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

Biochemical and Molecular Analysis of Field Resistance to Spirodiclofen in Panonychus citri (McGregor)

Spirodiclofen is one of the most widely used acaricides in China. The citrus red mite, Panonychus citri (McGregor) (Acari: Tetranychidae), is one of the most destructive citrus pests worldwide and has developed a high resistance to spirodiclofen. However, the molecular mechanism of spirodiclofen resistance in P. citri is still unknown. In this study, we identified a field spirodiclofen-resistant strain (DL-SC) that showed 712-fold resistance to spirodiclofen by egg bioassay compared to the susceptible strain. Target-site resistance was not detected as non-synonymous mutations were not found by amplification and sequencing of the ACCase gene of resistant and susceptible strains; in addition, the mRNA expression levels of ACCase were similar in both resistant and susceptible strains. The activity of detoxifying enzymes P450s and CCEs in the resistant strain was significantly higher than in the susceptible strain. The transcriptome expression data showed 19 xenobiotic metabolisms genes that were upregulated. Stage-specific expression profiling revealed that the most prominent upregulated gene, CYP385C10, in transcriptome data was significantly higher in resistant strains in all stages. Furthermore, functional analysis by RNAi indicated that the mortality caused by spirodiclofen was significantly increased by silencing the P450 gene CYP385C10. The current results suggest that overexpression of the P450 gene, CYP385C10, may be involved in spirodiclofen resistance in P. citri.

Spiromesifen contributes vascular developmental toxicity via disrupting endothelial cell proliferation and migration in zebrafish embryos

Spiromesifen (SPF) is a specific contact pesticide, which has been widely used to control the growth of sucking insects like mites and whiteflies on crops. Although its residues in crops and effects on organisms has been extensively reported, its impact on the vasculature is still not being reported. In the present study, using human umbilical vein endothelial cells (HUVECs) and zebrafish embryos, we investigated the effects of SPF on blood vessel development and its mechanism of action. SPF exposure triggered abnormal blood vessel development, including vascular deletions and malformations, inhibition of CCV remodeling, and decrease of SIV areas. SPF exposure also obstructed the migration of endothelial cell from caudal hematopoietic tissue in zebrafish embryos. SPF damaged cytoskeleton, caused cell cycle arrest, inhibited the viability and migration of HUVECs. In addition, SPF also inhibited the expression of the VEGF/VEGFR pathway-related genes (hif1a, vegfa, flt1, and kdrl), cell cycle-related genes (ccnd1, ccne1, cdk2, and pcna), and Rho/ROCK pathway-related genes (itgb1, rho, rock, mlc-1, and vim-1). Taken together, SPF may inhibit the proliferation and migration of vascular endothelial cells through disturbing cytoskeleton via the Rho/ ROCK pathway, resulting in vascular malformation. Our study contributes to potential insight into the mechanism of SPF toxicity in angiocardiopathy.

Spiro Derivatives in the Discovery of New Pesticides: A Research Review

Spiro compounds are biologically active organic compounds with unique structures, found in a wide variety of natural products and drugs. They do not readily lead to drug resistance due to their unique mechanisms of action and have, therefore, attracted considerable attention regarding pesticide development. Analyzing structure-activity relationships (SARs) and summarizing the characteristics of spiro compounds with high activity are crucial steps in the design and development of new pesticides. This review mainly summarizes spiro compounds with insecticidal, bactericidal, fungicidal, herbicidal, antiviral, and plant growth regulating functions to provide insight for the creation of new spiro compound pesticides.

Long-term studies on the evolution of resistance of Myzus persicae (Hemiptera: Aphididae) to insecticides in Greece

The aphid Myzus persicae s.l. (Hemiptera: Aphididae) is an important pest of many crops worldwide with a complex life cycle, intensely controlled by chemical pesticides, and has developed resistance to almost all used insecticides. In Greece, the aphid exhibits high genetic variation and adaptability and it is a classic example of evolution in the making. We have been studying M. persicae for over 20 years, on different host plants and varying geographical areas, analyzing its bio-ecology and the ability to develop resistance to insecticides. In this review, we present new and historical data on the effectiveness of insecticides from seven chemical groups used to control the aphid in Greece and the incidence of seven resistance mechanisms, including the new fast-spreading R81T point mutation of the postsynaptic nicotinic acetylcholine receptor. Thousands of samples were tested by biological, biochemical and molecular assays. The aphid populations were found to have developed and maintain resistance at medium to high levels to organophosphates, carbamates, pyrethroids and neonicotinoids for decades. In the latter group, a marked increase is recorded during an ~10-year period. The data analyzed and the extensive bibliography, advocate the difficulty to control the aphid making the design and application of IPM/IRM programs a challenge. We discuss principles and recommendations for the management of resistance, including the use of compounds such as flonicamid, spirotetramat, flupyradifurone and sulfoxaflor. We emphasize that resistance is a dynamic phenomenon, changing in time and space, requiring, therefore, continuous monitoring.

