Screening of larvicidal activity of seaweed extracts against the mosquito Aedes aegypt

Mosquito-borne diseases are a big global concern, being responsible for 300 million of annual cases of infection, according to World Health Organization (WHO), and being one of the main causes of deaths worldwide. Some of these diseases were reduced in the last centuries, particularly in the XX century, but many are remerging and also appearing in places that they were not used to. The genera of mosquitos with more medical importance are Anopheles, Aedes and Culex, but this work focus on Aedes aegypti (Ae. Aegypti). Most of these diseases do not have a vaccine or treatment, so the best way to fight against them is controlling the vectors – in this case Ae. aegypti, and some authors defend that this control is easier when applied in larvae stage. Four main strategies have been used: physical, genetic, chemical and biological control. Chemical control, namely the use of phytochemicals such as seaweed extracts, is receiving more attention lately, once the other methods pose problems like the environmental persistence, hazard in non-target organisms, and resistance by the insects. Seaweed are a big source of compounds that have a wide range of bioactivities serving several applications, as antibacterials, antifungals, antivirals, antitumorals, among others. In this work, the mosquito larvicidal potential of five organic extracts from two macroalgae species - Fucus vesiculosus (F. vesiculosus) and Ulva lactuca (U. lactuca) – were accessed in Ae. aegypti. The extracts were obtained with the solvents ethanol, methanol, chloroform, hexane and dichloromethane. Mortality assays, according WHO guidelines, were performed counting the dead larvae after 24 and 48 hours of exposure. Body length and the weight of surviving larvae were also measured to investigate potential gross impacts of the extracts on larval growth/development. From these assays, dichloromethane extract from F. vesiculosus was the only one that showed significant differences compared with the control, reaching 58% mortality after 48 hours of exposure. Body size measurements and weighings showed no differences between treatments. Assays with Artemia salina (A. salina) were also carried out with 24 hours of exposure, to access possible effects of these extracts in non-target aquatic organisms. In the same concentration used in mosquito larvae, no significant differences were observed between the extracts and the control. However, in a higher concentration, three extracts from F. vesiculosus showed toxicity, namely ethanol, chloroform and dichloromethane. The results showed that dichloromethane extract from F. vesiculosus could be a useful source of larvicidal compounds to fight mosquitos, the most important disease vectors for transmitting diseases to Humans. The A. salina data support that lethality in non-target species is only attained at higher concentrations of extract when compared with those for the mosquito. Anyway, a wider range of non-target organisms should be assayed. Having the present work as basis, further studies could try to elucidate the mode of action of the extracts effects when inducing lethality, in addition to isolate and characterize the compounds present in the most promising extracts. By other hand, screening tests using combinations of extracts could be useful to pinpoint yet unknown synergy or potentiation effects. At last, our data support that more studies should be done on a wider range of seaweeds, as extracts from different species do present quite different bioactivities.