Pymetrozine inhibits reproductive behavior of brown planthopper Nilaparvata lugens and fruit fly Drosophila melanogaster

Insect transient receptor potential vanilloid channels as potential targets of insecticides

Chordotonal organs are miniature sensory organs present in insects. Chordotonal organs depend on transient receptor potential (TRP) channels. Transient receptor potential vanilloid (TRPV) channels are the only TRPs identified that can act as targets of insecticides. By binding with TRPV channels, insecticides targeting the chordotonal organs trigger the inflow of calcium ions, resulting in abnormal function of the chordotonal organ to achieve the goal of eliminating pests. TRPV channels are highly expressed in various developmental stages and tissue parts of insects and play an important role in the whole life history of insects. In this review, we will discuss the structure and types of TRPV channels as well as their genetic relationships in different species. We also systematically reviewed the recent progress of TRPV channels as insecticide targets, demonstrating that TRPV channels can be used as the target of new high-efficiency insecticides.

Reverse genetic study reveals the molecular targets of chordotonal organ TRPV channel modulators

Insecticides have been widely used for the control of insect pests that have a significant impact on agriculture and human health. A better understanding of insecticide targets is needed for effective insecticide design and resistance management. Pymetrozine, afidopyropen and flonicamid are reported to target on proteins that located on insect chordotonal organs, resulting in the disruption of insect coordination and the inhibition of feeding. In this study, we systematically examined the susceptibility of six Drosophila melanogaster mutants (five transient receptor potential channels and one mechanoreceptor) to three commercially used insecticides, in order to identify the receptor subunits critical to the insect's response to insecticides. Our results showed that iav1, nan36aand wtrw1 mutants exhibited significantly reduced susceptibility to pymetrozine and afidopyropen, but not to flonicamid. The number of eggs produced by the three mutant females were significantly less than that of the w1118 strain. Meanwhile, the longevity of all male mutants and females of nan36a and wtrw1 mutants was significantly shorter than that of the w1118 strain as the control. However, we observed no gravitaxis defects in wtrw1 mutants and the anti-gravitaxis of wtrw1 mutants was abolished by pymetrozine. Behavioral assays using thermogenetic tools further confirmed the bioassay results and supported the idea that Nan as a TRPV subfamily member located in Drosophila chordotonal neurons, acting as a target of pymetrozine, which interferes with Drosophila and causes motor deficits with gravitaxis defects. Taken together, this study elucidates the interactions of pymetrozine and afidopyropen with TRPV channels, Nan and Iav, and TRPA channel, Wtrw. Our research provides another evidence that pymetrozine and afidopyropen might target on nan, iav and wtrw channels and provides insights into the development of sustainable pest management strategies.

Resistance Monitoring of Nilaparvata lugens to Pymetrozine Based on Reproductive Behavior

On the basis of the inhibition effects of pymetrozine on the reproductive behavior of N. lugens, we established a bioassay method to accurately evaluate the toxicity of pymetrozine in N. lugens and clarified the level of pymetrozine resistance of N. lugens in the field. In this study, pymetrozine's effects on the fecundity of N. lugens were evaluated using the topical application method and rice-seedling-dipping method. Moreover, the resistance of N. lugens to pymetrozine in a pymetrozine-resistant strain (Pym-R) and two field populations (YZ21 and QS21) was determined using the rice-seedling-dipping method and fecundity assay methods. The results showed that treatment of N. lugens third-instar nymphs with LC₁₅, LC₅₀, and LC₈₅ doses of pymetrozine resulted in a significantly reduced fecundity of N. lugens. In addition, N. lugens adults treated with pymetrozine, using the rice-seedling-dipping and topical application method, also exhibited a significantly inhibited fecundity. Using the rice-stem-dipping method, pymetrozine resistance levels were shown to be high in Pym-R (194.6-fold), YZ21 (205.9-fold), and QS21 (212.8-fold), with LC₅₀ values of 522.520 mg/L (Pym-R), 552.962 mg/L (YZ21), and 571.315 (QS21) mg/L. However, when using the rice-seedling-dipping or topical application fecundity assay method, Pym-R (EC₅₀: 14.370 mg/L, RR = 12.4-fold; ED₅₀: 0.560 ng/adult, RR = 10.8-fold), YZ21 (EC₅₀: 12.890 mg/L, RR = 11.2-fold; ED₅₀: 0.280 ng/adult; RR = 5.4-fold), and QS21 (EC₅₀: 13.700 mg/L, RR = 11.9-fold) exhibited moderate or low levels of resistance to pymetrozine. Our studies show that pymetrozine can significantly inhibit the fecundity of N. lugens. The fecundity assay results showed that N. lugens only developed low to moderate levels of resistance to pymetrozine, indicating that pymetrozine can still achieve effective control on the next generation of N. lugens populations.