Fitness cost, realized heritability and stability of resistance to spiromesifen in house fly, Musca domestica L. (Diptera: Muscidae)

The house fly, Musca domestica L. (Diptera: Muscidae), is an insect pest of public health and veterinary importance. Spiromesifen is a new chemistry insecticide widely used for the management of sucking insect pests of vegetables and crops. In the present study, assessment of resistance risk and fitness costs associated with spiromesifen resistance in M. domestica was studied. Moreover, stability of resistance to spiromesifen and other tested insecticides (fipronil, spinosad, and bifenthrin) was evaluated in the spiromesifen-selected-strain (SPIRO-SEL-POP). After 7-generations of selection with spiromesifen, SPIRO-SEL-POP developed 108.76-fold resistance compared with the unselected strain (UNSEL-POP). The estimated value of realized heritability was 0.59 for spiromesifen resistance. Due to withdrawal of spiromesifen selection for five generations (F₆-F₁₀) on SPIRO-SEL-POP, a decline in LC₅₀ values against spiromesifen, spinosad and bifenthrin was 0.16, 0.14 and 0.13-folds, respectively. In biological trait experiments, larval weight of Cross₁ (SPIRO-SEL-POP♀ × UNSEL-POP♂) and SPIRO-SEL-POP was significantly lower than that of Cross₂ (SPIRO-SEL-POP♂ × UNSEL-POP♀) and UNSEL-POP. Pupal weight of SPIRO-SEL-POP was higher when compared with Cross₁ while it was similar to that of Cross₂ and UNSEL-POP. Adult emergence rate of UNSEL-POP was higher than Cross₁, but similar to the Cross₂ and SPIRO-SEL-POP. The SPIRO-SEL-POP and Cross₁ showed the lowest relative fitness when compared with USEL-POP and Cross₂. Intrinsic rate of natural increase of SPIRO-SEL-POP was much lower than that of UNSEL-POP and Cross₂ followed by Cross₁. The SPIRO-SEL-POP exhibited lower biotic potential when compared with UNSEL-POP and Cross₂ but similar to Cross₁. Fecundity and hatching rates were lower in SPIRO-SEL-POP compared to UNSEL-POP. It could be concluded that spiromesifen resistance in M. domestica comes with a cost and is instable. Therefore, spiromesifen rotation with other insecticides and withdraw of its usage for some period could help to sustain its efficacy by delaying the development of resistance.

Inhibition of acetyl-CoA carboxylase by spirotetramat causes growth arrest and lipid depletion in nematodes

Plant-parasitic nematodes pose a significant threat to agriculture causing annual yield losses worth more than 100 billion US$. Nematode control often involves the use of nematicides, but many of them including non-selective fumigants have been phased out, particularly due to ecotoxicological concerns. Thus new control strategies are urgently needed. Spirotetramat (SPT) is used as phloem-mobile systemic insecticide targeting acetyl-CoA carboxylase (ACC) of pest insects and mites upon foliar application. However, in nematodes the mode of action of SPT and its effect on their development have not been studied so far. Our studies revealed that SPT known to be activated in planta to SPT-enol acts as a developmental inhibitor of the free-living nematode Caenorhabditis elegans and the plant-parasitic nematode Heterodera schachtii. Exposure to SPT-enol leads to larval arrest and disruption of the life cycle. Furthermore, SPT-enol inhibits nematode ACC activity, affects storage lipids and fatty acid composition. Silencing of H. schachtii ACC by RNAi induced similar phenotypes and thus mimics the effects of SPT-enol, supporting the conclusion that SPT-enol acts on nematodes by inhibiting ACC. Our studies demonstrated that the inhibition of de novo lipid biosynthesis by interfering with nematode ACC is a new nematicidal mode of action addressed by SPT, a well-known systemic insecticide for sucking pest control.

Using CRISPR/Cas9 genome modification to understand the genetic basis of insecticide resistance: Drosophila and beyond

Chemical insecticides are a major tool for the control of many of the world's most damaging arthropod pests. However, their intensive application is often associated with the emergence of resistance, sometimes with serious implications for sustainable pest control. To mitigate failure of insecticide-based control tools, the mechanisms by which insects have evolved resistance must be elucidated. This includes both identification and functional characterization of putative resistance genes and/or mutations. Research on this topic has been greatly facilitated by using powerful genetic model insects like Drosophila melanogaster, and more recently by advances in genome modification technology, notably CRISPR/Cas9. Here, we present the advances that have been made through the application of genome modification technology in insecticide resistance research. The majority of the work conducted in the field to date has made use of genetic tools and resources available in D. melanogaster. This has greatly enhanced our understanding of resistance mechanisms, especially those mediated by insensitivity of the pesticide target-site. We discuss this progress for a series of different insecticide targets, but also report a number of unsuccessful or inconclusive attempts that highlight some inherent limitations of using Drosophila to characterize resistance mechanisms identified in arthropod pests. We also discuss an experimental framework that may circumvent current limitations while retaining the genetic versatility and robustness that Drosophila has to offer. Finally, we describe examples of direct CRISPR/Cas9 use in non-model pest species, an approach that will likely find much wider application in the near future.

