The toxicity of flonicamid to cotton leafhopper, Amrasca biguttula (Ishida), is by disruption of ingestion: an electropenetrography study

Involvement of Inwardly Rectifying Potassium (Kir) Channels in the Toxicity of Flonicamid to Drosophila melanogaster

Inwardly rectifying potassium (Kir) channels regulate essential physiological processes in insects and have been identified as potential targets for developing new insecticides. Flonicamid has been reported to inhibit Kir channels, disrupting the functions of salivary glands and renal tubules. However, the precise molecular target of flonicamid remains debated. It is unclear whether flonicamid directly targets Kir channels or acts on other sites involved in the activation of transient receptor potential vanilloid (TRPV) channels. In this study, we observed that flonicamid is more toxic to flies than its metabolite, flumetnicam. This higher toxicity is difficult to reconcile if nicotinamidase is the active target, as flonicamid does not inhibit nicotinamidase. An alternative explanation is that flonicamid and flumetnicam may have distinct targets or act on multiple targets. Furthermore, reducing the expression of three individual Kir genes in the salivary glands of D. melanogaster significantly decreased the flies' susceptibility to both flonicamid and flumetnicam. The double knockdown of Kir1 with Kir3 or Kir2 with Kir3 further reduced the flies' sensitivity to both compounds. These findings confirm the involvement of Kir channels in mediating the toxic effects of flonicamid on flies. Overall, this study offers new insights into the physiological roles of insect Kir channels and flonicamid toxicity.

Complicated gene network for regulating feeding behavior: novel efficient target for pest management

Feeding behavior is a fundamental activity for insects, which is essential for their growth, development and reproduction. The regulation of their feeding behavior is a complicated process influenced by a variety of factors, including external stimuli and internal physiological signals. The current review introduces the signaling pathways in brain, gut and fat body involved in insect feeding behavior, and provides a series of target genes for developing RNA pesticides. Additionally, this review summaries the current challenges for the identification and application of functional genes involved in feeding behavior, and finally proposes the future research direction. © 2024 Society of Chemical Industry.

Nitrogen input reduces the physical defense of rice plant against planthopper, Nilaparvata lugens (Hemiptera: Delphacidae)

Nitrogen has important effects on plant growth and defense. Although studies on the alternation in plant chemical defense by nitrogen fertilization have been extensively reported, how it affects physical defense is poorly understood. Two rice (Oryza sativa L.) (Poales: Poaceae) varieties (LDQ7 and YLY1) were applied with varying nitrogen regimes (0.90 and 180 kg ha-1) to study their physical defense against the brown planthopper (BPH) Nilaparvata lugens (Hemiptera: Delphacidae) in this study. Results of the electrical penetration graph showed that BPH searching and penetrating duration time was shortened with increasing nitrogen application. Also, the tubercle papicle of rice leaves decreased with increasing nitrogen application, while rice leaves' surface structure and waxy composition changed with increasing nitrogen application. In field experiments, BPH populations increased with the application of nitrogen fertilizer. These findings suggest that nitrogen input can affect plant-insect interactions by reducing the physical defense of plants, which provides new ideas for the organic combinations of yield increase and pest control in rice fields.

Toxicity of Flonicamid to Diaphorina citri (Hemiptera: Liviidae) and Its Identification and Expression of Kir Channel Genes

