Abstract
The increased application of chemical control programs has led to the emergence and spread of insecticide resistance in mosquitoes. Novel environmentally safe control strategies are currently needed for the control of disease vectors. The use of entomopathogenic fungi could be a suitable alternative to chemical insecticides. Currently, Beauveria spp. and Metarhizium spp. are the most widely used entomopathogenic fungi for mosquito control, but increasing the arsenal with additional fungi is necessary to mitigate the emergence of resistance. Entomopathogenic fungi are distributed in a wide range of habitats. We have performed a comprehensive screen for candidate mosquitocidal fungi from diverse outdoor environments in Maryland and Puerto Rico. An initial screening of 22 fungi involving exposure of adult Anopheles gambiae to 2-weeks-old fungal cultures identified five potent pathogenic fungi, one of which is unidentified and the remaining four belonging to the three genera Galactomyces sp., Isaria sp. and Mucor sp. These fungi were then screened against Aedes aegypti, revealing Isaria sp. as a potent mosquito killer. The entomopathogenic effects were confirmed through spore-dipping assays. We also probed further into the killing mechanisms of these fungi and investigated whether the mosquitocidal activities were the result of potential toxic fungus-produced metabolites. Preliminary assays involving the exposure of mosquitoes to sterile filtered fungal liquid cultures showed that Galactomyces sp., Isaria sp. and the unidentified isolate 1 were the strongest producers of factors showing lethality against An. gambiae. We have identified five fungi that was pathogenic for An. gambiae and one for Ae. aegypti, among these fungi, four of them (two strains of Galactomyces sp., Mucor sp., and the unidentified isolate 1) have never previously been described as lethal to insects. Further characterization of these entomopathogenic fungi and their metabolites needs to be done to confirm their potential use in biologic control against mosquitoes.
Highlights
Vector-borne disease accounts for more than 17% of all infectious disease, causing more than 700,000 deaths annually
Once these were separated and streaked on individual plates, several isolates did not grow for more than one generation and were excluded from the study. Possible explanations for this could be that these isolates had an obligate symbiosis relationship with microorganisms in the collected sample, or that they could not produce a sufficient number of spores necessary to propagate under laboratory conditions
The failure of disease control programs resulting from insecticide resistance among mosquito populations highlights the urgent need for new tools for mosquito control, including new insecticides
Summary
Vector-borne disease accounts for more than 17% of all infectious disease, causing more than 700,000 deaths annually. Since one of the mode of action of entomopathogenic fungi is mediated through surface contact with adult mosquitoes, these agents would be applicable to a variety of deployment strategies, some of which are already in use for chemical insecticides (Scholte et al, 2005; Farenhorst et al, 2011). They are effective against mosquito strains that have developed resistance to the available chemical insecticides (Farenhorst et al, 2009, 2010; Blanford et al, 2011). In contrast to chemical insecticides that generally kill mosquitoes within 24 h, fungal biopesticides usually require more than a week to kill exposed mosquitoes and thereby the probability is decreased that resistance will emerge (Ffrench-Constant, 2005; Read et al, 2009)
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