Identification and functional characterization of a novel acetyl-CoA carboxylase mutation associated with ketoenol resistance in Bemisia tabaci

Insecticides of the tetronic/tetramic acid family (cyclic ketoenols) are widely used to control sucking pests such as whiteflies, aphids and mites. They act as inhibitors of acetyl-CoA carboxylase (ACC), a key enzyme for lipid biosynthesis across taxa. While it is well documented that plant ACCs targeted by herbicides have developed resistance associated with mutations at the carboxyltransferase (CT) domain, resistance to ketoenols in invertebrate pests has been previously associated either with metabolic resistance or with non-validated candidate mutations in different ACC domains. A recent study revealed high levels of spiromesifen and spirotetramat resistance in Spanish field populations of the whitefly Bemisia tabaci that was not thought to be associated with metabolic resistance. We confirm the presence of high resistance levels (up to >640-fold) against ketoenol insecticides in both Spanish and Australian B. tabaci strains of the MED and MEAM1 species, respectively. RNAseq analysis revealed the presence of an ACC variant bearing a mutation that results in an amino acid substitution, A2083V, in a highly conserved region of the CT domain. F1 progeny resulting from reciprocal crosses between susceptible and resistant lines are almost fully resistant, suggesting an autosomal dominant mode of inheritance. In order to functionally investigate the contribution of this mutation and other candidate mutations previously reported in resistance phenotypes, we used CRISPR/Cas9 to generate genome modified Drosophila lines. Toxicity bioassays using multiple transgenic fly lines confirmed that A2083V causes high levels of resistance to commercial ketoenols. We therefore developed a pyrosequencing-based diagnostic assay to map the spread of the resistance alleles in field-collected samples from Spain. Our screening confirmed the presence of target-site resistance in numerous field-populations collected in Sevilla, Murcia and Almeria. This emphasizes the importance of implementing appropriate resistance management strategies to prevent or slow the spread of resistance through global whitefly populations.

UDP-glycosyltransferases contribute to spirotetramat resistance in Aphis gossypii Glover

Uridine diphosphate (UDP)-glycosyltransferases (UGTs) catalyze the conjugation of small lipophilic endogenous and exogenous compounds with sugars to produce water-soluble glycosides, playing an important role in insect endobiotic regulation and xenobiotic detoxification. In this study, two UGT-inhibitors, sulfinpyrazone and 5-nitrouracil, significantly increased spirotetramat toxicity against third instar nymphs of resistant Aphis gossypii, whereas there were no synergistic effects in apterous adult aphids, suggesting UGT involvement in spirotetramat resistance in cotton aphids. Furthermore, the UHPLC-MS/MS was employed to determine the content of spirotetramat and its four metabolites (S-enol, S-glu, S-mono, S-keto) in the honeydew of resistant cotton aphids under spirotetramat treatment. No residual spirotetramat was detected in the honeydew, while its four metabolites were detected at a S-enol: S-glu: S-mono: S-keto ratio of 69.30: 6.54: 1.44: 1.00. Therefore, glycoxidation plays a major role in spirotetramat inactivation and excretion in resistant aphids. Compared with the susceptible strain, the transcriptional levels of UGT344M2 were significantly upregulated in nymphs and adults of the resistant strain. RNA interference of UGT344M2 dramatically increased spirotetramat toxicity in nymphs, but no such effect were found in the resistant adult aphids. Overall, UGT-mediated glycoxidation were found to be involved in spirotetramat resistance. The suppression of UGT344M2 significantly increased the sensitivity of resistant nymphs to spirotetramat, suggesting that UGT344M2 upregulation might be associated with spirotetramat detoxification. This study provides an overview of the involvement of metabolic factors, UGTs, in the development of spirotetramat resistance.

Laboratory and field evaluation of the aphidicidal activity of moso bamboo (Phyllostachys pubescens) leaf extract and identification of the active components

BACKGROUND

Botanical pesticides increasingly play important roles in the control of agricultural pests. In this study, the aphidicidal effect of moso bamboo (Phyllostachys pubescens) extract against mustard aphid was confirmed, the main active compounds identified, and aphidicidal mechanism of the most active compound established.

RESULTS

When the treatment concentration was 10.0 g L^-1 , the corrected mortality of bamboo leaf extract (BE) was 53.22 ± 5.20% and the petroleum ether component of bamboo leaf extract (PE) reached 82.76 ± 4.50%, which also showed a synergistic effect with imidacloprid. Four flavonoids were identified as the main active components in the BE via activity tracking and phytochemical method. Isoorientin had an LC₅₀ of 313.22 mg L^-1 , and affected the activities of acetylcholinesterase and peroxidase significantly, revealing the possible aphidicidal mechanism. When the treatment of 11.1% PE·imidacloprid was 200 mL, the control effect was 99.07%, which was better than that observed with 10% of imidacloprid or 0.5% of matrine.

CONCLUSIONS

These data provide a better understanding of the aphidicidal activity and aphidicidal mechanism of moso bamboo leaf extract and the most active compound, isoorientin. This will help in developing a more effective botanical aphicide. © 2019 Society of Chemical Industry.

The lipid metabolism alteration of three spirocyclic tetramic acids on zebrafish (Danio rerio) embryos

Spirocyclic tetramic acids are widely used in controlling phytophagous mite species throughout the world. the data set is incomplete and provides insufficient evidence for drawing the same conclusion for fish. To fill the gap whether these acaricides alter lipid metabolism on vertebrates, zebrafish embryos exposed to a series concentration of pesticides, the developmental effects, enzyme activities and levels of gene expression were assessed, battery of biomarker utilized by the integrated biomarker response (IBRv2) model. The 96 h-LC₅₀ of spirodiclofen, spiromesifen and spirotetramat were 0.14, 0.12 and 5.94 mg/L, respectively. Yolk sac deformity, pericardial edema, spinal curvature and tail malformation were observed. Three spirocyclic acids were unfavouring the lipid accumulation of by inhibited the acetyl-CoA carboxylase (ACC), fatty acid synthesis (FAS), fatty acid binding proteins (FABP2) and lipoprotein lipase (LPL) activity. The total cholesterol (TCHO) level significantly decreased in the 0.072 mg/L spirodiclofen group and 0.015 and 0.030 mg/L in the spiromesifen groups. No expected change in spirotetramat group on the TCHO and triglycerides (TGs) levels for any of the treatments. The mRNA levels of the genes related to lipid metabolism also significantly altered. In both spirodiclofen and spiromesifen, ACC achieved the highest scores among a battery of biomarkers using integrated biomarker response (IBRv2). The results suggest that spiromesifen was the most toxic for embryos development and spirodiclofen was the most toxic for lipid metabolism in embryos. The 0.07 mg/L of spirodiclofen, 0.05 mg/L of spiromesifen and 2.00 mg/L would cause malformation on zebrafish embryos. This study will provide new insight that fatty acid metabolism may be a suitable biomarker for the spirocyclic tetramic acids in fish species.

