ABSTRACT
A series of aromatic thiosemicarbazone-oxime [TP1 and TP2] derivatives and their Ni(II), Cu(II), and Co(II) complexes were synthesized, and their larvicidal activity was evaluated against Aedes aegypti and Aedes albopictus larvae. The efficacy of these substances to Aedes albopictus larvae has been demonstrated for the first time.
Laboratory colonized Aedes aegypti and Aedes albopictus larvae were subjected to larvicidal activity tests. Larval mortality rates at 24 and 48 hours were recorded and LC50 values were calculated. The study was carried out at Aydın Adnan Menderes University in 2021.
For Aedes aegypti, LC50 of TP1 and its Co(II) complex were 15.41, 9.75, μg/mL whereas for TP2 and its Co(II) complex, LC50 were 21.62, 20.50 μg/mL after 24 and 48 h respectively. For Aedes albopictus, TP1 and its Co(II) complex showed an LC50 of 12.06, 8.75 μg/mL, whereas TP2 and its Co(II) complex showed an LC50 of 32.87, 25.48 μg/mL, for 24, and 48 h respectively.
Both TP1 and TP2 compounds and their Co(II) complexes presented high efficacy against the larvae; it can be said that C=S groups in thiosemicarbazone derivatives are effective in showing activity and for this reason, studies should be continued to make these components effective.
Keywords: Mosquito, larvicidal effect, synthetic compounds, larval control
INTRODUCTION
Mosquitoes (Diptera: Culicidae), insects at the center of entomological research worldwide, are vectors of important diseases. Species such as Aedes aegypti (Linnaeus, 1762) and Aedes albopictus (Skuse, 1895) play important roles in the transmission of Chikungunya, Zika and Dengue infections (1). The incidence of Dengue, which causes the most death and illness among all other arthropod-borne viral diseases, has increased dramatically over the years with ≥350 million cases and nearly half a million deaths reported annually worldwide (2).
These Aedes mosquito species are spreading from their native areas to higher latitudes in North America and Europe (3,4). In Europe Ae. aegypti has been detected in Greece, Cyprus, Croatia, Italy, Spain, France, Türkiye, and Portugal; in countries south of the Sahara Desert, Western Morocco and Western Sahara, Northern Algeria and Tunisia, Egypt and Sudan in Africa; as well as in countries in Central and Southern Asia (5-8). Ae. albopictus, likewise, has spread from southern Asia to several countries in Africa, Europe, and America (9-11). The existence of Ae. albopictus was first recorded in Türkiye in 2011 (12). Then, respectively reported in Black Sea, Marmara and Aegean region in Türkiye (8,13-15).
Control of mosquito-transmitted diseases remains a big challenge. No vaccine is available for most of them, so the mainstay of disease management is vector control. Personal protection and chemical insecticides are also used extensively to control mosquitoes. Although chemical insecticides are efficacious to a certain degree, they are costly, pose serious risks to humans and the environment, and mosquitoes continue to develop resistance against most extant insecticides (16). Scientist across the globe continue to advocate the use of insect bio-pathogens such as Bacillus thuringiensis subsp. israelensis, Metarhizium anisopliae (Sorokin 1883) (Hypocreales: Clavicipitaceae) etc. against larvae and adults (17) as well as the search for safer alternatives.
Hydrazones are a class of organic compounds which are characterized by the presence of an azomethine-NHN=CH group, whereas thiosemicarbazones have the formula H2NC(S)NHN=CR2 (18). These functional groups play a major role in their biological activities (19-21). Hydrazones have various biological and clinical properties such as antituberculosis, anticonvulsant, anticancer, anti-inflammatory, antiviral, antiplatelet, antitumour, antimicrobial, antimalarial and antioxidant activity (22-26). Some thiosemicarbazones derivatives possess good larvicidal/insecticidal activity against various insects like Culex pipiens pallens (Linnaeus, 1758) (Diptera: Culicidae), Plutella xylostella (Lepidoptera: Plutellidae), Laphygma exigua (Hübner, 1808) (Lepidoptera: Noctuidae), Rhopalosiphum maidis (Fitch, 1856) (Homoptera: Aphididae), Nilaparvata lugens (Stal, 1854) (Hemiptera: Delphacidae) (27-31). Despite the availability of many insecticides, there has been a search for novel and more efficient agents with less bio-toxicity. The goal of developing new agents is valuable because currently very limited chemical classes are available for vector control.
In this study, we assessed a series of new aromatic thiosemicarbazone-oxime derivatives and their metalic complexes for their larvicidal effects against Ae. aegypti and Ae. albopictus larvae.
