The Toxicology and Biochemical Characterization of Cantharidin on Cydia pomonella

Biogenetic cantharidin is a promising leading compound to manage insecticide resistance of Mythimna separata (Lepidoptera: Noctuidae)

Cantharidin (CTD) is a natural toxin with effective toxicity to lepidopteran pests. Nevertheless, little information is available on whether pests develop resistance to CTD. After being exposed to CTD (50 mg/L to 90 mg/L) or 10 generations, the resistance ratio of laboratory selected cantharidin-resistant Mythimna separata (Cantharidin-SEL) strain was only elevated 1.95-fold. Meanwhile, the developmental time for M. separata was prolonged (delayed1.65 in males and 1.84 days in females). The reported CTD target, the serine/threonine phosphatases (PSPs), have not been shown significant activity variation during the whole process of CTD-treatment. The activity of detoxification enzymes (cytochrome monooxygenase P450, glutathione-S-transferase (GST) and carboxylesterase) were affected by CTD selection, but this change was not mathematically significant. More importantly, no obvious cross-resistance with other commonly used insecticides was observed in the M. separata population treated with CTD for 10 generations (resistance ratios were all lower 2.5). Overall, M. separata is unlikely to produce target-site insensitivity resistance, metabolic resistance to CTD. Meanwhile, cantharidin-SEL is not prone to develop cross-resistance with other insecticides. These results indicate that CTD is a promising biogenetic lead compound which can be applied in the resistance management on M. separata.

The Inhibition of Serine/Threonine Protein Phosphatase Type 5 Mediates Cantharidin Toxicity to Control Periplaneta americana (L.)

The American cockroach, Periplaneta americana (L.), is a notorious urban pest. It has developed insecticidal resistance to commonly used insecticides. Cantharidin (CTD) is a defensive toxin derived from blister beetles. It has been verified to have insecticidal toxicity in a range of pests. In this study, we determined the ingestion toxicity of CTD and norcantharidin (NCTD) to P. americana to test whether they had the potential to be effective against P. americana. Bioassays revealed that CTD produces toxicity against P. americana. The median lethal concentration (LC₅₀) value of CTD was 50.92 μg/mL, while NCTD displayed nearly no toxicity against P. americana. The inhibition assays of serine/threonine protein phosphatases (PSPs) in P. americana indicated that CTD and NCTD could inhibit PSPs. The value of the half maximal inhibitory concentration (IC₅₀) of CTD was 7.21 ± 0.94 μM, whereas that of NCTD was higher, at 31.65 ± 3.87 μM. Furthermore, the inhibition effect of CTD on the serine/threonine protein phosphatase type 5 of P. americana (PaPP5) was superior to that of NCTD. Specifically, the IC₅₀ of CTD reached 0.39 ± 0.04 μM, while the IC₅₀ of NCTD was 1.87 ± 0.23 μM. This study paves the way for insect-derived agents (CTD) to be applied toward controlling P. americana and contributes to the development of novel insecticides based on PP5 as a target.

Effects of Cantharidin and Norcantharidin on Larval Feeding and Adult Oviposition Preferences of the Diamondback Moth (Lepidoptera: Plutellidae)

The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is a destructive insect pest of cruciferous plants that has developed resistance to almost every listed commercial insecticide. Cantharidin as an animal-derived biopesticide is a natural defensive compound produced by Meloidae insects with toxicity to many insects including P. xylostella. Norcantharidin is an important substitute of cantharidin and has similar insecticidal activities to cantharidin. Although the toxicity of cantharidin and norcantharidin to P. xylostella has been reported, little research has focused on the effects of cantharidin or norcantharidin on the behavior of P. xylostella. In this study, we investigated the feeding behavior of third-instar larvae and the oviposition preference of adult P. xylostella in order to explore the effects of different concentrations of cantharidin and norcantharidin. Results show that cantharidin and norcantharidin have antifeedant effect on P. xylostella larvae. The values for AFC50 were 13.0228 and 149.4210 mg/ml, respectively. Furthermore, the oviposition deterrence rate of cantharidin on P. xylostella ranged from 49.37 to 58.24% and that of norcantharidin was from 20.88 to 33.33%. These results suggest cantharidin and norcantharidin may have repellent and antifeedant effect on P. xylostella, which could contribute toward using biopesticides to manage P. xylostella and may provide a new strategy for integrated pest management.