Spiromesifen and spirotetramat resistance in field populations of Bemisia tabaci Gennadius in Spain

BACKGROUND

Spiromesifen and spirotetramat are novel insecticides belonging to the chemical class of tetronic and tetramic acid derivatives. Both compounds have proven very effective against field populations of Bemisia tabaci around the world. However, several growers have recently reported control failures in Spain. Therefore, we studied the resistance level to these insecticides in field populations reporting control failures. In addition, we further selected a spiromesifen-resistant strain to study the mechanisms involved and the cross-resistance pattern.

RESULTS

All the new field populations collected were significantly more resistant to spiromesifen than the susceptible population, confirming the presence of resistance. Several populations showing high levels of resistance to spiromesifen (>10 000-fold), exhibited cross-resistance to spirotetramat, but resistance ratios were much lower (130-fold). The spiromesifen laboratory-selected strain was very resistant to spiromesifen (LC₅₀ > 30 000 mg L^-1 ) and spirotetramat (LC₅₀ = 368.1 mg L^-1 ), but lacks any cross-resistance to other insecticides, thus providing options for resistance management. None of the synergists tested significantly restored the susceptibility of B. tabaci to either spiromesifen or spirotetramat.

CONCLUSION

This is the first report of resistance to spiromesifen and spirotetramat in B. tabaci, and such high levels of resistance have not been reported before in any field collected pest. Our results suggest that enhanced detoxification does not critically contribute to resistance to ketoenols in B. tabaci. The obvious lack of a metabolic resistance mechanism either suggests a target-site resistance mechanism or a metabolic mechanism insensitive to the synergists tested. © 2018 Society of Chemical Industry.

Baseline susceptibility of Mediterranean strains of Trialeurodes vaporariorum (Westwood) to cyantraniliprole

BACKGROUND

Cyantraniliprole is a novel anthranilic diamide insecticide that acts on a broad spectrum of insect pests, exclusively by activating their ryanodine receptors. Cyantraniliprole is very effective against whitefly and it presents a favorable ecotoxicological profile. In this study, the baseline susceptibility to cyantraniliprole of Trialeurodes vaporariorum populations from the Mediterranean area was established in nymphal systemic uptake bioassays.

RESULTS

The bioassay data showed that the susceptibility to cyantraniliprole varied among the strains collected across the Mediterranean basin. The 50% lethal concentration (LC₅₀ ) range of cyantraniliprole for 16 field populations was from 0.017 to 0.194 mg L^-1 , a 11.4-fold natural variability between the least and most sensitive populations. These LC₅₀ values are similar to those reported in a previous study of the use of cyantraniliprol against another species of whitefly, Bemisia tabaci [LC₅₀  = 0.048 (0.034-0.063) mg L^-1 ].

CONCLUSION

The current study confirmed the effectiveness of cyantraniliprole against T. vaporarioum strains, adding to the evidence that cyantraniliprole is a promising tool for use in integrated pest management programs. Future shifts in the susceptibility of whitefly field populations to cyantraniliprole may be documented according to the baseline susceptibility range of the populations tested in this research. © 2018 Society of Chemical Industry.

Insecticide resistance in Trialeurodes vaporariorum populations and novel diagnostics for kdr mutations

BACKGROUND

Neonicotinoids, pyrethroids and ketoenols are currently used for the control of Trialeurodes vaporariorum (Hemiptera: Aleyrodidae). In this study, insecticide resistance status and mechanisms were investigated using classical bioassays and molecular techniques.

RESULTS

Dose-response bioassays were performed on 19 Greek populations, among the 35 different whitefly populations used for the whole analysis. Resistance factors scaled up to 207-, 4657- and 59-fold for imidacloprid, bifenthrin and spiromesifen, respectively. Molecular assays were used to investigate the frequency of known resistance mutations. A simple polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay was developed for detecting the pyrethroid-resistant alleles r1 (mutation L925I) and r2 (mutation T929I) of the para-type voltage-gated sodium channel gene (VGSC). Both alleles were present at high frequencies (on average 65% and 33%, respectively) in 14 populations from Greece. The M918 L pyrethroid resistance mutation was not detected in any of the Greek populations. Sequencing and a Taqman allelic discrimination were used to monitor the frequency of the mutation E645K of the acetyl-coenzyme A carboxylase gene (ACC) recently linked to spiromesifen resistance. This mutation was detected in 20 of the 24 populations examined in ∼38% frequency among the 433 individuals tested. However, its association with the spiromesifen resistance phenotype was not confirmed in the Greek populations. Finally, two homologues of the CYP6CM1 Bemisia tabaci P450, the known neonicotinoid metabolizer, were found upregulated in two T. vaporariorum neonicotinoid-resistant populations; they were both functionally expressed in Escherichia coli, but the recombinant proteins encoded did not metabolize those neonicotinoid insecticides tested.