A sensitive and specific enzyme-linked immunosorbent assay for the detection of pymetrozine in vegetable, cereal, and meat

Pymetrozine is a neonicotinoid insecticide with high efficacy against aphids and planthoppers, and has been used worldwide. To monitor its residue in food, a highly specific and sensitive monoclonal antibody (McAb) was prepared, and an indirect competitive enzyme-linked immunosorbent assay (icELISA) was developed to detect pymetrozine, with a 50% inhibition value (IC₅₀) of 7.70 μg/L. The McAb showed little affinity for acetamiprid, hexazinone, metamitron, nitenpyram, metribuzin, and imidacloprid. The limits of detection (LOD) calculated from the analysis of broccoli, cabbage, wheat, maize, rice, chicken, fish, and crayfish samples were from 1.56 to 2.72 μg/kg and the average recoveries were from 81.25 to 103.19%. icELISA was confirmed using liquid chromatography-tandem mass spectrometry (LC-MS/MS). These results demonstrated that the optimised icELISA is a convenient and effective analytical tool for monitoring pymetrozine residues in food.

Fitness costs of resistance to insecticide pymetrozine combined with antimicrobial zhongshengmycin in Nilaparvata lugens (Stål)

The brown planthopper, Nilaparvata lugens (Stål), is a major pest of rice crops, and its control is critical for food security. Pymetrozine has been recommended as an alternative to imidacloprid for controlling N. lugens, but the pest has developed high resistance to it, making its prohibition and restriction urgent. To address this issue, we conducted a study using a mixture of pymetrozine and zhongshengmycin with the effective ratio of 1:40, to evaluate the fitness costs in N. lugens. Our results showed that N. lugens had a relative fitness of 0.03 under this ratio, with significantly reduced longevity, female and male adult periods, total pre-oviposition days, and fecundity. Moreover, the expression levels of the uricase gene (EC1.7.3.3) and farnesyl diphosphate farnesyl transferase gene (EC2.5.1.21) were reduced in N. lugens. These genes are involved in urea metabolism and steroid biosynthesis pathway, respectively, and their suppression can interfere with the normal nutritional function of N. lugens. Our study demonstrates that the combination of chemical insecticides and antimicrobials can delay the development of resistance and improve the efficiency of pest control. This information is valuable for researchers developing management strategies to delay the development of pymetrozine resistance in N. lugens.

Mining Lygus hesperus (western tarnished plant bug) transcriptomic data for transient receptor potential channels: Expression profiling and functional characterization of a Painless homolog

The transient receptor potential (TRP) family of cation channels are evolutionarily conserved proteins with critical roles in sensory physiology. Despite extensive studies in model species, knowledge of TRP channel functional diversity and physiological impact remains limited in many non-model insect species. To assess the TRP channel repertoire in a non-model agriculture pest species (Lygus hesperus), publicly available transcriptomic datasets were mined for potential homologs. Among the transcripts identified, 30 are predicted to encompass complete open reading frames that encode proteins representing each of the seven TRP channel subfamilies. Although no homologs were identified for the Pyrexia and Brivido channels, the TRP complement in L. hesperus exceeded the 13-16 channels reported in most insects. This diversity appears to be driven by a combination of alternative splicing, which impacted members of six subfamilies, and gene expansion of the TRPP subfamily. To validate the in silico data and provide more detailed analyses of L. hesperus TRP functionality, the putative Painless homolog was selected for more in depth analysis and its functional role in thermosensation examined in vitro. RT-PCR expression profiling revealed near ubiquitous expression of the Painless transcript throughout nymphal and adult development. Electrophysiological data generated using a Xenopus oocyte recombinant expression system indicated activation parameters for L. hesperus Painless homolog that are consistent with a role in noxious heat (40°-45 °C) thermosensation.