METHODS
Chemistry
The heteroaromatic thiosemicarbazone-oxime derivatives [TP1 and TP2], and their metal complexes [Ni(II), Cu(II) and Co(II)] were synthesized (32,33). The synthesis of these groups and their complexes is illustrated and outlined in Figure 1a, b. [TP1; 3-acetylthiosemicarbazone glyoxime; TP2:4‐ acetylthiosemicarbazone glyoxime]. Aromatic thiosemicarbazone derivatives consist of pyridine rings.
Compounds structures were determined using mass spectroscopy, UV, H-NMR, IR, C-NMR methods. All compounds obtained were purified by recrystallization from ethanol and verified by TLC and elemental analysis (34). Both analytical and spectral data of compounds were the same as proposed structures. These compounds were diluted to concentrations of 100, 75, 50 and 25 ppm in dimethyl sulfoxide (DMSO) and used in larvicidal activity bioassays (35).
Larvicidal Activity Assays
Aedes aegypti, Bora Bora strain was obtained from Hacettepe University Biology Department and has been reared in insectarium in the Vector Control Laboratory, Aydın Adnan Menderes University, Türkiye. Aedes albopictus larvae were sampled from Muğla populations in the Aegean Region and adult laboratory colonies were obtained from these larvae and reared in the insectarium. This species are susceptible strains that have no insecticide resistance. Both species are reared in an insectarium at 25±2 °C, 70±10 relative humidity, and 12 h light-dark photoperiod. Eggs collected were hatched in distilled water in plastic containers and larvae were provided ground fish food (Tetramin). Emerged adults fed on soaked cotton balls containing 10% sucrose.
Bioassays on the larvicidal activity of the compounds were carried out with some modifications using described by World Health Organization in 24-well plates (Corning Falcon microplates) against Ae. aegypti and Ae. albopictus larvae (36). Four different concentrations [100, 75, 50, 25 ppm (mg/L)] were tested against late 3rd-4th stage larvae of Ae. aegypti and Ae. albopictus mosquito.
Required concentrations of the compounds were dissolved and diluted in DMSO and used in larvicidal activity bioassays. Experiments were done under laboratory conditions in an incubator maintained at a constant temperature of 28 °C and 80% RH under light and dark conditions for 12 h each. Three independent replicates were carried out. Negative control had DMSO (1%) and the commercial Bti (0.05 g/L), (Bacillus thuringiensis var. israelensis), (VectoBac 12AS, Valent Biosciences, USA) was used as positive control. After treatment application, contents were evenly mixed by swirling plates. Larvae that failed to move after probing with a brush were recorded as dead. Larval mortality was calculated for each concentration and after 24 h and 48 h, larval mortality was recorded and the lethal concentration LC50 was calculated.
Statistical Analysis
Larval mortality data was adjusted (37) and analysed using SAS, Proc Probit (Version 9.2) to obtain LC50 (the lethal concentration median) value and chi-square values. We used SPSS 23.0 software and a 95% confidence level; results with p<0.05 were considered significant. Analysis was done if control mortality was <10%.
RESULTS
Aromatic thiosemicarbazone derivatives and their Ni(II), Cu(II) and Co(II) complexes were synthesized and evaluated for Ae. aegypti and Ae. albopictus mosquitoes for their larvicidal activity.
Aedes albopictus
The LC50 for aromatic thiosemicarbazone (TP1) and its Ni(II), Cu(II) and Co(II) complexes were 22.717, 26.265, 13.806 and 12.065 µg/mL respectively after 24 h of exposure. The LC50 values were recorded as 21.125, 23.308, 12.004, 8.755 µg/mL after 48 h of exposure (Table 1). Ae. albopictus larvae were more susceptible to the TP1-Co complex than the other metal complexes [Ni (II), Cu (II)] of thiosemicarbazone oxime derivatives.
The LC50 for TP2 and its Ni (II), Cu (II) and Co (II) complexes were 51.594, 44.913, 35.719, 32.875 µg/mL after 24 h of exposure while the values were found to be 48.764, 39.037, 27.516, 25.489 µg/mL after 48 h of exposure (Table 1).
Aedes aegypti
Against Ae. aegypti, the LC50 for aromatic thiosemicarbazone (TP1) was 44.919 µg/mL and its Ni (II), Cu (II) and Co (II) complexes LC50 were 40.644,18.514,15.417 µg/mL after 24 h of exposure. After 48 h these values decline to 31.059, 32.038, 13.665, 9.755 µg/mL respectively (Table 2).
