Monitoring, Cross-Resistance, Inheritance, and Synergism of Plutella xylostella (Lepidoptera: Plutellidae) Resistance to Pyridalyl in China

Fitness costs of resistance to insecticides in insects

The chemical application is considered one of the most crucial methods for controlling insect pests, especially in intensive farming practices. Owing to the chemical application, insect pests are exposed to toxic chemical insecticides along with other stress factors in the environment. Insects require energy and resources for survival and adaptation to cope with these conditions. Also, insects use behavioral, physiological, and genetic mechanisms to combat stressors, like new environments, which may include chemicals insecticides. Sometimes, the continuous selection pressure of insecticides is metabolically costly, which leads to resistance development through constitutive upregulation of detoxification genes and/or target-site mutations. These actions are costly and can potentially affect the biological traits, including development and reproduction parameters and other key variables that ultimately affect the overall fitness of insects. This review synthesizes published in-depth information on fitness costs induced by insecticide resistance in insect pests in the past decade. It thereby highlights the insecticides resistant to insect populations that might help design integrated pest management (IPM) programs for controlling the spread of resistant populations.

Resistance Monitoring for Six Insecticides in Vegetable Field-Collected Populations of Spodoptera litura from China

The common cutworm, Spodoptera litura (Fabricius), is a notorious and damaging insect pest of horticultural crops in China, the management of which largely relies on chemical agents that are limited by the development of chemical resistance in target populations. As such, resistance monitoring of S. litura populations is a necessary part of management strategies of insecticide resistance. In the current work, we monitored resistance to six insecticides in field-collected populations of S. litura sampled from eleven provinces across China in 2021. The results show that S. litura populations developed significant resistance against chlorantraniliprole, cyantraniliprole, metaflumizone, and pyridalyl and low levels of resistance to chromafenozide. However, S. litura populations were susceptible or exhibited minimal resistance to tetraniliprole. Possible cross-resistances between chlorantraniliprole, cyantraniliprole, metaflumizone, pyridalyl, and chromafenozide were found by pairwise correlation, which also revealed that tetraniliprole lacked cross-resistance with all insecticides tested. Our results suggest suspending the use of chemical agents against which S. litura displayed significant field-evolved resistance, such as chlorantraniliprole, metaflumizone, and pyridalyl, in favor of pesticides against which S. litura was susceptible or exhibited minimal resistance, such as tetraniliprole and chromafenozide, which may help slow the development of insecticide resistance, and in which field management programs aimed at controlling S. litura in China would benefit from the integration of such survey-informed insecticide application strategies. Moreover, the baseline susceptibility confirmed for the six tested insecticides can contribute to design strategies of resistance management for S. litura.

Resistance and fitness costs in diamondback moths after selection using broflanilide, a novel meta-diamide insecticide

The diamondback moth (DBM) Plutella xylostella (L.) (Lepidoptera: Plutellidae) is an insect pest found around the world that feeds on cruciferous crops. The DBM has become resistant to most insecticides in current use in the field. Broflanilide is a novel meta-diamide insecticide that binds to a new site on the γ-aminobutyric acid receptor and very efficiently protects against most pests in the order Lepidoptera, including DBM. In this study, the resistance of a laboratory-bred susceptible strain of DBM to broflanilide and the fitness costs posed by broflanilide to the DBM were evaluated. The DBM had no obvious resistance to broflanilide after 10 generations of selection. The realized heritability h² was 0.033, suggesting a low risk of resistance developing in this strain. The F10 generation had no cross-resistance to the insecticides abamectin and endosulfan (which target the γ-aminobutyric acid receptor) and chlorantraniliprole (which targets a non-γ-aminobutyric acid receptor). The specific activities of important detoxification enzymes (cytochrome P450 monooxygenase, esterase, and glutathione S-transferase) were not obviously altered. However, the larval stage was prolonged and the adult stage was shortened significantly in F11 generation than the F0 generation. The total preoviposition period TPOP significantly prolonged 1.90 d in F11 generation. The fitness value R_f (0.93) was lower for the F11 generation than the F0 generation. The results indicated that long-term exposure to broflanilide exerts clear fitness costs in the DBM. This information will be useful in identifying reasonable broflanilide application guidelines for managing broflanilide resistance in the DBM.

Bacillus thuringiensis and Chlorantraniliprole Trigger the Expression of Detoxification-Related Genes in the Larval Midgut of Plutella xylostella

