euphoria longan (longan) in China. Control of C. sinensis is difficult because of its cryptic life habit; thus, an effective ovicide could be beneficial. The larvicidal effects of diflubenzuron (DFB) have been documented in many insect pest species. Therefore, DFB might be a useful ovicide to control C. sinensis. However, the detailed mode of action of DFB interference with insect molting and egg hatching is unclear. Thus, we studied alterations in expression of all genes potentially affected by DFB treatment using a transcriptome approach in 2-d-old C. sinensis eggs. Clean reads were assembled to generate 203 455 unigenes and 440 558 transcripts. A total of 4625 differently expressed genes, which included 2670 up-regulated and 1955 down-regulated unigenes, were identified. Chitin binding and chitin metabolic processes were among the most significant enriched pathways according to Gene Ontology analyses. Most of the genes that encode enzymes involved in the chitin biosynthesis pathway were unaffected, whereas genes that presumably encode cuticle proteins were up-regulated. Furthermore, altered expression patterns of 10 genes involved in the chitin biosynthesis pathway of C. sinensis embryos were observed in response to DFB treatment at different time points by quantitative reverse transcription polymerase chain reaction. We also observed abnormal development; there was reduced chitin content and modulated chitin distribution of newly hatched larvae, and altered egg hatching. Our findings illustrate an ovicidal effect of DFB on C. sinensis, and reveal more molecular consequences of DFB treatment on insects.

Compatibility between Leptolegnia chapmanii and diflubenzuron and neem oil for the control of Aedes aegypti / Compatibilidad de Leptolegnia chapmanii con diflubenzurón y aceite de neem parael control de Aedes aegypti

Abstract Leptolegnia chapmanii is pathogenic to mosquito larvae. The compatibility betweenL. chapmanii and two insect growth regulators (IGR), diflubenzuron and neem oil, was evaluated. L. chapmanii was grown on culture media containing different concentrations of eachIGR. The mycelial growth was significantly reduced with the highest concentrations of IGR(F6,98= 268, p < 0.05). However, the production of zoospores (F6,56= 0.93, p > 0.05) and the lar-val mortality of Aedes aegypti (F6,56= 0.95, p > 0.05) were not significantly different amongtreatments. Furthermore, the percentage of adult emergence in the presence of different con-centrations of diflubenzuron or a neem formulation was determined, and the pathogenic activityof zoospores was evaluated at the concentrations that inhibit the emergence at 30, 50 and 90%.The pathogenicity of zoospores was not significantly different among treatments (F6,14= 0.54,p > 0.05), and the larval mortalities were above 90% in all cases.

Resumen Leptolegnia chapmanii es un patógeno de larvas de mosquitos. Se evaluó la com-patibilidad entre L. chapmanii y dos reguladores del crecimiento de insectos (IGR, por sus siglasen inglés), diflubenzurón y aceite de neem. L. chapmanii creció en medios de cultivo con diferentes concentraciones de estos IGR. El crecimiento micelial fue menor con las concentraciones más altas (F6,98= 268; p < 0,05). No hubo diferencias significativas en la producción de zoosporas (F6,56= 0,93; p > 0,05) ni en la mortalidad de las larvas de Aedes aegypti (F6,56= 0,95; p > 0,05).Se determinó el porcentaje de emergencia de adultos en presencia de diferentes concentra-ciones de diflubenzurón o de un formulado a base del aceite de neem. También se evaluó lapatogenicidad de las zoosporas a las concentraciones de los IGR que inhibieron la emergenciadel 30, 50 y 90% de adultos. No hubo diferencias significativas en la actividad de las zoosporasal comparar los tratamientos (F6,14= 054; p > 0,05). La mortalidad de las larvas fue superior al90%.

Bioassay and molecular monitoring of insecticide resistance status in Aedes albopictus populations from Greece, to support evidence-based vector control.

