How long is long enough? Decreasing on plant extract lethal effects over time affects Aedes aegypti larvae mortality

The mosquito Aedes aegypti has overcome all kinds of human being mosquito control attempts over the last century. Strategies for vector population control resorts to the use of synthetic insecticides, which can lead to problems of intoxication in humans and environmental contamination. We evaluated the effects of Bacillus thuringiensis var. israelensis (Bti), Ilex paraguariensis (mate-herb) and Ilex theezans (cauna-herb) extracts against A. aegypti larvae mortality. The bioassays were conducted under controlled laboratory conditions of temperature (27{+/-}3{degrees}C) and photoperiod (12h). Hydroalcoholic I. theezans leaves extract displayed better residual effect compared to I. paraguariensis fruit aqueous extract. Variation in larval mortality was also observed in the exposure periods (low after a few weeks). Low mortality after a few weeks may mean increased the food for mosquito in a oppose effect over time. The residual effect of Bti was observed during the 56 days of the study duration (100% of mortality). The strongest residual effect of I. theezans was probably due to the presence of chemical on its leaves, such coumarins, hemolytic saponins and cyanogenic glucosides, absent in I. paraguariensis. On the other hand, alternative methods to vector control present risks in a long term scale by reversal of larvicide effect into food resource. Our results contributed to the prospection of natural insecticides and open the possibility for subsequent studies of the use of plant extracts in field situations in a short time scale.