Flonicamid is a selective insecticide effective against piercing-sucking insects. Although its molecular target has been identified in other species, the specific effects and detailed mechanism of action in Diaphorina citri Kuwayama remain poorly understood. In this study, we determined that the LC50 of flonicamid for D. citri adults was 16.6 mg AI L-1 after 4 days of exposure. To explore the relevant mechanisms, the treatments with acetone and with 20 mg AI L-1 flonicamid for 96 h were collected as samples for RNA-Seq. The analysis of the transcriptomes revealed 345 differentially expressed genes (DEGs) in D. citri adults subjected to different treatments. Among these DEGs, we focused on the inward-rectifying potassium (Kir) channel genes, which have been extensively studied as potential targets of flonicamid. Three Kir subunit genes (Dckir1, Dckir2, Dckir3) in D. citri were successfully cloned and identified. Furthermore, the expression profiles of these DcKirs were investigated using RT-qPCR and showed that their expression significantly increased after D. citri eclosion to adulthood, particularly for DcKir3. The DcKirs were predominantly expressed in gut tissues, with DcKir1 and DcKir2 exhibiting high expression levels in the hindgut and midgut, respectively, while DcKir3 showing high expression in the midgut and Malpighian tubules. This study provides insights into the potential roles of Kir subunits in D. citri and enhances our understanding of the physiological effects of flonicamid in this pest.

Hormetic effects of thiamethoxam on Schizaphis graminum: demographics and feeding behavior

In agroecosystems, insects contend with chemical insecticides often encountered at sublethal concentrations. Insects' exposure to these mild stresses may induce hormetic effects, which has consequences for managing insect pests. In this study, we used an electrical penetration graph (EPG) technique to investigate the feeding behavior and an age-stage, two-sex life table approach to estimate the sublethal effects of thiamethoxam on greenbug, Schizaphis graminum. The LC5 and LC10 of thiamethoxam significantly decreased longevity and fecundity of directly exposed adult aphids (F0). However, the adult longevity, fecundity, and reproductive days (RPd)-indicating the number of days in which the females produce offspring - in the progeny generation (F1) exhibited significant increase when parental aphids (F0) were treated with LC5 of the active ingredient. Subsequently, key demographic parameters such as intrinsic rate of increase (r) and net reproductive rate (R0) significantly increased at LC5 treatment. EPG recordings showed that total durations of non-probing (Np), intercellular stylet pathway (C), and salivary secretion into the sieve element (E1) were significantly increased, while mean duration of probing (Pr) and total duration of phloem sap ingestion and concurrent salivation (E2) were decreased in F0 adults exposed to LC5 and LC10. Interestingly, in the F1 generation, total duration of Np was significantly decreased while total duration of E2 was increased in LC5 treatment. Taken together, our results showed that an LC5 of thiamethoxam induces intergenerational hormetic effects on the demographic parameters and feeding behavior of F1 individuals of S. graminum. These findings have important implications on chemical control against S. graminum and highlight the need for a deeper understanding of the ecological consequences of such exposures within pest management strategies across the agricultural landscapes.

Thiamethoxam-Induced Intergenerational Sublethal Effects on the Life History and Feeding Behavior of Rhopalosiphum padi

Thiamethoxam, a second-generation neonicotinoid insecticide is widely used for controlling sap-sucking insect pests including Rhopalosiphum padi. The current study aimed to investigate the life-history parameters and feeding behavior of R. padi following treatments with sublethal concentrations of thiamethoxam. The lethal concentration 50 (LC50) value of thiamethoxam against adult R. padi was 11.458 mg L-1 after 48 h exposure. The sublethal concentrations of thiamethoxam (LC5 and LC10) significantly decreased the adult longevity, fecundity, and reproductive days in the directly exposed aphids (F0 generation). In the progeny R. padi (F1), the developmental durations and total prereproductive period (TPRP) were decreased while the adult longevity, fecundity, and reproductive days (RPd) were increased at both thiamethoxam concentrations. The demographic parameters including the net reproductive rate (R0), intrinsic rate of increase (r), and finite rate of increase (λ) were prolonged only at the LC5 of thiamethoxam. The EPG results indicated that the sublethal concentrations of thiamethoxam increases the total duration of non-probing (Np) while reducing the total duration of E2 in directly exposed aphids (F0). Interestingly, the E2 were significantly increased in the progeny generation (F1) descending from previously exposed parental aphids (F0). Overall, this study showed that thiamethoxam exhibited high toxicity against directly exposed individuals (F0), while inducing intergenerational hormetic effects on the progeny generation (F1) of R. padi. These findings provided crucial details about thiamethoxam-induced hormetic effects that might be useful in managing resurgences of this key pest.