Monitoring, cross-resistance, inheritance, and fitness costs of brown planthoppers, Nilaparvata lugens, resistance to pymetrozine in China

BACKGROUND

The brown planthopper, Nilaparvata lugens, is considered the most destructive pest of rice in many Asian countries including China. Use of pymetrozine in insect resistance management (IRM) has been one strategy to control this pest. In this study, we reported the status of pymetrozine resistance in Nilaparvata lugens (Stål) collected from China over the period 2017-2021 and selected a strain of N. lugens resistant to pymetrozine and evaluated the cross-resistance, inheritance and fitness costs of the resistance.

RESULTS

Monitoring data (2017-2021) showed that field populations of N. lugens in China developed moderate- to high-level pymetrozine resistance during these 5 years. By continuous selection with pymetrozine in the lab, the pymetrozine selected N. lugens strain (Pym-R₉₈ ) developed a 225.2-fold resistance compared to a susceptible strain. The Pym-R₉₈ strain showed high cross-resistance to dinotefuran (66.6-fold) and low cross-resistance to nitenpyram (5.2-fold) and sulfoxaflor (5.8-fold). Inheritance pattern analysis of Pym-R₉₃ revealed that resistance to pymetrozine was polygenic, autosomal and incompletely dominant. Fitness costs of pymetrozine resistance were present in Pym-R₉₀ and WA2020 strains with a relative fitness of 0.72 and 0.60, respectively. The developmental duration of Pym-R₉₀ and WA2020 was significantly longer and hatchability was significantly lower compared to pymetrozine-susceptible strain (Pym-S).

CONCLUSIONS

N. lugens has developed high level of resistance to pymetrozine. Pymetrozine-resistance brown planthopper had cross-resistance with some of neonicotinoids such as dinotefuran, nitenpyram and sulfoxaflor. The autosomal, incompletely dominant and polygenic resistance to pymetrozine in N. lugens and the fitness costs associated with this resistance can be exploited in IRM strategies to preserve the lifetime of pymetrozine for control of N. lugens in China. © 2022 Society of Chemical Industry.

Mechanism of metabolic resistance to pymetrozine in Nilaparvata lugens: over-expression of cytochrome P450 CYP6CS1 confers pymetrozine resistance

BACKGROUND

Pymetrozine is commonly used for the control of Nilaparvata lugens, and resistance to pymetrozine has been frequently reported in the field populations in recent years. However, the mechanism of brown planthopper resistance to pymetrozine is still unknown.

RESULTS

In this study, a pymetrozine-resistant strain (PMR) was established, and the potential biochemical resistance mechanism of N. lugens to pymetrozine was investigated. Pymetrozine was synergized by the inhibitor piperonyl butoxide (PBO) in the PMR with 2.83-fold relative synergistic ratios compared with the susceptible strain (Sus). Compared with the Sus, the cytochrome P450 monooxygenase activity of PMR was increased by 1.7 times, and two P450 genes (NlCYP6CS1 and NlCYP301B1) were found to be significantly overexpressed more than 6.0-fold in the PMR. Pymetrozine exposure induced upregulation of NlCYP6CS1 expression in the Sus, but the expression of NlCYP301B1 did not change significantly. In addition, RNA interference (RNAi)-mediated suppression of NlCYP6CS1 gene expression dramatically increased the toxicity of pymetrozine against N. lugens. Moreover, transgenic lines of Drosophila melanogaster expressing NlCYP6CS1 were less susceptible to pymetrozine, and had a stronger ability to metabolize pymetrozine.

CONCLUSIONS

Taken together, our findings indicate that the overexpression of NlCYP6CS1 is one of the key factors contributing to pymetrozine resistance in N. lugens. And this result is helpful in proposing a management strategy for pymetrozine resistance.

Cholecystokinin-like peptide mediates satiety by inhibiting sugar attraction

Feeding is essential for animal survival and reproduction and is regulated by both internal states and external stimuli. However, little is known about how internal states influence the perception of external sensory cues that regulate feeding behavior. Here, we investigated the neuronal and molecular mechanisms behind nutritional state-mediated regulation of gustatory perception in control of feeding behavior in the brown planthopper and Drosophila. We found that feeding increases the expression of the cholecystokinin-like peptide, sulfakinin (SK), and the activity of a set of SK-expressing neurons. Starvation elevates the transcription of the sugar receptor Gr64f and SK negatively regulates the expression of Gr64f in both insects. Interestingly, we found that one of the two known SK receptors, CCKLR-17D3, is expressed by some of Gr64f-expressing neurons in the proboscis and proleg tarsi. Thus, we have identified SK as a neuropeptide signal in a neuronal circuitry that responds to food intake, and regulates feeding behavior by diminishing gustatory receptor gene expression and activity of sweet sensing GRNs. Our findings demonstrate one nutritional state-dependent pathway that modulates sweet perception and thereby feeding behavior, but our experiments cannot exclude further parallel pathways. Importantly, we show that the underlying mechanisms are conserved in the two distantly related insect species.