CONCLUSION

The development of simple diagnostics and their use alongside classical and molecular techniques for the early detection of resistant populations are of great importance for pest management strategies. The practical implications of our results are discussed in light of whitefly control. © 2017 Society of Chemical Industry.

Evolution of imidacloprid resistance in Myzus persicae in Greece and susceptibility data for spirotetramat

BACKGROUND

Myzus persicae s.l. is a major crop pest globally and has evolved resistance to a range of insecticide classes making it increasingly difficult to control in some areas. Here we compare bioassay monitoring data for two important compounds, imidacloprid and spirotetramat, on field samples/clones collected in Greece.

RESULTS

A total of 122 aphid samples/clones from central and northern Greece were examined in dose-response bioassays with imidacloprid. There was an overall increase in the level of resistance (resistance factor = 15-40) within tobacco-collected samples from 78.7% in 2007 to 86.7% in 2015. The corresponding frequencies for peach samples were 13.3% and 6.7%. These results were confounded however by the first identification of the R81T target mutation in Greece during 2015 (4.3% as heterozygotes in peach) and 2016 (21.3% as heterozygotes in peach). No resistance to spirotetramat was found at the 60 clones collected in 2015.

CONCLUSION

Resistance to imidacloprid is continuing to increase within Greek M. persicae s.l. populations and the situation is likely to deteriorate further with the recent identification of the R81T resistance mutation. Resistance to spirotetramat has not been found and is therefore a good alternative to neonicotinoids for resistance management. © 2017 Society of Chemical Industry.

Novel mutations and expression changes of acetyl-coenzyme A carboxylase are associated with spirotetramat resistance in Aphis gossypii Glover

Acetyl-coenzyme A carboxylase (ACC) catalyses the carboxylation of acetyl-coenzyme A (acetyl-CoA) to produce malonyl-CoA during the de novo synthesis of fatty acids. Spirotetramat, an inhibitor of ACC, is widely used to control a range of sucking insects, including the Aphis gossypii. In the present study, Reverse transcription quantitative real-time PCR (RT-qPCR) results demonstrated that ACC was significantly overexpressed in a laboratory-selected spirotetramat-resistant strain compared with the susceptible strain. ACC RNA interference significantly suppressed fecundity and led to cuticle formation deficiencies in resistant adults and nymphs compared with the control. The full-length ACC gene was sequenced from both resistant and susceptible cotton aphids, and a strong association was found between spirotetramat resistance and 14 amino acid substitutions in the biotin carboxylase domain and carboxyl transferase domain of the ACC gene. Furthermore, ACC activity was higher in resistant aphids than in the susceptible strain, and ACC in the resistant aphids exhibited significant insensitivity to spirotetramat and spirotetramat-enol. The results indicate that the overexpressed insensitive (mutated) ACC target played an important role in the high levels of spirotetramat resistance observed here. This association of amino acid substitution with resistance is the first report of a potential target site mechanism affecting spirotetramat in the cotton aphid.

Molecular diagnostics for detecting pyrethroid and abamectin resistance mutations in Tetranychus urticae

Avermectin and pyrethroid resistance mutations (the G314D and the G326E in the glutamate gated chloride channels, and the F1538I in the voltage gated sodium channel) have been reported in the spider mite Tetranychus urticae, one of the most devastating pests of protected and open field crops worldwide. We developed three TaqMan molecular diagnostic assays for monitoring the presence and frequency of these mutations in T. urticae field populations. The TaqMan assays were validated against known genotypes and subsequently used to monitor the frequency of the resistance mutations in eleven T. urticae populations from Greece and Cyprus, with variable history of avermectin and pyrethroids applications. The frequency of the F1538I pyrethroid resistance mutation largely varied among samples, with highest frequencies (75%-97%) detected in four populations derived from protected and open field crops from Crete and Peloponnesus, low frequencies in three populations (2.5%-11%) from Attiki, Cyprus and Crete and not detected in four populations from Crete, Peloponnesus and Cyprus. The frequency of the abamectin resistance mutations G314D and G326E also varied across populations (from 0 to 100%), showing fixation in two populations (>97.5% for the G314D and 100% for the G326E), originating from rose greenhouses from Greece, low frequencies in three populations (5%-12.5%) also originating from rose greenhouses (Crete, Peloponnesus and Cyprus) and not detected in six populations from protected and open field vegetable crops. The TaqMan diagnostics showed higher resolution in detecting specific alleles in low frequency, compared to massive quantitative sequencing approaches previously employed. They can be used, together with classical bioassays, to support evidence - based insecticide resistance management strategies.

miR-276 and miR-3016-modulated expression of acetyl-CoA carboxylase accounts for spirotetramat resistance in Aphis gossypii Glover

