Selective activity of diselenides against Aedes aegypti (Diptera: Culicidae) larvae

Abstract INTRODUCTION: Aedes aegypti (L.) is the major vector of arboviruses that causes serious public health concerns in tropical and subtropical countries. METHODS: We examined the larvicidal activity of 1,2-diphenyldiselenide [(PhSe)2] and 1,2-bis(4-chlorophenyl) diselenide [(p-ClPhSe)2] and determine its toxicity to different non-target organisms. RESULTS: (PhSe)2 and (p-ClPhSe)2 killed Ae. aegypti L3 larvae with LC50/24h values of 65.63 µM (20.48 mg/L) and 355.19 µM (135.33 mg/L), respectively. (PhSe)2 was not toxic to the four model organisms. CONCLUSIONS: (PhSe)2 is a larvicidal compound with selective action against Ae. aegypti larvae. The mechanisms of action of (PhSe)2 under field conditions remain to be investigated.

Aedes aegypti (L.) is a mosquito widely distributed in tropical, subtropical, and temperate regions during warm seasons. The mosquito transmits human arboviruses, including dengue, chikungunya, and Zika virus 1 . It is estimated that the dengue virus infects over 390 million people in at least 100 countries on different continents and causes significant number of deaths and public health concerns. Zika virus infection has been associated with malformations in human fetal development 1 . The circulation of and co-infection by the dengue, chikungunya, and Zika viruses create a worrying scenario that demands effective measures to control the mosquito vector population 2 .
The Ae. aegypti populations can be controlled by different methods, such as the physical elimination of containers with larvae and pulp, chemical larvicides, and insecticides; the use of biological control agents such as Bacillus thuringiensis var. israelensis (Bti), Wolbachia bacteria, fish, and genetically modified male mosquitoes 2 . Larvicides are among the most important control agents because they eliminate immature forms and prevent the development of the adult form that transmits arboviruses. Bti is a larvicide of limited use because of its low persistence and the need for reapplication for effectiveness 3 . The chemical agent Pyriproxyfen acts more like a growth regulator than larvicide, and the agricultural pests Bemisia tabaci and Musca domestica (L.) are resistant to its action 4 . The development of new and efficient larvicides is important to broaden the arsenal of methods to control Ae. aegypti population. Larvicides must cause rapid larval death at low concentrations and no toxicity to non-target organisms. Sodium selenite suppresses the growth of Culex quinquefasciatus larvae and kills 50% of the larvae population at a concentration of 11 mg/L 5 . Selenium is a trace element essential for the antioxidant activity of selenoenzymes but can be toxic at high doses. Selenium compounds such as 1,2-diphenyldiselenide [(PhSe) 2 ] exert various biological activities, including neuroprotective, anti-inflammatory, antiulcer, antidepressant, and anxiolytic activities. Its structural analog 1,2-bis(4-chlorophenyl) diselenide [(p-ClPhSe) 2 ] prevents memory loss and treats metabolic disorders 6 . We hypothesize that low concentrations of selenium compounds (PhSe) 2 and (p-ClPhSe) 2 are toxic to Ae. aegypti larvae, but not other non-target organisms.
Ae. aegypti eggs were provided by Professor Margareth de Lara Capurro Guimarães, Ph.D., from the Institute of Biomedical Sciences of the University of São Paulo (USP), São Paulo, SP, Brazil. The eggs were hatched in glass jars containing autoclaved distilled water at 28 °C exposed to a 12 h photoperiod and fed with crushed fish feed (Tetramin ® ). Newly hatched larvae (L1) and third instar larvae (L3) were obtained approximately 12 h and 72 h after egg contact with water, respectively 7 .
The selenium compounds, (PhSe) 2 and (p-ClPhSe) 2 , were synthesized according to a modified literature procedure 8 . The identity and purity of the compounds were confirmed by nuclear magnetic resonance (NMR) spectroscopy. 1 H and 13 C NMR spectra were recorded on a Bruker Ascend™ 500 MHz instrument with tetramethylsilane as an internal standard. Column chromatography was performed using Merck silica gel (230-400 mesh). Thin-layer chromatography was performed using Merck silica gel GF254 of 0.25 mm thickness, and the plates were revealed with either iodine vapor or acidic vanillin. (PhSe) 2 : yellow solid; yield: 74%. 1  The tests were performed in plastic cups (180 mL) containing 20 mL of tap water and 20 L3 of Ae. Aegypti 7 , in the presence of different concentrations of (PhSe) 2 and (p-ClPhSe) 2 or 10% DMSO (control). The cups were incubated at 28°C with a 12 h photoperiod. The number of dead larvae were counted after 24 h and 48 h, and the larval mortality percentage was calculated. The (PhSe) 2 and (p-ClPhSe) 2 concentrations capable of killing 50% (LC 50 ) and 90% (LC 90 ) of the L3 larval population were estimated from the concentration-dependent mortality curves.
The eggs were kept in vials containing selenium compounds at a concentration corresponding to the LC 90/24h or 10% DMSO (control) to determine the death time of L1. The time from egg hatching to 100% L1 larvae death was recorded.
The toxicity of (PhSe) 2 and (p-ClPhSe) 2 to different model organisms was assessed at concentrations corresponding to the LC 90/24h . The compounds' environmental toxicity was estimated from the growth rate of the algae Chlorella vulgaris BR017 and the viability of the protozoan Tetrahymena pyriformis. The toxicity to non-target organisms was determined in Caenorhabditis elegans N2 and Galleria mellonella 9 .
Despite the availability of chemical control methods, the incidence of arboviruses transmitted by Ae. aegypti, such as dengue, chikungunya, and Zika, has increased annually, demanding the development of new larvicidal agents with low environmental toxicity and high selectivity and efficiency. Here, we report the selective larvicidal action of diphenyldiselenide (PhSe) 2 against Ae. aegypti larvae and its low environmental impact.