BACKGROUND: Aedes albopictus has a well-established presence in southern European countries, associated with recent disease outbreaks (e.g. chikungunya). Development of insecticide resistance in the vector is a major concern as its control mainly relies on the use of biocides. Data on the species' resistance status are essential for efficient and sustainable control. To date the insecticide resistance status of Ae. albopictus populations from Greece against major insecticides used in vector control remains largely unknown. METHODS: We investigated the insecticide resistance status of 19 Ae. albopictus populations from 11 regions of Greece. Bioassays were performed against diflubenzuron (DFB), Bacillus thuringiensis var. israelensis (Bti), deltamethrin and malathion. Known insecticide resistance loci were molecularly analysed, i.e. voltage-gated sodium channel (VGSC) mutations associated with pyrethroid resistance; presence and frequency of carboxylesterases 3 (CCEae3a) and 6 (CCEae6a) gene amplification associated with organophosphate (OP) resistance and; chitin synthase-1 (CHS-1) for the possible presence of DFB resistance mutations. RESULTS: Bioassays showed full susceptibility to DFB, Bti and deltamethrin, but resistance against the OP malathion (range of mortality: 55.30-91.40%). VGSC analysis revealed a widespread distribution of the mutations F1534C (in all populations, with allelic frequencies between 6.6-68.3%), and I1532T (in 6 populations; allelic frequencies below 22.70%), but absence of V1016G. CCE gene amplifications were recorded in 8 out of 11 populations (overall frequency: 33%). Co-presence of the F1534C mutation and CCEae3a amplification was reported in 39 of the 156 samples analysed by both assays. No mutations at the CHS-1 I1043 locus were detected. CONCLUSIONS: The results indicate: (i) the suitability of larvicides DFB and Bti for Ae. albopictus control in Greece; (ii) possible incipient pyrethroid resistance due to the presence of kdr mutations; and (iii) possible reduced efficacy of OPs, in a scenario of re-introducing them for vector control. The study highlights the need for systematic resistance monitoring for developing and implementing appropriate evidence-based control programmes.

Identification and detection of a novel point mutation in the Chitin Synthase gene of Culex pipiens associated with diflubenzuron resistance.

BACKGROUND: Diflubenzuron (DFB) is one of the most used insecticides in mosquito larval control including that of Culex pipiens, the proven vector of the recent West Nile Virus epidemics in Europe. Two mutations (I1043L and I1043M) in the chitin synthase (CHS) putative binding site of DFB have been previously reported in Cx. pipiens from Italy and associated with high levels of resistance against this larvicide. METHODOLOGY/PRINCIPAL FINDINGS: Here we report the identification of a third mutation at the same I1043 position of the CHS gene resulting in the substitution of Isoleucine to Phenylalanine (I1043F). This mutation has also been found in agricultural pests and has been functionally validated with genome editing in Drosophila, showing to confer striking levels (>15,000 fold) of DFB resistance. The frequency of the I1043F mutation was found to be substantially higher in Cx. pipiens mosquitoes surviving DFB doses largely exceeding the recommended field dose, raising concerns about the future efficient use of this insecticide. We monitored the presence and frequency of DFB mutations in Cx. pipiens mosquitoes from several Mediterranean countries, including Italy, France, Greece, Portugal and Israel. Among the Cx. pipiens populations collected in Northern Italy all but one had at least one of the three DFB mutations at allele frequencies reaching 93.3% for the I1043M, 64.8% for the I1043L and 10% for the I1043F. The newly reported I1043F mutation was also identified in two heterozygote individuals from France (4.2% allelic frequency). In contrast to Italy and France, no DFB resistant mutations were identified in the Cx. pipiens mosquitoes sampled from Greece, Portugal and Israel. CONCLUSIONS/SIGNIFICANCE: The findings of our study are of major concern for mosquito control programs in Europe, that rely on the use of a limited number of available larvicides, and highlight the necessity for the development of appropriate Insecticide Resistance Management (IRM) programs, to ensure the sustainable use of DFB.

Effects of diflubenzuron on associated insect fauna with Anopheles(Diptera: Culicidae) in laboratory, partial-field, and field conditions in the Central Amazon.