Background: Diamondback moth (DBM), Plutella xylostella (L.), has developed resistance to many insecticides. The molecular mechanism of DBM resistance to Bt-G033A combined with chlorantraniliprole (CL) remains undefined. Methods: In this study, field-resistant strains of Plutella xylostella to three pesticides, namely, Bacillus thuringiensis (Bt) toxin (Bt-G033A), CL, and a mixture of Bt + CL, were selected to evaluate the resistance level. Additionally, transcriptomic profiles of a susceptible (SS-DBM), field-resistant (FOH-DBM), Bt-resistant (Bt-DBM), CL-resistant (CL-DBM), and Bt + CL-resistant (BtC-DBM) strains were performed by comparative analysis to identify genes responsible for detoxification. Results: The Bt-G033A was the most toxic chemical to all the DBM strains among the three insecticides. The comparative analysis identified 25,518 differentially expressed genes (DEGs) between pairs/combinations of strains. DEGs were enriched in pathways related to metabolic and catalytic activity and ABC transporter in resistant strains. In total, 17 metabolic resistance-related candidate genes were identified in resistance to Bt-G033A, CL, and Bt + CL by co-expression network analysis. Within candidate genes, the majority was upregulated in key genes including cytochrome P450, glutathione S-transferase (GST), carboxylesterase, and acetylcholinesterase in CL- and BtC-resistant strains. Furthermore, aminopeptidase N (APN), alkaline phosphatase (ALP), cadherin, trypsin, and ABC transporter genes were eminent as Bt-resistance-related genes. Expression patterns of key genes by the quantitative real-time PCR (qRT-PCR) proved the credibility of transcriptome data and suggest their association in the detoxification process. Conclusion: To date, this study is the most comprehensive research presenting functional transcriptome analysis of DBM using Bt-G033A and CL combined insecticidal activity.

Historical changes in the lethal effects of insecticides against the diamondback moth, Plutella xylostella (L.)

BACKGROUND

The diamondback moth, Plutella xylostella (L.), is one of the most destructive pests owing to the rapid development of resistance to various groups of insecticides. Although the rotation strategy has become common, resistance management continues to be a critical issue.

RESULTS

In Japan, historical changes in the susceptibility of P. xylostella to insecticides were estimated using 82 references. The meta-analysis using generalized linear mixed models suggested that each mode of action (MoA) had different patterns of changes to the susceptibility, which might be due to the prevalence of the rotation strategy since the 2010s and the fitness cost of the resistance gene. For example, spinosyns and bacterial insecticide Bacillus thuringiensis (Bt), for which resistance has a high fitness cost, maintained high lethal effects on P. xylostella throughout several decades. However, pyrethroids and benzoylureas, for which resistance has no or little fitness cost, remained at low levels of susceptibility regardless of the rotation strategy.

CONCLUSION

Our findings suggest optimal combinations of MoA for a sustainable rotation strategy based on their stability and recovery of effectiveness. Susceptibility monitoring using our method and further study of fitness costs will improve the rotation strategy concerning P. xylostella in the future. © 2021 Society of Chemical Industry.

Cross-resistance, fitness costs, and biochemical mechanism of laboratory-selected resistance to tenvermectin A in Plutella xylostella

BACKGROUND

Tenvermectin A is a new avermectin derivative that has good insecticidal and acaricidal effects. In order to study the resistance of Plutella xylostella to tenvermectin A, a sensitive strain (SS) and a laboratory-selected tenvermectin A-resistant strain (RS, 33.57-fold) were used to evaluate cross-resistance and fitness costs as well as to determine the resistance mechanism.

RESULTS

There was no cross-resistance with common pesticides except for moderate cross-resistance with cypermethrin (resistance ratio = 10.26-fold) observed in RS. The activities of metabolic enzymes were measured, and the results showed that mixed function oxidase (MFO) and carboxylate esterase (CarE) in RS increased significantly by 2.92- and 2.86-fold, respectively, compared with SS. In addition, there was no obvious difference in glutathione-S-transferase (GST), which indicated that enhanced MFO and CarE activities may be the main mechanisms of detoxification. In the four typical resistance-related genes, expression of GluCl (4.86-fold), ABCC2 (3.85-fold), and CYP6 (2.94-fold) in RS were significantly promoted, but expression of GST was not. The clone and sequence of the PxGluClα subunit displayed six mutations that could lead to changes in the amino acid residues.

CONCLUSION

High suitability related to tenvermectin A resistance was observed in RS, and it was found that the developmental stages of RS were significantly shortened and the survival rate of females was reduced. In addition, the mechanism of resistance to tenvermectin A may be regulated by the glutamate-gated chloride channel, ATP-binding cassette transporter, and MFO. In general, the study of resistance and biochemical mechanisms can provide beneficial and rational information for the management of resistance in P. xylostella. © 2021 Society of Chemical Industry.

Characterization of field-evolved resistance to pyridalyl in a near-isogenic line of diamondback moth, Plutella xylostella

BACKGROUND

Plutella xylostella has developed resistance to a variety of pesticides in the field. Selection, inheritance, a near-isogenic line, cross-resistance and biochemical mechanisms of pyridalyl resistance were characterized in a field-collected resistant population of P. xylostella from China.