Characterized cantharidin distribution and related gene expression patterns in tissues of blister beetles, Epicauta chinensis

Cantharidin (CTD), a terpenoid defensive toxin mainly produced by blister beetles, is widely known by its toxicity to both cancer cells and pests. However, little information is known about its biosynthesis in blister beetles. In this study, first we determined the CTD content in various tissues of adult blister beetles on different days after mating, and then detected the temporal and spatial expression patterns of genes related to CTD biosynthesis in Epicauta chinensis. Results revealed that the accessory gland is the source of the highest CTD production. The second highest level was in the fat body in male blister beetles after mating. In females, the highest CTD content was in the reproductive system except the ovary after mating. As revealed by messenger RNA expression level analysis, the highest levels of 3-hydroxy-3-methylglutary-CoA reductase (HMGR) and juvenile hormone epoxide hydrolase (JHEH) transcripts of E. chinensis were observed in the fat body in males after mating. However, the highest transcript level of EcHMGR was in the ovary and EcJHEH was maintained at a nearly similar level in females. The transcript level of methyl-farnesoate epoxide was significantly higher in the head and that of CYP4BM1 in the midgut in both male and female E. chinensis. We speculate that the fat body may play a more important role than other tissues on the CTD biosynthesis in male E. chinensis after mating. There may be multiple tissues involved in the process of CTD biosynthesis. These four genes probably play regulatory roles in different tissues in males.

Formulation and evaluation of norcanthridin nanoemulsions against the Plutella xylostella (Lepidotera: Plutellidae)

Background

Norcantharidin (NCTD), a demethylated derivative of cantharidin (defensive toxin of blister beetles), has been reported to exhibit insecticidal activity against various types of agricultural pests. However, NCTD applications are limited by its poor water solubility and high dosage requirement. Nanoemulsions have attracted much attentions due to the transparent or translucence appearance, physical stability, high bioavailability and non-irritant in nature. In general, nanoemulsions with small droplet size can enhance the bioavailability of drugs, whereas this phenomenon is likely system dependent. In present study, NCTD nanoemulsions were developed and optimized to evaluate and improve the insecticidal activity of NCTD against Plutella xylostella (Lepidotera: Plutellidae) by a spontaneous emulsification method.

Results

Triacetin, Cremophor EL and butanol were selected as the constituents of NCTD nanoemulsions via solubility determination, emulsification efficiency and ternary phase diagram construction. Insecticidal activity of NCTD nanoemulsion was associated with the content of surfactant and cosurfactant: (1) Higher effective toxicity exhibited at Smix (surfactant to cosurfactant mass ratio) = 3:1 that may be associated with the changes in interfacial tension; (2) NCTD nanoemulsion at 3:7 < SOR (surfactant to oil mass ratio) < 6:4 was more effective at lower surfactant level, which was attributed to the relatively slow diffusion rate of NCTD hindering by excess surfactant. Interestingly, nanoemulsions with smaller droplets were not found to be more effective in our study.

Conclusions

The optimized NCTD nanoemulsion (triacetin/Cremophor EL/butanol (60/20/20, w/w)) exhibited effective insecticidal activity (LC₅₀ 60.414 mg/l, LC₉₀ 185.530 mg/l, 48 h) than the NCTD acetone solution (LC₅₀ 175.602 mg/L, LC₉₀ 303.050 mg/L, 48 h). Spontaneous emulsifying nanoemulsion employed to formulate this poor water-soluble pesticide is a potential system for agriculture application.

Electronic supplementary material

The online version of this article (10.1186/s12896-019-0508-8) contains supplementary material, which is available to authorized users.

Characterization of Vitellogenin and Vitellogenin Receptor of Conopomorpha sinensis Bradley and Their Responses to Sublethal Concentrations of Insecticide

Conopomorpha sinensis Bradley is the dominant borer pest of Litchi chinesis and Euphoria longan. Current management of C. sinensis relies upon insecticide application to adult moths. In addition to the direct mortality induced by insecticides, a sublethal dose of insecticides also affects growth, survival, and reproduction in the exposed insects. Vitellogenin (Vg) and vitellogenin receptor (VgR) are normally identified as essential reproduction-related proteins in insects. In this study, we characterized these two genes from C. sinensis, and investigated their differential responses to sublethal concentrations of insecticide. Cloned CsVg and CsVgR consist of 5391 and 5424-bp open reading frames, which encode proteins of 1796 and 1807 amino acid residues, respectively. The CsVg protein contains the typical vitellogenin, DUF1943 and VWFD domains as other reported lepidopteran Vgs. The CsVgR was characterized as a typical low density lipoprotein receptor with two highly conserved LBD and EGF precursor domains, one hydrophobic transmembrane domain, one cytoplasmic domain, and 13 putative N-glycosylation sites. We next assessed the sublethal effect of four major insecticides on egg-laying in C. sinensis. The toxicity against C. sinensis varied among the insecticides tested, with LC₅₀ values ranging from 0.23 ppm for chlorpyrifos to 20.00 ppm for β-cypermethrin, among which emamectin benzoate (EB) showed a significant negative impact on egg-laying, survival rate, ovarian development, and mating rate of C. sinensis at LC₃₀ doses. Further investigation showed that the transcriptional level of CsVg and CsVgR were impaired in different way at 24, 48, and 72 h after EB exposure, and this result was in agreement with the diminished egg-laying of C. sinensis in the sublethal concentration EB-treated group. A repressed transcription level of CsVgR was observed at 48 h after treatment, suggesting that EB elicits a delayed response in the abundance of CsVgR. These results established different roles of CsVg and CsVgR in response to the sublethal effect of insecticides. CsVg might be a better parameter than CsVgR for assessing the effect of sublethal insecticides on reproduction in C. sinensis.