Impact of sublethal concentrations of flonicamid on key demographic parameters and feeding behavior of Schizaphis graminum

Flonicamid is a novel systemic insecticide that efficiently controls sap-sucking insect pests. However, the impact of sublethal concentrations of flonicamid on key demographic parameters and the feeding behavior of greenbug, Schizaphis graminum has not yet been studied. In this study, we used the age stage, two-sex life table approach, and electrical penetration graphs (EPGs) to investigate the sublethal effects of flonicamid on the biological traits and feeding behavior of S. graminum. Bioassays showed that flonicamid possesses high toxicity to adult S. graminum with LC50 of 5.111 mg L-1 following 48 h exposure. Sublethal concentrations of flonicamid (LC5 and LC10) significantly decreased the longevity and fecundity of directly exposed parental aphids (F0), while the reproductive days were reduced only at LC10. The pre-adult stage and total pre-reproductive period (TPRP) increased in F1 individuals after exposure of F0 aphids to the sublethal concentrations of flonicamid. Furthermore, the adult longevity, fecundity and key demographic parameters (R0, r, and λ) were significantly reduced in progeny generation (F1). EPG recordings showed that the total duration of phloem sap ingestion and concurrent salivation (E2) decreased substantially in F0 and F1 aphids after exposure to LC5 and LC10 of flonicamid. Taken together, our results showed that the sublethal concentrations of flonicamid affect the demographic parameters and feeding behavior that ultimately suppress the population growth of S. graminum. This study provides in-depth information about the overall effects of flonicamid on S. graminum that might help to manage this key pest.

Reducing Expression of Salivary Protein Genes by Flonicamid Partially Contributed to Its Feeding Inhibition of the Brown Planthopper on Rice

Flonicamid inhibits the feeding of piercing-sucking pests as a selective systemic insecticide. The brown planthopper (BPH), Nilaparvata lugens (Stål), is one of the most serious pests on rice. During feeding, it uses its stylet to collect sap by penetrating the phloem, and at the same time, it delivers saliva into the rice plant. Insect salivary proteins play important roles in feeding and interacting with plants. Whether flonicamid affects the expression of salivary protein genes and then inhibits the feeding of BPH is not clear. Here, from 20 functionally characterized salivary proteins, we screened five salivary proteins (NlShp, NlAnnix5, Nl16, Nl32, and NlSP7) whose gene expressions were significantly inhibited by flonicamid. We performed experimental analysis on two of them (Nl16 and Nl32). RNA interference of Nl32 significantly reduced the survival rate of BPH. Electrical penetration graph (EPG) experiments showed that both flonicamid treatment and knockdown of Nl16 and Nl32 genes significantly reduced the feeding activity of N. lugens in the phloem and also reduced the honeydew excretion and fecundity. These results suggested that the inhibition of flonicamid on the feeding behavior in N. lugens might be partially attributed to its effect on the expression of salivary protein genes. This study provides a new insight into the mechanism of action of flonicamid on insect pests.

Flubendiamide Resistance and Its Mode of Inheritance in Tomato Pinworm Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae)

