Significance and Impact of the Study: The entomopathogenic fungus Metarhizium anisopliae has been extensively studied as a potential agent for biological mosquito control. This study demonstrates the potential of M. anisopliae s.l. strain IP 46, which can infect adults, larvae and eggs of Aedes aegypti, against the eggs of this important vector species by indirect applications of oil-based conidial formulations.
AbstractIncreasing needs for innovative control tools against the dengue vector Aedes aegypti have prompted investigations into the development of specific mycoinsecticides. The entomopathogenic fungus Metarhizium anisopliae attacks both larval and adult stages, but its ovicidal activity against A. aegypti is still little explored. This study reports important findings about the effectiveness of conidia formulated in water and oil-in-water emulsions and of direct and indirect application techniques against A. aegypti eggs. The ovicidal activity of M. anisopliae increased with higher conidial concentrations regardless of the application technique, and larvae elimination concentrations were lowest with oil-in-water-formulated conidia (LEC 50 ≤ 4Á8 9 10 3 conidia cm À2 and LEC 90 ≤ 1Á9 9 10 5 conidia cm À2 , respectively). Conidia eventually stimulated larval eclosion. Consequently, the indirect application of oil-based fungal formulations onto substrates where oviposition will later occur appears to be a more efficient means to infect those eggs than the direct fungal application to previously deposited eggs.
Little is known about the ovicidal effects of fungi that attack nymphs and adults of triatomine vectors. A combined formulation of Metarhizium anisopliae IP 46 conidia prepared with diatomaceous earth (DE) and vegetable oil was tested against eggs of Triatoma infestans. Eggs were highly susceptible to fungal infection at relative humidity close to saturation [>98% relative humidity (RH)] but not at 75% RH regardless of the formulation applied. Susceptibility of eggs decreased with longer post‐ovipositional embryonation periods before treatments. The eventual eclosion of nymphs was best suppressed by application of conidia prepared with DE + oil and at a >98% RH incubation. Moreover, nymphs were less affected by the fungus when exposed for only a 24‐h period after eclosion to a treated surface than individuals that were in constant contact with the conidia. These findings contribute to a better understanding of the potential of M. anisopliae as an agent against all developmental stages of T. infestans.
Physically disturbed Triatoma infestans (Hemiptera: Reduviidae) adults, as well as adults of other Chagas' disease vectors, secrete a mix of volatile organic compounds (VOCs) with alarm and possible sexual and defence functions. The aim of the present research was to test whether infection with the entomopathogenic fungus Beauveria bassiana (Ascomycota: Hypocreales: Clavicipitaceae) has an effect on VOC secretion in disturbed T. infestans and on the expression of two genes (Ti-brnq and Ti-bckdc) potentially involved in VOC biosynthesis. The volatiles released by insects at different time periods after fungal treatment were identified and their relative amounts measured. Isobutyric acid was the most abundant volatile found in both healthy and fungus-infected insects and underwent no significant relative changes through the infection process. The secretion of propionic acid, however, was significantly higher at 1-4 days post-infection (d.p. i.) compared with that in controls. A slight induction of both Ti-brnq and Ti-bckdc genes was found by real-time polymerase chain reaction at 4 d.p. i., with expression values reaching up to three-fold those in controls. The early stages of fungal infection seem to affect the composition of the alarm pheromone by changing the expression pattern of both genes analysed. These results help to elucidate the impact of fungal infections on the chemical ecology of triatomine bugs.
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