This study aimed to analyze the possible effects of diflubenzuron on the associated insect fauna under laboratory, semi-field and field conditions. Laboratory bioassays were performed in aquariums with mortality readings (%) every 24h until 96h, and in semi-fields, insects were kept in basins with readings every 24h for up to 12 days, in triplicates and a control. In the field experiment, a collection was performed before the application of diflubenzuron in ten brick factory pits (25m²) and 15 post-application. The values of LC50 and LC90 for Chironomus were 2.77x10-3g/L and 0.86g/L, respectively, and for Buenoa, they were 0.019g/L and 0.92g/L, a strong relationship was observed between mortality and exposure time (r²>0.8) in all concentrations used. In semi-field, similar mean values of emergency inhibition were observed, except for Euthyplociidae, which was more sensitive. There was no significant difference between the data of richness and diversity of aquatic insects in the field experiment, considering an interval of 15 days (p=0.32). Finally, the data suggest that diflubenzuron may have a negative effect on the associated insect fauna, but in the field experiment the environmental conditions of Anopheles breeding sites may have affected its efficiency.

Compatibility between Leptolegnia chapmanii and diflubenzuron and neem oil for the control of Aedes aegypti.

Leptolegnia chapmanii is pathogenic to mosquito larvae. The compatibility between L. chapmanii and two insect growth regulators (IGR), diflubenzuron and neem oil, was evaluated. L. chapmanii was grown on culture media containing different concentrations of each IGR. The mycelial growth was significantly reduced with the highest concentrations of IGR (F6,98=268, p<0.05). However, the production of zoospores (F6,56=0.93, p>0.05) and the larval mortality of Aedes aegypti (F6,56=0.95, p>0.05) were not significantly different among treatments. Furthermore, the percentage of adult emergence in the presence of different concentrations of diflubenzuron or a neem formulation was determined, and the pathogenic activity of zoospores was evaluated at the concentrations that inhibit the emergence at 30, 50 and 90%. The pathogenicity of zoospores was not significantly different among treatments (F6,14=0.54, p>0.05), and the larval mortalities were above 90% in all cases.

Detection of diflubenzuron and pyrethroid resistance mutations in Culex pipiens from Mugla, Turkey.

Chemical insecticides are the main tool for vector control worldwide with pyrethroids and the larvicide diflubenzuron (DFB) used extensively against the primary West Nile virus vector Culex pipiens. In this study we monitored the presence, frequency and geographical distribution of the Chitin synthase 1 gene mutations I1043L, I1043M associated with striking diflubenzuron resistance and the knock down resistance mutation L1014F associated with pyrehtroid resistance, in Cx. pipiens populations from Turkey. DFB resistance mutations I1043L, I1043M were found at high frequencies ranging from 15.7% to 37.5% and 25% to 52.7% in all study sites (first report for the presence of DFB resistant mutations, anywhere outside Italy). The L1014F pyrethroid resistance mutation was also present at relatively high frequencies, ranging from 40% to 50%. The high frequency of both DFB and pyrethroid resistance mutations are possibly associated with the intense use of these insecticides in agricultural and mosquito control applications in the region. Given the limited number of available insecticides for vector control these findings are of major importance for public health in Turkey and neighboring countries.

Transcriptional plasticity of different ABC transporter genes from Tribolium castaneum contributes to diflubenzuron resistance.

The development of insecticide resistance challenges the sustainability of pest control and several studies have shown that ABC transporters contribute to this process. ABC transporters are known to transport a large range of chemically diverse molecules across cellular membranes, and therefore the identification of ABC transporters involved in insecticide resistance is difficult. Here, we describe a comprehensive strategy for the identification of whole sets of ABC transporters involved in insecticide resistance using the pest beetle, Tribolium castaneum (Tc) as a model. We analyzed the expression of ABCA to ABCC genes in different tissues and developmental stages using larvae that were sensitive or resistant to diflubenzuron (DFB). The mRNA levels of several ABC genes expressed in excretory or metabolic tissues such as midgut, Malpighian tubules or fat body were markedly upregulated in response to DFB. Next, we monitored mortality in the presence of the ABC inhibitor verapamil, and found that it causes sensitization to DFB. We furthermore established a competitive assay for the elimination of DFB, based on Texas Red (TR) fluorescence. We monitored TR elimination in larvae that were treated with DFB or different ABC inhibitors, and combinations of them. TR elimination was decreased significantly in the presence of DFB, verapamil and the ABCC inhibitor MK-571. The effect was synergized when DFB and verapamil were both present suggesting that the transport of TR and DFB involves overlapping sets of ABC transporters. Finally, we silenced the expression of DFB-responding ABC genes by RNA interference and then followed the survival rates after DFB exposure. Mortality increased particularly when specific ABCA and ABCC genes were silenced. Taken together, we were able to show that different ABC transporters expressed in metabolic and excretory tissues contribute to the elimination of DFB. Up- or down-regulation of gene expression occurs within a few days already at very low DFB concentrations. These results suggests that transcriptional plasticity of several ABC genes allows adaptation of the efflux capacity in different tissues to eliminate insecticides and/or their metabolites.