Acetyl-coenzyme A carboxylase (acetyl-CoA carboxylase, ACC) catalyses the carboxylation of acetyl-CoA to produce malonyl-CoA during de novo fatty acid synthesis. A laboratory-selected spirotetramat-resistant strain (SR) of cotton aphid was used in this study. RT-qPCR results demonstrated significant increases in the levels of ACC transcript in the resistant strain compared to the susceptible strain. Depletion of overexpressed ACC transcripts by RNAi also significantly enhanced the sensitivity of the resistant aphid to spirotetramat. We hypothesized that ACC gene expression is subject to post-transcriptional regulation. To investigate the underlying mechanism, the 66 known miRNAs of Aphis gossypii were used for target prediction, eight of which were predicted to target ACC. Validation identified two miRNAs, miR-276 and miR-3016, with abundance levels that were highly inversely correlated with ACC transcript levels. This result suggests that the miRNAs miR-276 and miR-3016 may play major roles in the post-transcriptional regulation of the ACC gene. Modulation of the abundance of miR-276 and miR-3016 through addition of inhibitors/mimics of miR-276 or miR-3016 to the artificial diet significantly altered both ACC transcript levels and the tolerance of A. gossypii to spirotetramat, thus confirming the roles of these two miRNAs in the regulation of spirotetramat resistance.

Over-expression of CYP6A2 is associated with spirotetramat resistance and cross-resistance in the resistant strain of Aphis gossypii Glover

A laboratory-selected spirotetramat-resistant strain (SR) of cotton aphid developed 579-fold and 15-fold resistance to spirotetramat in adult aphids and 3rd instar nymphs, respectively, compared with a susceptible strain (SS) [26]. The SR strain developed high-level cross-resistance to alpha-cypermethrin and bifenthrin and very low or no cross-resistance to the other tested insecticides. Synergist piperonyl butoxide (PBO) dramatically increased the toxicity of spirotetramat and alpha-cypermethrin in the resistant strain. RT-qPCR results demonstrated that the transcriptional levels of CYP6A2 increased significantly in the SR strain compared with the SS strain, which was consistent with the transcriptome results [30]. The depletion of CYP6A2 transcripts by RNAi also significantly increased the sensitivity of the resistant aphid to spirotetramat and alpha-cypermethrin. These results indicate the possible involvement of CYP6A2 in spirotetramat resistance and alpha-cypermethrin cross-resistance in the cotton aphid. These together with other cross-resistance results have implications for the successful implementation of resistance management strategies for Aphis gossypii.

Transcriptome profiling of a spirodiclofen susceptible and resistant strain of the European red mite Panonychus ulmi using strand-specific RNA-seq

BACKGROUND

The European red mite, Panonychus ulmi, is among the most important mite pests in fruit orchards, where it is controlled primarily by acaricide application. However, the species rapidly develops pesticide resistance, and the elucidation of resistance mechanisms for P. ulmi has not kept pace with insects or with the closely related spider mite Tetranychus urticae. The main reason for this lack of knowledge has been the absence of genomic resources needed to investigate the molecular biology of resistance mechanisms.

RESULTS

Here, we provide a comprehensive strand-specific RNA-seq based transcriptome resource for P. ulmi derived from strains susceptible and resistant to the widely used acaricide spirodiclofen. From a de novo assembly of the P. ulmi transcriptome, we manually annotated detoxification enzyme families, target-sites of commonly used acaricides, and horizontally transferred genes implicated in plant-mite interactions and pesticide resistance. In a comparative analysis that incorporated sequences available for Panonychus citri, T. urticae, and insects, we identified radiations for detoxification gene families following the divergence of Panonychus and Tetranychus genera. Finally, we used the replicated RNA-seq data from the spirodiclofen susceptible and resistant strains to describe gene expression changes associated with resistance. A cytochrome P450 monooxygenase, as well as multiple carboxylcholinesterases, were differentially expressed between the susceptible and resistant strains, and provide a molecular entry point for understanding resistance to spirodiclofen, widely used to control P. ulmi populations.

CONCLUSIONS

The new genomic resources and data that we present in this study for P. ulmi will substantially facilitate molecular studies of underlying mechanisms involved in acaricide resistance.

Spirotetramat resistance adaption analysis of Aphis gossypii Glover by transcriptomic survey

A resistant strain of the cotton aphid (SR) developed 441.26-fold and 11.97-fold resistance to spirotetramat for adult aphids and nymphs, respectively, compared with the susceptible (SS) strain. Solexa sequencing technology was employed to identify differentially expressed genes (DEGs) in the spirotetramat-resistant cotton aphid. Respective totals of 22,430,522 and 21,317,732 clean reads were obtained from SR and SS cDNA libraries and assembled into 35,222 non-redundant (Nr) consensus sequences. A total of 14,913, 9,220, 7,922, 4,314 and 4,686 sequences were annotated using Nr, Swiss-Prot, Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and Clusters of Orthologous Groups (COG), respectively. Compared with the SS strain, the SR strain had 1287 significantly changed unigenes, of which 130 genes were up-regulated and 1157 genes were down-regulated (P ≤ 0.001). Among these genes, 440 unigenes were annotated, consisting of 114 up-regulated and 326 down-regulated genes. The expression levels of heat shock protein 70 (Hsp70) and UDP-glucuronosyltransferase were significantly up-regulated in the SR strain compared to the SS strain. The genes encoding cuticle proteins, salivary glue protein, fibroin heavy chain, energy ATP synthase, and cytochrome c oxidase were dramatically decreased. Among the DEGs, cytochrome P450 6A2 (c20965.graph_c0) was the only P450 gene up-regulated in the SR strain. The expression levels of 10 DEGs were confirmed by real-time qPCR, and the trends in gene expression observed by qPCR matched those of the Solexa expression profiles. The acetyl-CoA carboxylase (ACC) genes in the SR and SS libraries both contain four single nucleotide polymorphisms (SNPs), with three common SNPs: 1227 (C/T), 1811 (A/T: F/Y) and 3759 (C/T); however, 7540 (A/T) and 108 (G/A) occurred solely in the SS and SR strains, respectively.