Tomato pinworm, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) is the major pest of tomato crops in Pakistan. Insecticides are commonly used for the management of this insect-pest. To develop a better insecticide resistance management strategy and evaluate the risk of resistance evolution, a field collected population of the tomato pinworm was selected with flubendiamide in the laboratory. We investigated the genetics of flubendiamide resistance and concentration-mortality response to other insecticides by selecting a field strain of tomato pinworm with commercial flubendiamide formulation. Tuta absoluta was reciprocally crossed with resistant strain (Fluben-R) and was selected up to 13 generations, while F₁ progeny was back-crossed with resistant parent (Fluben-R). The results of LC₅₀ and Resistance Ratio (RR) demonstrated a higher resistance developed in field and laboratory-selected strains (G₂ and G₁₃, respectively). Field-collected and laboratory-selected (Fluben-R) strains demonstrated higher intensity of concentration-mortality response against chlorantraniliprole, thiamethoxam, permethrin, abamectin and tebufenozide compared to susceptible ones. Based on the overlapping of 95% FL, it demonstrated significant differences, revealing that it was not sex linked (autosomal) with no maternal effects. The backcross analysis of the F₁´ resistant parent resulting in significant differences at all concentrations suggests that resistance is controlled by more than one factor; the null hypothesis was rejected and inheritance was under polygenic control. Resistance progression from 38 to 550 folds demonstrated that T. absoluta can develop a higher level of resistance to flubendiamide. Concentration-mortality response experiments demonstrated that the LC₅₀ of some tested insecticides was higher for field-collected and laboratory-selected strains, suggesting that resistance mechanisms should be studied at a molecular level for better understanding. These results could be helpful to design resistance management strategies against the tomato pinworm.

Transcriptome analysis unravels RNAi pathways genes and putative expansion of CYP450 gene family in cotton leafhopper Amrasca biguttula (Ishida)

Cotton Leafhopper, Amrasca biguttula is an important pest of cotton and okra in the Indian subcontinent. Presently limited genomic/transcriptomic information is available for this insect in any of open source databases. The present study reports the first assembled and annotated de novo transcriptome of cotton leafhopper. Out of 75,551 transcripts, 39,613 CDS (Coding Sequence) were predicted with 35,282 showing positive blast hits with NCBI nr database. The Gene ontology (GO) analysis annotated 7431 CDS  with KEGG pathway categorizing these CDS into 22 different functional groups. The majority of CDS were annotated in signal transduction and transport catabolism pathways. The sequence data was screened for RNAi pathway genes and presence of 37 transcripts associated with this process confirmed the existence of robust RNAi machinery. The role of core RNAi machinery genes (Dicer-2, Ago-2, Piwi and Staufen) has been validated through dsRNA feeding studies. The data resource has also been used to identify potential RNAi targets and genes associated with insecticide detoxification specifically CYP 450 family. The current study provides a useful sequence resource which can be used to initiate molecular studies in this insect with emphasis on insecticide resistance, RNAi and functional genomics.

Flonicamid and knockdown of inward rectifier potassium channel gene CsKir2B adversely affect the feeding and development of Chilo suppressalis

BACKGROUND

The selective insecticide flonicamid shows highly insecticidal activities against piercing-sucking insects and has been widely used for the control of Hemipteran insect pests, whereas its effects on Lepidopteran insect pests remain largely unknown. Recently, inward rectifier potassium (Kir) channel has been verified to be a target of flonicamid, however, functional characterization of Lepidopteran Kir genes is still lacking.

RESULTS

Flonicamid shows no insecticidal toxicity against Chilo suppressalis larvae. However, the feeding and growth of larvae were reversibly inhibited by flonicamid (50-1200 mg L^-1 ). Flonicamid treatment also remarkably reduced and delayed the pupation and eclosion of Chilo suppressalis. Additionally, five distinct Kir channel genes (CsKir1, CsKir2A, CsKir2B, CsKir3A and CsKir3B) were cloned from Chilo suppressalis. Expression profiles analysis revealed that CsKir2A was predominately expressed in the hindgut of larvae, whereas CsKir2B had high expressions in the Malpighian tubules and hindgut. RNA interference (RNAi)-mediated knockdown of CsKir2B significantly reduced the growth and increased the mortalities of larvae, whereas silencing of CsKir2A had no obvious effects on Chilo suppressalis.

CONCLUSION

Flonicamid exhibits adverse effects on the growth and development of Chilo suppressalis. CsKir2B might be involved in the feeding behavior of Chilo suppressalis. These results provide valuable information on the effects of flonicamid on non-target insects as well as the function of insect Kir channels, and are helpful in developing new insecticide targeting insect Kir channels. © 2020 Society of Chemical Industry.