Effects of chitin synthesis inhibitor treatment on Lepeophtheirus salmonis (Copepoda, Caligidae) larvae.

The salmon louse (Lepeophtheirus salmonis) is an ectoparasite infecting Atlantic salmon (Salmo salar), which causes substantial problems to the salmon aquaculture and threatens wild salmon. Chitin synthesis inhibitors (CSIs) are used to control L. salmonis in aquaculture. CSIs act by interfering with chitin formation and molting. In the present study, we investigated the action of four CSIs: diflubenzuron (DFB), hexaflumuron (HX), lufenuron (LF), and teflubenzuron (TFB) on larval molt. As the mode of action of CSIs remains unknown, we selected key enzymes in chitin metabolism and investigated if CSI treatment influenced the transcriptional level of these genes. All four CSIs interfered with the nauplius II molt to copepodids in a dose-dependent manner. The EC50 values were 93.2 nM for diflubenzuron, 1.2 nM for hexaflumuron, 22.4 nM for lufenuron, and 11.7 nM for teflubenzuron. Of the investigated genes, only the transcriptional level of L. salmonis chitin synthase 1 decreased significantly in hexaflumuron and diflubenzuron-treated larvae. All the tested CSIs affected the molt of nauplius II L. salmonis larvae but at different concentrations. The larvae were most sensitive to hexaflumuron and less sensitive to diflubenzuron. None of the CSIs applied had a strong impact on the transcriptional level of chitin synthesis or chitinases genes in L. salmonis. Further research is necessary to get more knowledge of the nature of the inhibition of CSI and may require methods such as studies of protein structure and enzymological studies.

Silencing of the Chitin Synthase Gene Is Lethal to the Asian Citrus Psyllid, Diaphorina citri.

Chitin synthase is a critical enzyme that catalyzes N-acetylglucosamine to form chitin, which plays an important role in the growth and development of insects. In this study, we identified a chitin synthase gene (CHS) with a complete open reading frame (ORF) of 3180 bp from the genome database of Diaphorina citri, encoding a protein of 1059 amino acid residues with the appropriate signature motifs (EDR and QRRRW). Reverse transcription-quantitative PCR (RT-qPCR) analysis suggested that D. citri CHS (DcCHS) was expressed throughout all developmental stages and all tissues. DcCHS had the highest expression level in the integument and fifth-instar nymph stage. Furthermore, the effects of diflubenzuron (DFB) on D. citri mortality and DcCHS expression level were investigated using fifth-instar nymph through leaf dip bioassay, and the results revealed that the nymph exposed to DFB had the highest mortality compared with control group (Triton-100). Silencing of DcCHS by RNA interference resulted in malformed phenotypes and increased mortality with decreased molting rate. In addition, transmission electron microscopy (TEM) and fluorescence in situ hybridization (FISH) also revealed corresponding ultrastructural defects. Our results suggest that DcCHS might play an important role in the development of D. citri and can be used as a potential target for psyllid control.

RNAi-based bioinsecticide for Aedes mosquito control.

Zika virus infection and dengue and chikungunya fevers are emerging viral diseases that have become public health threats. Their aetiologic agents are transmitted by the bite of genus Aedes mosquitoes. Without effective therapies or vaccines, vector control is the main strategy for preventing the spread of these diseases. Increased insecticide resistance calls for biorational actions focused on control of the target vector population. The chitin required for larval survival structures is a good target for biorational control. Chitin synthases A and B (CHS) are enzymes in the chitin synthesis pathway. Double-stranded RNA (dsRNA)-mediated gene silencing (RNAi) achieves specific knockdown of target proteins. Our goal in this work, a new proposed RNAi-based bioinsecticide, was developed as a potential strategy for mosquito population control. DsRNA molecules that target five different regions in the CHSA and B transcript sequences were produced in vitro and in vivo through expression in E. coli HT115 and tested by direct addition to larval breeding water. Mature and immature larvae treated with dsRNA targeting CHS catalytic sites showed significantly decreased viability associated with a reduction in CHS transcript levels. The few larval and adult survivors displayed an altered morphology and chitin content. In association with diflubenzuron, this bioinsecticide exhibited insecticidal adjuvant properties.