For aromatic thiosemicarbazone (TP2) and its Ni(II), Cu(II) and Co(II) complexes, LC50 values were found to be 44.913, 42.308, 32.875, 21.612 µg/mL after 24 h of exposure, while the values were found to be 42.494, 33.683, 25.489, 20.504 µg/mL after 48 h of exposure, respectively.
DISCUSSION
We observed that TP1-Co complex (II) and TP2-Co complex (II) presented higher toxicity against both species than the other compounds named TP1, TP1-Ni complex (II), TP1-Cu complex (II), TP2, TP2-Ni complex (II), TP2-Cu complex (II) after 24 and 48 h of exposure and these compounds were found more effective for Ae. albopictus than Ae. aegypti larvae.
According to Rayms-Keller et al. (38) showed that copper edetate in nanostructures and chitosan microcapsules exhibited larvicidal activity against Ae. aegypti larvae with LC90 of 60 and 20 mg/L, respectively. Also, Tabanca et al. (29) tested ten hydrazone derivatives against Ae. aegypti larvae. They showed that compound 9 [CH(CH3)2] exhibited larvicidal activity with LD50 values of 57.4 ppm and LD90 of 297.8 ppm after 24 h treatment. Similarly, Rochelly et al. (39), showed that tested Aedes aegypti and Anopheles darlingi larvae were sensitive to benzoyl thiosemicarbazone and to the metal nickel complex; benzoyl thiosemicarbazone was more toxic than nickel complex. Kaplancikli et al. (40) showed that hydrazide-hydrazone derivates namely 4-Fluorobenzoic acid [(2-furanyl) methylene] hydrazide), 11(3-Acetyl-5-(4-fluorophenyl)-2-phenyl-2,3-dihydro-1,3,4-oxadiazole), 12(3-Acetyl-5-(4-fluorophenyl)-2-(4-bromophenyl)-2,3-dihydro-1,3,4-oxadiazole) and 17 (3-Acetyl-5-(4-fluorophenyl)-2-[4-(dimethyl amino) phenyl]-2,3-dihydro-1,3,4-oxadiazole at a concentration of 100 ppm caused 100% mortality against Ae. aegypti larvae. In contrast, our benzaldehyde derivative and hydrazones carrying oxime group were not effective against the mosquitoes in our study. Bursalı et al. (35) investigated the larvicidal effects of Co(II) complexes with barbiturate derivatives against Ae. aegypti and demonstrated that bis[(2-phenoxyphenyl)(2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene)methyl]amidocopper(II) showed high larvicidal activity with LC50 of 37.91 ppm.
Aromatic thiosemicarbazone and derivatives displayed high toxicity against mosquito larvae so have the potential to be used as larvicides against important mosquito species. However, more studies are needed to elucidate their mode of action as well as to assess the impacts such metals will have on the other organism and the environment and other insects. Insecticide resistance has necessitated the development new compounds, but this is challenging (41). Integrated management programmers aimed at mosquito larvae serves as one of the most efficacious means to control populations and reduce vector-borne disease incidence in endemic areas.
Aromatic thiosemicarbazone derivatives and their Ni(II), Cu(II) and Co(II) complexes were synthesized and evaluated for their larvicidal activity against Aedes mosquitoes. This is the first study done with Aedes albopictus with these substances. The data results indicating that the aromatic thiosemicarbazone complex has insecticidal potential against Ae. aegypti and Ae. albopictus larvae and may be used to design new derivatives.
CONCLUSION
The search for new active compounds continues as resistance develops to pesticides used to combat existing vectors. Mosquito management programs targeting larvae serve as one of the most effective ways to control insect populations and reduce the number of vector-borne diseases. A series of aromatic thiosemicarbazone and hydrazone derivatives their Ni(II), Cu(II) and Co(II) complexes were synthesized and evaluated for their larvicidal activity against Ae. aegypti and Ae. albopictus. The data results indicated that the aromatic thiosemicarbazone complex has insecticidal potential and may be used to design new derivatives to increase the bioefficacy of these compounds against Ae. aegypti and Ae. albopictus larvae. However, elucidating the mode of action of these compounds in larvae and developing new compounds with pharmalogical potential are necessary.
Ethics
Ethics Committee Approval: This article does not contain any studies with animals performed by any of the authors.
Informed Consent: This article does not contain any studies with animals performed by any of the authors.
Authorship Contributions
Concept: F.B., İ.B.B., F.M.Ş., İ.D., Design: F.B., İ.B.B., F.M.Ş., Data Collection or Processing: F.B., Analysis or Interpretation: F.B., Literature Search: F.B., Writing: F.B., İ.B.B., F.M.Ş.
Conflict of Interest: No conflict of interest was declared by the authors.
Financial Disclosure: The authors declared that this study received no financial support.
