The impact of yeast-encapsulated orange oil in Aedes aegypti oviposition.

The yeast-encapsulated orange oil (YEOO) is a novel larvicide under development against vector mosquitoes. Despite its efficiency against Aedes aegypti (L.) in small scale experiments, its applicability in vector control can be influenced by other effects on mosquito behaviour or physiology. For this reason, the impact of YEOO particles in mosquito oviposition was evaluated in laboratory and semi-field conditions. Oviposition assays with one gravid Aedes aegypti female were carried under laboratory and semi-field conditions with natural light and temperature fluctuation. For all ovitraps, the number of eggs was manually counted in the wooden paddle and in the solution of each ovitrap. The proportion of eggs between substrates (wooden paddle and solution) varied between conditions, with females in laboratory presenting a lower preference to lay eggs in paddles when compared with studies in semi-field. This behaviour shifts in laboratory can create challenges to extrapolate results from laboratory to the field. Here, studies in both conditions indicate a similar impact of YEOO particles in Aedes aegypti oviposition. The potential treatment concentration of YEOO particles presents a strong repellent/deterrent effect (-0.559 > OAI > -0.760) within the initial 72h of application when compared with water, and weak repellent/deterrent signal (OAI = -0.220) when compared against inactivated yeast. Control ovitraps with water were more positive for egg presence than treated ovitraps, while ovitraps with YEOO particles and inactivated yeast present similar number of positive ovitraps. It is possible that the repellent/deterrent action is partially driven by the delivery system, since most times Citrus sinensis EO oviposition repellent/deterrent signal is weak, and it seem influenced by solvent/delivery used. However, it is unclear how the yeast wall that protect/surrounds the orange oil will negatively affect oviposition since live yeast are normally consider an attractant for mosquito oviposition.

High larvicidal efficacy of yeast-encapsulated orange oil against Aedes aegypti strains from Brazil.

BACKGROUND: Botanical substances such as essential oils (EOs) have demonstrated insecticidal properties and are a valid option for vector control. However, free EOs are unreliable as mosquito larvicides due their easy degradation by environmental exposure to ultraviolet light and higher temperatures. Here, we assessed the efficacy of a mosquito larvicide based on orange oil in a yeast-based delivery system against Aedes aegypti strains with different resistance status towards chemical neurotoxic insecticides. This larvicide preparation was physicochemically characterized in a previous report. METHODS: Larvae of four Ae. aegypti strains from different regions of Brazil and different resistance profiles for deltamethrin (pyrethroid) and temephos (organophosphate) were tested against yeast-encapsulated orange oil (YEOO) in laboratory conditions for measurement of LC50 and LC90 values. The same assays were performed with the Belo Horizonte strain under environmental conditions (natural light and temperature). The resistance profiles of these strains were compared to the Rockefeller reference strain in all conditions. RESULTS: YEOO was found to be a highly active larvicide (LC50 < 50 mg/L) against all Ae. aegypti strains tested in both laboratory conditions (LC50 = 8.1-24.7 mg/L) and environmental conditions with natural light and temperature fluctuation (LC50 = 20.0-49.9 mg/L). Moreover, all strains were considered susceptible (RR < 5) to YEOO, considering resistance ratios calculated based on the Rockefeller strain. The resistance ratios were only higher than 2.5 for LC90-95 of Belo Horizonte in the laboratory, probably due the higher heterogeneity associated with older egg papers (> 5 months). CONCLUSION: YEOO demonstrates high larvicidal activity against Ae. aegypti strains with resistant phenotypes for deltamethrin (PY) and temephos (OP). This larvicidal activity suggests the potential for the development of YEOO as an alternative intervention to synthetic insecticides in integrated vector management programs, for populations with resistance to commonly used insecticides.

Teste do óleo essencial de laranja (Citrus sinensis) encapsulado em leveduras para o controle da população de Aedes aegypti em Belo Horizonte - MG / Test of orange essential oil (Citrus sinensis) encapsulated in yeasts to control the population of Aedes aegypti in Belo Horizonte - MG

Arbovírus como dengue (DENV), Zika (ZIKV) e chikungunya (CHIKV) são hoje uma importante e constante ameaça à saúde humana em regiões tropicais e subtropicais do mundo. As epidemias causadas por esses vírus estão intimamente relacionadas à disseminação do seu vetor primário, o Aedes aegypti. Na ausência de terapia específica e vacinas amplamente disponíveis para a população contra DENV, ZIKV e CHIKV, a redução da ocorrência das arboviroses causadas por estes vírus deve ocorrer especialmente através do controle do vetor. Para tal, inseticidas químicos neurotóxicos foram utilizados nas últimas décadas de forma contínua e indiscriminada, culminando na seleção de populações de A. aegypti resistentes a estes compostos. Sendo assim, o desenvolvimento de novos produtos com ação inseticida, sobretudo que associem facilidade de aplicação, eficiência e baixo impacto ambiental, é atualmente de grande interesse. Nesse contexto, esse estudo pretende testar a estabilidade, a atividade larvicida e a influência no comportamento de oviposição do óleo essencial da laranja encapsulado em célula de levedura (OELE) considerando populações de A. aegypti como alvo. A atividade larvicida do OELE foi avaliada sob condições ambiente de temperatura, umidade e luz. Foram realizados bioensaios do tipo dose-resposta para a determinação das concentrações letais e a mortalidade foi monitorada após 24 horas

Já a estabilidade do OELE foi avaliada semanalmente durante 21 dias em reservatórios plásticos dispostos em ambiente parcialmente exposto a luz solar. Os ensaios para investigar o impacto do larvicida OELE na escolha do local de oviposição por fêmeas individualizadas foram realizados no laboratório, em gaiolas teladas. O larvicida OELE foi considerado altamente ativo (CL50 < 50 mg/L) contra A. aegypti das duas populações testadas, porém, a população de Belo Horizonte apresentou valores superiores para CL50 (49,66 mg/L), CL90 (130,02 mg/L) e CL95 (180,36mg/L) quando comparada à linhagem Rockefeller (CL50=20,04 /CL90=57.3 e CL95= 81,92 mg/L). O período de estabilidade do OELE no ambiente com índices de mortalidade próximos a 100% não ultrapassou os 14 dias para 160 e 280mg/L. O OELE nas armadilhas influenciou a escolha do local de oviposição das fêmeas repelindo ou inibindo a postura de ovos. Estas observações suportam a hipótese de que o OELE pode vir a ser uma alternativa viável no controle de populações de A. aegypti, porém a instabilidade e a degradação do biolarvicida demandam mais avaliações, assim como a influência de fatores ambientais. (AU)