Abstract
BackgroundThe entomopathogenic fungus Metarhizium anisopliae is a candidate for the integrated management of the disease vector mosquito Aedes aegypti. Metarhizium anisopliae is pathogenic and virulent against Ae. aegypti larvae; however, its half-life is short without employing adjuvants. Here, we investigated the use of neem oil to increase virulence and persistence of the fungus under laboratory and simulated field conditions.MethodsNeem was mixed with M. anisopliae and added to recipients. Larvae were then placed in recipients at 5-day intervals for up to 50 days. Survival rates were evaluated 7 days after exposing larvae to each treatment. The effect of neem on conidial germination following exposure to ultraviolet radiation was evaluated under laboratory conditions. Statistical tests were carried out using ANOVA and regression analysis.ResultsLaboratory bioassays showed that the fungus alone reduced survival to 30% when larvae were exposed to the treatment as soon as the suspension had been prepared (time zero). A mixture of fungus + neem resulted in 11% survival at time zero. The combination of fungus + neem significantly reduced larval survival rates even when suspensions had been maintained for up to 45 days before adding larvae. For simulated-field experiments 1% neem was used, even though this concentration is insecticidal, resulting in 20% survival at time zero. However, this toxic effect was reduced over time. When used alone under simulated-field conditions the fungus rapidly lost virulence. The formulation fungus + neem effectively maintained fungal virulence, with larval survival rates significantly reduced for up to 45 days after preparation of the suspensions. The effective half-life of the fungus or neem when used separately was 6 and 13 days, respectively. The half-life of fungus formulated in 1% neem was 34 days. Conidia suspended in neem maintained high levels of germination even following a 2-h exposure to ultraviolet radiation.ConclusionsA combination of the entomopathogenic fungus M. anisopliae with neem oil effectively increases the half-life and virulence of the fungus when tested against Ae. aegypti larvae, even under simulated field conditions. Neem oil also protected the fungus from the damaging effects of ultraviolet radiation.
Highlights
The entomopathogenic fungus Metarhizium anisopliae is a candidate for the integrated management of the disease vector mosquito Aedes aegypti
We have recently shown that combining neem oil with M. anisopliae significantly increases the virulence of this fungus against Ae. aegypti larvae [9], even when this oil was used at low concentrations
Fungal persistence under laboratory conditions Before testing fungal persistence in the field, bioassays were carried out under laboratory conditions using 0.01% neem (v/v), fungus (1 × 109 conidia ml-1) and a combination of neem + fungus, with larvae exposed to the different treatments immediately following preparation or following different time periods of up to 50 days
Summary
The entomopathogenic fungus Metarhizium anisopliae is a candidate for the integrated management of the disease vector mosquito Aedes aegypti. Arboviruses vectored by the mosquito Aedes aegypti, namely dengue, Zika, chikungunya and urban yellow fever, are of growing concern, in tropical and semi-tropical regions of the world, and in southern Europe where it is a threat [1]. The 2018 yellow fever epidemic in Brazil [4] was probably vectored by mosquitoes of the genus Haemagogus and Sabethes, which are only found in forested areas, in what is termed the sylvatic cycle [5]. This epidemic resulted in the death of 338 people in 2018. Yellow fever epidemics will become even more serious if the virus enters the urban cycle, where it is normally vectored by Ae. aegypti
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