RESULTS

Compared with a susceptible IVF-S strain, the field-collected FZ population showed ~ 350-fold resistance to pyridalyl. The FZ-PY strain, selected from the FZ population using pyridalyl, developed ~ 640-fold resistance to pyridalyl. Inheritance tests indicated that pyridalyl resistance in the FZ-PY strain was autosomal and incompletely recessive. Through successive backcrossing to IVF-S, a near-isogenic strain (NIL-PY) was established that exhibited 191.21-fold resistance to pyridalyl and no cross-resistance to other tested popular insecticides. No significant effects of synergists and higher activities of metabolic enzymes were observed in NIL-PY compared with IVF-S. Furthermore, the survival rate of NIL-PY larvae, and female oviposition, fecundity and egg viability were markedly reduced in NIL-PY compared with IVF-S. The fitness of NIL-PY was found to be 0.56 compared with IVF-S.

CONCLUSION

Considering that no relevant effects of synergists or oxidative metabolism were observed in NIL-PY, and that pyridalyl resistance results in significant fitness costs compared with IVF-S, further research will be conducted on the mechanism of target-site resistance. © 2020 Society of Chemical Industry.

Identification and Expression of Inward-Rectifying Potassium Channel Subunits in Plutella xylostella

In insects, inward-rectifying potassium (Kir) channels regulate vital physiological functions, such as feeding behavior, silk secretion, renal excretion, and immune function. Therefore, they offer promising potential as targets for insecticides. Three types of Kir subunits have been identified in Diptera and Hemiptera, but the Kir subunits of Lepidoptera still remain unclear. This study identified five Kir subunit genes (pxkir1, pxkir2, pxkir3A, pxkir3B, and pxkir4) in the transcriptome of Plutella xylostella. Phylogenetic analysis identified pxkir1, pxkir2, pxkir3A, and pxkir3B as orthologous genes of kir1–3 in other insects. Interestingly, pxkir4 may be encoding a new class of Kir subunit in Lepidoptera that has not been reported to date. To identify further Kir channel subunits of P. xylostella, the gene expression profiles of five pxkir genes were studied by quantitative real-time PCR. These pxkir genes are expressed throughout the development of P. xylostella. pxkir1 and pxkir2 were highly expressed in thoraxes and legs, while pxkir3 (3A and 3B) and pxkir4 had high expression levels in the midgut and Malpighian tubules. This study identified the composition and distribution of Kir subunits in P. xylostella for the first time, and provides useful information for the further study of Kir channel subunits in Lepidoptera.

Cross-resistance, biochemical mechanism and fitness costs of laboratory-selected resistance to pyridalyl in diamondback moth, Plutella xylostella

Pyridalyl belongs to one novel type of insecticides with uncertain mode of action, and it showed significant efficacy against Plutella xylostella, which has been considered as one notorious insect pest in the world. To characterize pyridalyl resistance in P. xylostella, one susceptible strain XY-PS and one laboratory-selected pyridalyl-resistant strain XY-PR (34.4-fold) were used to establish cross-resistance patterns, and no cross-resistance to a series of popular insecticides in the XY-PR was observed. Activities of metabolic enzymes were measured and results showed that there was an approximate 5.2-fold significant increase in cytochrome P450 monooxygenase (P450) and no significant differences in glutathione S-transferase (GST) and esterase between XY-PR and XY-PS, indicating that the enhanced activity of P450 could be dominant mechanism of detoxification. Furthermore, expression profiles of three previously published resistance-associated P450 genes were established but no one was significantly different expression. Besides, fitness costs associated with pyridalyl resistance was observed in XY-PR, and it had been found that survival rate of larvae and hatchability were reduced in XY-PR. Then, by calculating the net replacement rate (R₀) of XY-PS, the fitness of XY-PR was established as 0.64. In conclusion, above results provided helpful data and information for studying further on mechanism of pyridalyl resistance, and will be conductive to design rational strategies of resistance management in P. xylostella.

ABCC2 participates in the resistance of Plutella xylostella to chemical insecticides

The ABCC2 protein of Plutella xylostella is an important target of Cry1A toxins from Bacillus thuringiensis (Bt), but whether this protein is involved in the resistance of P. xylostella to other insecticides remains unclear. In this study, the abcc2 gene of P. xylostella was cloned and the expression levels of Pxabcc2 in susceptible and resistant strains were investigated. ABCC2 was found to be expressed 3.2-6.7-fold higher in the resistant strain than in the susceptible strain; in the surviving P. xylostella, ABCC2 levels were significantly higher when treated with indoxacarb, avermectin, and beta-cypermethrin. We constructed a stable ABCC2-expressing HEK-293 cell line to reveal the contribution of ABCC2 to insecticide resistance. The avermectin and chlorfenapyr sensitivities of the stably-transfected cell line were significantly lower than those of the control cells. The intracellular avermectin concentration was significantly lower in the stably-transfected cell line than in the control cells after four hours of exposure. This study shows that up-regulated ABCC2 expression is related to insecticide resistance in P. xylostella. Moreover, we used RNA interference technology to reduce ABCC2 levels in P. xylostella. Down-regulating ABCC2 expression did not significantly affect avermectin or chlorfenapyr resistance in P. xylostella. We speculate that increased ABCC2 expression can enhance metabolic resistance in P. xylostella. This study also provides new insights into cross-resistance between B. thuringiensis toxins and chemical insecticides.