Focal distribution of diflubenzuron resistance mutations in Culex pipiens mosquitoes from Northern Italy.

Insecticide resistance is a major threat for vector control and prevention of mosquito borne diseases. In the Culex pipiens mosquitoes, resistance against diflubenzuron (DFB) was firstly detected in Ravenna (Emilia-Romagna region, Northern Italy), in 2015. The resistant phenotypes were associated with two mutations, I1043 M and I1043 L, at the amino acid 1043 of the chitin synthase gene. In this study, we monitored the presence, frequency and geographical distribution of the DFB resistant mutations in Cx. pipiens populations from Northern Italy, and in populations from Greece and France. In the Emilia-Romagna region, the resistant mutations were detected in 20 out of the 30 populations analysed, reaching allelic frequencies over 70%. The presence and distribution of the resistance mutations was highly focal, with a clear pattern of increasing resistant allelic frequencies moving from the Western towards the Eastern provinces of Emilia-Romagna. Contrary to Italy, DFB resistant alleles were not detected in the Cx. pipiens mosquitoes sampled from Greece and France. Following statistical, literature and bibliographical database analyses on the history of DFB insecticide use in the study areas, we suggest that the selection pressures from the intense agricultural DFB applications occurring throughout the' 80-'90 s against orchard pests, followed, from 2000s onwards by mosquito control DFB applications, may account for the high mutation frequencies observed in the Cx. pipiens populations of the Eastern provinces of Emilia-Romagna. The findings are of major concern for public health in Italy and Europe, as DFB remains a very important insecticide used for controlling arbovirus mosquito vectors, where alternative larvicides are extremely limited.

The impact of insecticide applications on the dynamics of resistance: The case of four Aedes aegypti populations from different Brazilian regions.

BACKGROUND: In the tropics, the utilization of insecticides is still an important strategy for controlling Aedes aegypti, the principle vector of dengue, chikungunya and Zika viruses. However, increasing insecticide resistance in Ae. aegypti populations might hinder insecticide efficacy on a long-term basis. It will be important to understand the dynamics and evolution of insecticide resistance by assessing its frequency and the mechanisms by which it occurs. METHODOLOGY/PRINCIPAL FINDINGS: The insecticide resistance status of four Brazilian Ae. aegypti populations was monitored. Quantitative bioassays with the major insecticides employed in the country was performed: the adulticide deltamethrin (a pyrethroid-PY) and the larvicides, temephos (an organophosphate) and diflubenzuron (a chitin synthesis inhibitor). Temephos resistance was detected in all populations although exhibiting a slight decrease over time probably due to the interruption of field use. All vector populations were susceptible to diflubenzuron, recently introduced in the country to control Ae. aegypti. Resistance against deltamethrin was extremely high in three populations. Molecular assays investigated substitutions in the voltage gated sodium channel (NaV), the PY target site, at positions 1011, 1016 and 1534. Elevated frequencies of substitutions Val1016Ile and Phe1534Cys related to high PY resistance levels were identified. Biochemical assays detected alterations in the activities of two detoxifying enzyme classes related to metabolic resistance, glutathion-S-transferases and esterases. The results obtained were evaluated in the context of both recent insecticide use and the records of dengue incidence in each locality. CONCLUSIONS/SIGNIFICANCE: The four Ae. aegypti populations evaluated were resistant to the neurotoxic insecticides, temephos and deltamethrin. However, they were still susceptible to diflubenzuron. A probable correlation between adult insect resistance to PY and the domestic application of insecticides is discussed, pointing to the need for awareness measures regarding the correct utilization by citizens. This work aims to contribute to the efficient and rational management of Ae. aegypti control of both larvae and adults.