Acute Toxicity and Sublethal Effects of Fenpyroximate to Amblyseius swirskii (Acari: Phytoseiidae)

Knowledge about the effects of pesticides on biological control agents is required in order to successfully implement integrated pest management programs. The predatory mite Amblyseius swirskii Athias-Henriot has been used to control thrips, whiteflies, and broad mites in vegetable production; however, effects of fenpyroximate, an acaricide and insecticide used in vegetable crops, on A. swirskii have not been evaluated. The effect of four residual concentrations of fenpyroximate on A. swirskii females was measured under laboratory conditions including its effect on their fecundity and larval survival. Fresh residues of fenpyroximate were significantly toxic to adult females and larvae. Mortality increased and fecundity decreased as the concentration (0.026-0.208 ml/50 ml of water) and time after treatment (24-120 h) increased. Fifty percent of the larvae survived on the two lower concentrations (0.026 and 0.052 ml/50 ml of water) after 120 h.

Genotype to phenotype, the molecular and physiological dimensions of resistance in arthropods

The recent accumulation of molecular studies on mutations in insects, ticks and mites conferring resistance to insecticides, acaricides and biopesticides is reviewed. Resistance is traditionally classified by physiological and biochemical criteria, such as target-site insensitivity and metabolic resistance. However, mutations are discrete molecular changes that differ in their intrinsic frequency, effects on gene dosage and fitness consequences. These attributes in turn impact the population genetics of resistance and resistance management strategies, thus calling for a molecular genetic classification. Mutations in structural genes remain the most abundantly described, mostly in genes coding for target proteins. These provide the most compelling examples of parallel mutations in response to selection. Mutations causing upregulation and downregulation of genes, both in cis (in the gene itself) and in trans (in regulatory processes) remain difficult to characterize precisely. Gene duplications and gene disruption are increasingly reported. Gene disruption appears prevalent in the case of multiple, hetero-oligomeric or redundant targets.

Proteomics-based identification and analysis proteins associated with spirotetramat tolerance in Aphis gossypii Glover

Spirotetramat were now widely used for control insecticides resistant aphids since 2011 in China. In order to elucidate the possible resistance mechanism, a laboratory selected resistant strain (SR) of cotton aphid was established with a 578.93-fold and 14.91-fold resistance ratio to spirotetramat for adult aphids and nymph, respectively, as compared with the susceptible strain (SS). In this study, a comparative proteomic analysis between SR and SS strains were conducted aims to better understand the resistant cotton aphids' spirotetramat tolerance mechanism. Approximately 493 protein spots were detected in the two-dimension polyacrylamide gel electrophoresis (2-DE). The intensities of 35 protein spots significantly changed, showing differences more than 2-fold in the SR strain compared with that in the SS strain. Of these spots, 20 protein spots were more abundant in the SR strain and 15 protein spots were more abundant in the SS strain. Twenty six differently expressed proteins were identified and categorized into several functional groups including carbohydrate and energy metabolism, antioxidant system, protein folding, amino acid metabolism, secondary metabolism and cytoskeleton protein, etc. Among these proteins, the acetyl-coA carboxylase (ACC), heat shock protein 70, ubiquitin-conjugating enzyme, fatty acid synthase, UDP-glucose 6-dehydrogenase, etc. are speculated confer the spirotetramat resistance in cotton aphids.

The cyclic keto-enol insecticide spirotetramat inhibits insect and spider mite acetyl-CoA carboxylases by interfering with the carboxyltransferase partial reaction

Acetyl-CoA carboxylase (ACC) catalyzes the committed and rate-limiting step in fatty acid biosynthesis. The two partial reactions, carboxylation of biotin followed by carboxyl transfer to the acceptor acetyl-CoA, are performed by two separate domains in animal ACCs. The cyclic keto-enol insecticides and acaricides have been proposed to inhibit insect ACCs. In this communication, we show that the enol derivative of the cylic keto-enol insecticide spirotetramat inhibited ACCs partially purified from the insect species Myzus persicae and Spodoptera frugiperda, as well as the spider mite (Tetranychus urticae) ACC which was expressed in insect cells using a recombinant baculovirus. Steady-state kinetic analysis revealed competitive inhibition with respect to the carboxyl acceptor, acetyl-CoA, indicating that spirotetramat-enol bound to the carboxyltransferase domain of ACC. Interestingly, inhibition with respect to the biotin carboxylase substrate ATP was uncompetitive. Amino acid residues in the carboxyltransferase domains of plant ACCs are important for binding of established herbicidal inhibitors. Mutating the spider mite ACC at the homologous positions, for example L1736 to either isoleucine or alanine, and A1739 to either valine or serine, did not affect the inhibition of the spider mite ACC by spirotetramat-enol. These results indicated different binding modes of the keto-enols and the herbicidal chemical families.

Novel mutations and mutation combinations of ryanodine receptor in a chlorantraniliprole resistant population of Plutella xylostella (L.)