Toxicity and Sublethal Effects of Cantharidin on Musca domestica (Diptera: Muscidae)

The house fly, Musca domestica L. (Diptera: Muscidae), is a major pest of medical and veterinary importance all over the world. Management efforts for house flies are usually compromised owing to their resistance to many groups of conventional insecticides. Cantharidin, a natural toxin produced by meloid beetles, is a biopesticide with a reported toxicity to some insect pests including house flies. However, the effects of cantharidin on biological and fitness parameters of house flies have not yet been investigated. In the present study, we investigated the toxicity and sublethal effects of cantharidin on biological parameters of house flies for two consecutive generations. The results revealed that the values of LC50, LC25, LC10, and LC2 against house flies were to be 2.45, 1.23, 0.66, and 0.30 mg/liter, respectively. Sublethal effects of these concentrations on the development and reproduction parameters of house flies revealed that cantharidin reduced population growth by affecting pupation rate, adult emergence, and by lengthening developmental time. The female ratio, fecundity, egg hatching, and survival of adult flies were significantly reduced at LC2, LC10, LC25, and LC50 of cantharidin when compared with the control group. Furthermore, the increase in concentration of cantharidin had a significant effect on reducing the mean values of mean relative growth rate, net reproductive rate (Ro), intrinsic rate of natural increase (rm), and biotic potential (bp). In conclusion, the results of this study revealed the toxicity of cantharidin against house flies and the adverse effects of sublethal concentrations on biological parameters which may have positive implications for effective management of house flies.

Toxicity of Six Insecticides on Codling Moth (Lepidoptera: Tortricidae) and Effect on Expression of Detoxification Genes

The codling moth, Cydia pomonella (L.), is a key worldwide fruit pest that has evolved high levels of resistance to almost all classes of conventional insecticides. Neonicotinoids, a new reduced-risk biorational insecticide class, have remained an effective control approach. In this study, the toxicity and sublethal effect of conventional and reduced-risk biorational insecticides on transcripts abundance of three detoxification genes in codling moth were determined. Bioassays on a codling moth laboratory strain suggested that acetamiprid had the highest oral toxicity against the third-instar larvae compared with the other five pesticides. Results also indicated that acetamiprid exhibits long-term efficacy against codling moth even at 120 h post feeding. Real-time quantitative polymerase chain reaction showed that the detoxification genes CYP9A61, CpGST1, and CpCE-1 were differentially induced or suppressed by deltamethrin, cypermethrin, methomyl, carbaryl, and imidacloprid, depending on the type of insecticides; in contrast, no significant difference in CYP9A61, CpGST1, and CpCE-1 expressions were observed after acetamiprid exposure, when compared with the control. These results suggest that the reduced-risk biorational insecticide acetamiprid is an effective insecticide with no induction of detoxification genes and can be integrated into the management of codling moth.

Chronic Sublethal Effects of Cantharidin on the Diamondback Moth Plutella xylostella (Lepidoptera: Plutellidae)

The diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae), is a major pest of cruciferous vegetables worldwide. Cantharidin, a natural toxin isolated from blister beetles, has been reported to be toxic to P. xylostella. However, little is known on the chronic sublethal effects of cantharidin on this species. In this study, we assessed the changes of susceptibility, development, reproduction and other demographic parameters in both the selected P. xylostella strain (Sub, selected by LC25 cantharidin for consecutive 12 generations) and the revertant strain (SubR, derived from the Sub strain without being exposed to cantharidin for 12 generations). Results revealed that the two strains maintained a relatively high-level susceptibility to cantharidin. Severe adverse effects on the population dynamics and fitness in Sub strain were observed. In addition, repeated exposure of P. xylostella to sublethal concentration of cantharidin resulted in negative effects on adult performance and deformities in adults. Although morphologically normal for individuals, the SubR strain exhibited a disadvantage in population growth rate. Our results showed that sublethal concentration of cantharidin exhibited severe negative effects on population growth for longtime. These findings would be useful for assessing the potential effects and risk of cantharidin on P. xylostella and for developing effective integrated pest management.