Date Published: October 16, 2015
Publisher: Public Library of Science
Author(s): Javier A. Garza-Hernández, Filiberto Reyes-Villanueva, Tanya L. Russell, Marieta A. H. Braks, Alberto M. Garcia-Munguia, Mario A. Rodríguez-Pérez, Mark Quentin Benedict. http://doi.org/10.1371/journal.pntd.0004144
Abstract: BackgroundDengue is the most prevalent arboviral disease transmitted by Aedes aegypti worldwide, whose chemical control is difficult, expensive, and of inconsistent efficacy. Releases of Metarhizium anisopliae—exposed Ae. aegypti males to disseminate conidia among female mosquitoes by mating represents a promising biological control approach against this important vector. A better understanding of fungus virulence and impact on reproductive parameters of Ae. aegypti, is need before testing auto-dissemination strategies.Methodology/Principal FindingsMortality, mating competitiveness, sperm production, and the capacity to auto-disseminate the fungus to females up to the 5thcopulation, were compared between Aedes aegypti males exposed to 5.96 x 107 conidia per cm2 of M. anisopliae and uninfected males. Half (50%) of fungus-exposed males (FEMs) died within the first 4 days post-exposure (PE). FEMs required 34% more time to successively copulate with 5 females (165 ± 3 minutes) than uninfected males (109 ± 3 minutes). Additionally, fungus infection reduced the sperm production by 87% at 5 days PE. Some beneficial impacts were observed, FEMs were able to successfully compete with uninfected males in cages, inseminating an equivalent number of females (about 25%). Under semi-field conditions, the ability of FEMs to search for and inseminate females was also equivalent to uninfected males (both inseminating about 40% females); but for the remaining females that were not inseminated, evidence of tarsal contact (transfer of fluorescent dust) was significantly greater in FEMs compared to controls. The estimated conidia load of a female exposed on the 5th copulation was 5,200 mL-1 which was sufficient to cause mortality.Conclusion/SignificanceOur study is the first to demonstrate auto-dissemination of M. anisopliae through transfer of fungus from males to female Ae. aegypti during mating under semi-field conditions. Our results suggest that auto-dissemination studies using releases of FEMs inside households could successfully infect wild Ae. aegypti females, providing another viable biological control tool for this important the dengue vector.
Partial Text: Aedes aegypti is the principal vector of the four dengue (DENV) virus serotypes . Although its control through larval source removal is effective, the only rapid but inconsistent way to interrupt epidemic transmission is by chemical insecticides [2,3]. The scarcity of natural enemies of Ae. aegypti [4,5] has led to promising research into biocontrol with entomopathogenic fungus. Metarhizium anisopliae and Beauveria bassiana have been examined by direct exposure of larvae to conidia (asexual, non-motile fungus spores) in water/oil, and through contact of resting adults on fungus-impregnated black clothes/nets [6–11]. Furthermore, M. anisopliae also reduces Ae. aegypti vectorial capacity by interfering with dengue virus replication; females co-infected with M. anisopliae and DENV-2 had lower viral loads in heads compared to females infected only with DENV-2 . Metarhizium anisopliae is a hyphomycetous insect-pathogenic fungus of which the conidia infect insects by penetrating the cuticle. Metarhizium spp. are endemic worldwide and are not harmful to birds, fish, or mammals including humans . Their pathogenicity/toxicity/allergenicity has been studied intensively representing only minimal risk to vertebrates, the environment and public health .
Regarding direct impacts on survival, the M. anisopliae strain (CBG-Ma2) had a LT50 of 4 days, which was similar to that reported for other strains [7–8, 10]. Although it is difficult to compare results across different studies, here the highest copulation rate (inseminated and not) of 75% recorded for FEMs is comparable to the 65%–85% range reported when 5 males were confined for 24 hours with 20 females in large field cages . Concerning the parameters pertaining to mating activity, both negative and beneficial results were recorded. The negative impacts were that a longer time was invested by FEMs to successively copulate with 5 females than uninfected males. Also fungus infection reduced the sperm production by 87% by day 5 PE.