AC-DC electropenetrography: fundamentals, controversies, and perspectives for arthropod pest management

Studying the intimate association of arthropods with their physical substrate is both important and challenging. It is important because substrate is a key determinant for organism fitness; challenging because the intricacies of this association are dynamic, and difficult to record and resolve. The advent of electropenetrography (EPG) and subsequent developments allowed researchers to overcome this challenge. Nonetheless, EPG research has been historically restricted to piercing-sucking hemipteran plant pests. Recently, its potential use has been greatly broadened for additional pests with instrument advances. Thus, blood-feeding arthropods and chewing feeders, as well as non-feeding behaviors like oviposition by both pests and parasitoids, are novel new targets for EPG research, with critical consequences for integrated pest management. EPG can explain mechanisms of crop damage, plant or animal pathogen transmission, and the effects of insecticides, antifeedants, repellents, or transgenic plants and animals, on specific behaviors of damage or transmission. This review broadly covers the principles and development of EPG technology, emphasizing controversies and challenges remaining with suggested research to overcome them. In addition, it summarizes 60+ years of basic and applied EPG research, and previews future directions for pest management. The goal is to stimulate new applications for this unique enabling technology. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.

AC-DC electropenetrography unmasks fine temporal details of feeding behaviors for two tick species on unsedated hosts

Ticks are significant nuisance pests and vectors of pathogens for humans, companion animals, and livestock. Limited information on tick feeding behaviors hampers development and rigorous evaluation of tick and tick-borne pathogen control measures. To address this obstacle, the present study examined the utility of AC–DC electropenetrography (EPG) to monitor feeding behaviors of adult Dermacentor variabilis and Amblyomma americanum in real-time. EPG recording was performed during early stages of slow-phase tick feeding using an awake calf host. Both tick species exhibited discernable and stereotypical waveforms of low-, medium-, and high-frequencies. Similar waveform families and types were observed for both tick species; however, species-specific waveform structural differences were also observed. Tick waveforms were hierarchically categorized into three families containing seven types. Some waveform types were conserved by both species (e.g., Types 1b, 1c, 2b, 2c) while others were variably performed among species and individually recorded ticks (e.g., Types 1a, 2a, 2d). This study provides a proof-of-principle demonstration of the feasibility for using EPG to monitor, evaluate, and compare tick feeding behaviors, providing a foundation for future studies aimed at correlating specific feeding behaviors with waveforms, and ultimately the influence of control measures and pathogens on tick feeding behaviors.

Biotransformation of insecticide flonicamid by Aminobacter sp. CGMCC 1.17253 via nitrile hydratase catalysed hydration pathway

AIMS

This study evaluates flonicamid biotransformation ability of Aminobacter sp. CGMCC 1.17253 and the enzyme catalytic mechanism involved.

METHODS AND RESULTS

Flonicamid transformed by resting cells of Aminobacter sp. CGMCC 1.17253 was carried out. Aminobacter sp. CGMCC 1.17253 converts flonicamid into N-(4-trifluoromethylnicotinoyl) glycinamide (TFNG-AM). Aminobacter sp. CGMCC 1.17253 transforms 31·1% of the flonicamid in a 200 mg l^-1 conversion solution in 96 h. Aminobacter sp. CGMCC 1.17253 was inoculated in soil, and 72·1% of flonicamid with a concentration of 0·21 μmol g^-1 was transformed in 9 days. The recombinant Escherichia coli expressing Aminobacter sp. CGMCC 1.17253 nitrile hydratase (NHase) and purified NHase were tested for the flonicamid transformation ability, both of them acquired the ability to transform flonicamid into TFNG-AM.

CONCLUSIONS

Aminobacter sp. CGMCC 1.17253 transforms flonicamid into TFNG-AM via hydration pathway mediated by cobalt-containing NHase.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report that bacteria of genus Aminobacter has flonicamid-transforming ability. This study enhances our understanding of flonicamid-degrading mechanism. Aminobacter sp. CGMCC 1.17253 has the potential for bioremediation of flonicamid pollution.

