Activities of some Brazilian plants against larvae of the mosquito Aedes aegypti
Introduction
In Brazil, the rates of incidence of dengue and yellow fever have attained levels that are of considerable concern to local authorities. The mosquito that carries the arbovirus responsible for these diseases is Aedes aegypti [1]. This vector is now to be found in all parts of Brazil, its distribution and abundance being strongly influenced by the presence of man and by the level of poverty of the population [2]. Increasing deforestation, which permits greater agricultural utilisation, decreases the rural habitats of the mosquito leading to its migration to urban centres [3], thus following a similar migration pattern to that of the poorer section of the human population.
For dengue, at least, there are currently no effective methods by which to control the advance of the epidemic within the country. Despite the immense resource presented by the natural flora of Brazil, control of A. aegypti still depends basically on the use of synthetic pesticides. This strategy is, however, becoming inefficient because of the genetic plasticity of the mosquito. Each year, larger doses of synthetic insecticides are required leading to increased dangers for man and progressive contamination of the ecosystem.
The development of techniques that would provide more efficient insect control without serious effects on the environment is clearly required in the fight against the spread of this disease.
As part of our continued search of the biodiversity resource available in Brazil for natural products with utilisable bioactivity, we have assayed larvicidal activity towards A. aegypti of extracts and oils derived from nineteen Brazilian plants. The species, distributed between ten botanical families, were selected based on ethnobotanical and chemosystematic information, a summary of which is presented in Table 1.
Section snippets
Chemicals
Solvents were of commercial grade and were distilled prior to use. Anhydrous sodium sulphate and dimethylsulphoxide (DMSO) (Merck, Darmstadt, Germany) were of analytical grade.
Plant material
Plant specimens (minimum of 500 g fresh weight for each species) were collected from their natural habitats in the northeast of Brazil, mainly in the states of Alagoas, Pernambuco and Bahia. Plants were identified by botanist Dr. Elias de Paula of the Universidade Federal de Brasilia (UnB, Brasilia, Brazil); voucher
Results and discussion
The results obtained in the preliminary assays of eighteen extracts or oils of seventeen Brazilian plants against larvae of A. aegypti (Table 2) showed that only six were active according to our accepted norm (i.e. > 50% lethality at 500 μg/l). Of the active samples, all of which caused 100% lethality of larvae at 500 μg/l, five were oils and one was an ethanolic extract of stem material. The most active sample was cashew nut (A. occidentalis) oil which showed an LC50 value of 14.5 μg/l (Table 3
Acknowledgements
The authors wish to thank Conselho Nacional de Pesquisa e Desenvolvimento and Fundação de Amparo a Pesquisa do Estado de Alagoas for their financial support.
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