Food intake is critical for animal survival and reproduction and is regulated both by internal states that signal appetite or satiety, and by external sensory stimuli. It is well known that the internal nutritional state influences the strength of the chemosensory perception of food signals. Thus, both gustatory and olfactory signals of preferred food are strengthened in hungry animals. However, the molecular mechanisms behind satiety-mediated modulation of taste are still not known. We show here that cholecystokinin-like (SK) peptide in brown planthopper and Drosophila signals satiety and inhibits sugar attraction by lowering the activity of sweet-sensing gustatory neurons and transcription of a sugar receptor gene, Gr64f. We show that SK peptide signaling reflects the nutritional state and inhibits feeding behavior. Re-feeding after starvation increases SK peptide expression and spontaneous activity of SK producing neurons. Interestingly, we found that SK peptide negatively regulates the expression of the sweet gustatory receptor and that activation of SK producing neurons inhibits the activity of sweet-sensing gustatory neurons (GRNs). Furthermore, we found that one of the two known SK peptide receptors is expressed in some sweet-sensing GRNs in the proboscis and proleg tarsi. In summary, our findings provide a mechanism that is conserved in distantly related insects and which explains how feeding state modulates sweet perception to regulate feeding behavior. Thus, we have identified a neuropeptide signal and its neuronal circuitry that respond to satiety, and that regulate feeding behavior by inhibiting gustatory receptor gene expression and activity of sweet sensing GRNs.

Identification of putative abdominal vibration-related genes through transcriptome analyses in the brown planthopper (Nilaparvata lugens)

The sexually mature female brown planthoppers (BPHs) send out abdominal vibration (AV) signals through the rice so that the males can obtain intraspecific, gender, and localization information to prepare for mating. Destroying vibration signals is an alternative biological method for pest control. However, the regulatory mechanism of AV in female BPHs remains elusive, which presents an obstacle to pest control. We observed that before mating female BHPs emitted abdominal vibration signals that disappeared immediately after mating and reappeared after 6 days. Therefore, ovarian and brain samples of female BPHs from Unmated-6h⁺ (with AV), Mated-6h^- (without AV) and Mated-6d⁺ (with AV) individuals were collected for transcript analyses. By transcriptional sequencing analyses, 33 candidate genes that might involve in the regulation of female AV were obtained. After selecting 4 candidate genes of them for verification by RNA interference (RNAi), it was found that interference of juvenile hormone binding protein (JHBP) could greatly reduce the probability and frequency of AV for female BPHs. In general, this study identified AV-related candidate genes in female BPHs through transcriptome analyses and provided an important basis for future research on pest control in BPHs.

Molecular characterizations and expression profiles of transient receptor potential channels in the brown planthopper, Nilaparvata lugens

Transient receptor potential (TRP) is a superfamily of important cation channels located on the cell membrane. It can regulate almost all sensory modality and control a series of behaviors, including hearing, locomotion, gentle touch, temperature sensation, dry air and food texture detection. The expression profiles of TRP channels have been well documented in the model insect Drosophila melanogaster. However, little is known about the TRP channels of agricultural pests. In this study, we cloned 9 TRP ion channel genes from brown planthopper. Their amino acid sequences are highly conserved with homologues of other insects and have typical TRP channel characteristics: six transmembrane domains (TM1 - TM6) and a pore region between TM5 and TM6. These TRP channels of N. lugens were expressed in all developmental stages and various body parts. The expression levels of almost all TRP channels were relatively higher in adults than nymph stages, and lowest in the eggs. Antenna and abdomen were the main body parts with high expression of these genes. Furthermore, the mRNA levels of these TRP genes were significantly decreased in the third-instar nymphs injected with double-stranded RNA (dsRNA). The survival rate of different TRP dsRNA injected nymphs all exceeded 81%, which was no significant difference compared with the control group. These results suggested that these 9 TRP channels are expressed throughout the body and all ages of the brown planthopper, and are involved in regulating multiple physiological and behavioral processes. The identification of TRP channel genes in this study not only provides a foundation for further exploring the potential roles of TRP channels, but also serves as targets to develop new insecticides for the control of agricultural pests.