Waveforms From Stylet Probing of the Mosquito Aedes aegypti (Diptera: Culicidae) Measured by AC-DC Electropenetrography

Electropenetrography (EPG) has been used for many years to visualize unseen stylet probing behaviors of plant-feeding piercing-sucking insects, primarily hemipterans. Yet, EPG has not been extensively used with blood-feeding insects. In this study, an AC-DC electropenetrograph with variable input resistors (Ri), i.e., amplifier sensitivities, was used to construct a waveform library for the mosquito arbovirus vector, Aedes aegypti (Linneaus), while feeding on human hands. EPG waveforms representing feeding activities were: 1) electrically characterized, 2) defined by visual observation of biological activities, 3) analyzed for differences in appearance by Ri level and type of applied signal (AC or DC), and 4) quantified. Electrical origins of waveforms were identified from five different Ri levels and AC versus DC. Mosquitoes produced short stylet probes ('bites') that typically contained five waveform families. Behaviors occurred in the following order: surface salivation (waveform family J), stylet penetration through the outer skin (K), penetration of deeper tissues and location of blood vessels/pathway activities (L), active ingestion with engorgement (M), and an unknown behavior that terminated the probe (N). Only K, L, and M were performed by every insect. A kinetogram of conditional probabilities for waveform performance demonstrated plasticity among individuals in L and M, which were alternated. Now that EPG waveforms for mosquito feeding have been defined, EPG can be used as a tool for improved biological understanding of mosquito-borne diseases.

Biodegradation of the Insecticide Flonicamid by Alcaligenes faecalis CGMCC 17553 via Hydrolysis and Hydration Pathways Mediated by Nitrilase

Flonicamid (N-cyanomethyl-4-trifluoromethylnicotinamide, FLO), a novel selective systemic pyridinecarboxamide insecticide, effectively controls hemipterous pests. However, microbial degradation of flonicamid, along with the enzymatic mechanism, has not been studied. Here, bacterial isolate PG13, which converts flonicamid into 4-(trifluoromethyl)nicotinol glycine (TFNG) and N-(4-trifluoromethylnicotinoyl)glycinamide (TFNG-AM), was isolated and identified as Alcaligenes faecalis CGMCC 17553. The genome of CGMCC 17553 contained five nitrilases but no nitrile hydratase, and recombinant Escherichia coli strains harboring CGMCC 17553 nitrilase gene nitA or nitD acquired the ability to degrade flonicamid. Purified NitA catalyzed flonicamid into both TFNG and TFNG-AM, indicating dual functionality, while NitD could only produce TFNG-AM. Three-dimensional homology modeling revealed that aromatic amino acid residues in the catalytic pocket affected nitrilase activity. These findings further our understanding of the enzymatic mechanism of flonicamid metabolism in the environment and may help develop a potential bioremediation agent for the elimination of flonicamid contamination.

Block of Kir channels by flonicamid disrupts salivary and renal excretion of insect pests

Flonicamid is a selective insecticide for the control of sap-sucking insects; it exerts toxic effects by inhibiting insect feeding. However, its molecular target remains elusive. In this study, we functionally characterized NlKir1 channels of the brown planthopper (Nilaparvata lugens) in HEK293 cells. Homomeric NlKir1 channels generated inward-rectifying K⁺ currents. Flonicamid inhibited NlKir1 channels at nanomolar concentrations. Furthermore, flonicamid inhibited honeydew and salivary secretions of planthoppers, and reduced the renal excretion of female mosquitoes in a dose-dependent manner. The inhibitory effect of flonicamid on fluid secretion of isolated Malpighian tubules from Culex pipiens pullens was comparable to that of the selective Kir1 inhibitor. The observed physiological alterations by flonicamid are likely mediated by Kir1 channels and could lead to the disruption of feeding behaviors and eventually lethality. Our study establishes the Kir1 channel as the target of flonicamid and provided new insights into the mode of action of flonicamid.