A previous study documented a glycine to glutamic acid mutation (G4946E) in ryanodine receptor (RyR) was highly correlated to diamide insecticide resistance in field populations of Plutella xylostella (Lepidoptera: Plutellidae). In this study, a field population collected in Yunnan province, China, exhibited a 2128-fold resistance to chlorantraniliprole. Sequence comparison between resistant and susceptible P. xylostella revealed three novel mutations including a glutamic acid to valine substitution (E1338D), a glutamine to leucine substitution (Q4594L) and an isoleucine to methionine substitution (I4790M) in highly conserved regions of RyR. Frequency analysis of all four mutations in this field population showed that the three new mutations showed a high frequency of 100%, while the G4946E had a frequency of 20%. Furthermore, the florescent ligand binding assay revealed that the RyR containing multiple mutations displayed a significantly lower affinity to the chlorantraniliprole. The combined results suggested that the co-existence of different combinations of the four mutations was involved in the chlorantraniliprole resistance. An allele-specific PCR based method was developed for the diagnosis of the four mutations in the field populations of P. xylostella.

Variation in mortality among populations is higher for pymetrozine than for imidacloprid and spiromesifen in Trialeurodes vaporariorum in greenhouses in Finland

BACKGROUND

Insecticide resistance in Trialeurodes vaporariorum W. is unknown in the species' northern distribution range where it inhabits mainly commercial greenhouses. Resistance development in whiteflies feeding on year-round crops in greenhouses is possible owing to the use of chemical treatments to back up biocontrol. The authors tested the response levels to spiromesifen, pymetrozine and imidacloprid in whiteflies collected from seven greenhouses within a 35 km radius in western Finland.

RESULTS

All except one (PR) population had LC50 values below the recommended concentrations for the tested compounds. However, some populations showed reduced susceptibility to pymetrozine in comparison with the reference susceptible population. Resistance ratios to pymetrozine were highly variable (resistance ratio 0.5-39.7), even among closely located greenhouses, and higher than those for imidacloprid (resistance ratio 1.05-10.5) and spiromesifen (resistance ratio 0.8-11.5). LC50 values and application frequencies of pymetrozine correlated positively among the sampled populations.

CONCLUSION

High variation in resistance levels to pymetrozine among populations within natural whitefly dispersal limits reflects variation in the usage of this compound among individual greenhouse crop producers. Thus, resistance management is recommended at the individual greenhouse crop producer level, even in a dense production cluster. © 2014 Society of Chemical Industry.

Genetic variation of the greenhouse whitefly, Trialeurodes vaporariorum (Hemiptera: Aleyrodidae), among populations from Serbia and neighbouring countries, as inferred from COI sequence variability

The greenhouse whitefly Trialeurodes vaporariorum Westwood, 1856 (Hemiptera: Aleyrodidae) is an invasive and highly polyphagous phloem-feeding pest of vegetables and ornamentals. Trialeurodes vaporariorum causes serious damage due to direct feeding and transmits several important plant viruses. Excessive use of insecticides has resulted in significantly reduced levels of susceptibility of various T. vaporariorum populations. To determine the genetic variability within and among populations of T. vaporariorum from Serbia and to explore their genetic relatedness with other T. vaporariorum populations, we analysed the mitochondrial cytochrome c oxidase I (COI) sequences of 16 populations from Serbia and six neighbouring countries: Montenegro (three populations), Macedonia (one population) and Croatia (two populations), for a total of 198 analysed specimens. A low overall level of sequence divergence and only five variable nucleotides and six haplotypes were found. The most frequent haplotype, H1, was identified in all Serbian populations and in all specimens from distant localities in Croatia and Macedonia. The COI sequence data that was retrieved from GenBank and the data from our study indicated that H1 is the most globally widespread T. vaporariorum haplotype. A lack of spatial genetic structure among the studied T. vaporariorum populations, as well as two demographic tests that we performed (Tajima's D value and Fu's Fs statistics), indicate a recent colonisation event and population growth. Phylogenetic analyses of the COI haplotypes in this study and other T. vaporariorum haplotypes that were retrieved from GenBank were performed using Bayesian inference and median-joining (MJ) network analysis. Two major haplogroups with only a single unique nucleotide difference were found: haplogroup 1 (containing the five Serbian haplotypes and those previously identified in India, China, the Netherlands, the United Kingdom, Morocco, Reunion and the USA) and haplogroup 3 (containing the single Serbian haplotype H3 and haplotypes from Costa Rica, the USA and Spanish Canary Islands). Collectively, our data indicate a rather limited value of COI as a genetic marker for discrimination between different T. vaporariorum populations in the investigated area. Possible explanations for the observed lack of COI sequence variability, such as specific genetics of biological invasion and/or the influence of bacterial symbionts that manipulate insect reproduction, are discussed.

The molecular genetics of insecticide resistance

The past 60 years have seen a revolution in our understanding of the molecular genetics of insecticide resistance. While at first the field was split by arguments about the relative importance of mono- vs. polygenic resistance and field- vs. laboratory-based selection, the application of molecular cloning to insecticide targets and to the metabolic enzymes that degrade insecticides before they reach those targets has brought out an exponential growth in our understanding of the mutations involved. Molecular analysis has confirmed the relative importance of single major genes in target-site resistance and has also revealed some interesting surprises about the multi-gene families, such as cytochrome P450s, involved in metabolic resistance. Identification of the mutations involved in resistance has also led to parallel advances in our understanding of the enzymes and receptors involved, often with implications for the role of these receptors in humans. This Review seeks to provide an historical perspective on the impact of molecular biology on our understanding of resistance and to begin to look forward to the likely impact of rapid advances in both sequencing and genome-wide association analysis.

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.