Impact of moisture on survival of Aedes aegypti eggs and ovicidal activity of Metarhizium anisopliae under laboratory conditions

Abstract

The effect of relative humidity (43%, 75%, 86% and > 98%) Aedes aegypti is the principal vector of dengue viruses in tropical and subtropical regions throughout the world. This closely human-associated mosquito uses a great variety of large to small sized breeding sites and occurs year round even in semi-arid regions. Females oviposit mainly above the waterline on damp surfaces, and unhatched larvae have to hold out until immersion of eggs in water will permit eclosion and further development. Populations decline during the dryer and colder season but mosquitoes never disappear completely using subterranean and other breeding sites resulting from watering behavior by local residents (Varejão et al. 2005, Maciel-de-Freitas et al. 2007). Moreover, eggs out of water resist prolonged desiccation during distinct dry seasons, and the surviving larvae contribute to a reestablishment of vector populations at the beginning of the rainy season, thus increasing the risk of dengue epidemics (Sota & Mogi 1992, Silva & Silva 1999. Inside eggs, diapausing larvae are exposed to predation and infection by pathogens, particularly fungi that may invade eggs actively through the egg shell or affect larvae with toxic metabolites produced on the egg surface (Russell et al. 2001, Luz et al. 2007). Moisture is a limiting factor for both egg survival and fungal activity but there is still little information about the impact of relative humidity (RH) on the survival of larvae inside eggs or a possible effect of entomopathogenic fungi.We report here on the impact of RH on egg survival and an ovicidal activity of M. anisopliae under laboratory conditions. This fungal species is pathogenic to larvae and adults of A. aegypti (Scholte et al. 2004, Silva et al. 2004, and recent studies indicated that it is also effective against eggs of this vector (Luz et al. 2007). The fungal isolate tested here, IP 46, originated from a soil sample collected in 2001 in the Cerrado of Central Brazil and was stored at the IPTSP, UFG, Goiânia, Brazil. Conidia were collected from 15 days old sporulating cultures grown in Petri dishes (100 x 20 mm) at 25°C and 12 h photophase on potato-dextrose-agar; hyphal bodies were produced in liquid Sabouraud-dextrose-yeast extract medium (Goettel & Inglis 1997) for five days on a rotary shaker at 25°C and 240 rpm.The population of A. aegypti originated from virusfree larvae, collected in 1991 in Goiânia, Brazil. Mosquitoes were reared under laboratory conditions as described by Silva et al. (1998). Eggs were prepared using the method related by Luz et al. (2007), and then topically treated with 50 µl water-suspended conidia or hyphal bodies, at 5x10 6 propagules/cm 2 or with water only for the controls and incubated for up to six months at 25°C and 43%, 75%, 86% or > 98% RH regulated in the test chambers with saturated solutions of K 2 CO 3 , NaCl, KCl and K 2 SO 4 , respectively (Winston & Bates 1960). Each month, fungal development on eggs and egg viability were tested. Eggs were submerged in water and quantitative ecl...

Effects of Metarhizium anisopliae conidia mixed with soil against the eggs of Aedes aegypti

“…Under septic conditions, an effect of the conidial concentrations on aedine eggs can be expected. The ovicidal effect of M. anisopliae against A. aegypti when mixed into soils was obviously reduced in comparison to direct treatments of eggs with conidial suspensions (Luz et al 2007(Luz et al , 2008Albernaz et al 2009;Santos et al 2009). Because in the present study conidia were applied as conidia per unit of soil and were uniformly mixed into the soil, the real dosages to these indirectly exposed eggs were much lower than those delivered by a direct application of fungus to the eggs.…”

“…Whereas the larvicidal and adulticidal activity of M. anisopliae is well documented (Scholte et al 2004a), there is still little information on the ovicidal activity of this and other fungi against A. aegypti. The eggs of this vector were highly susceptible to infection with M. anisopliae IP 46 under laboratory conditions when conidia were applied to eggs arranged on filter paper and incubated at humidities close to saturation (Luz et al 2007(Luz et al , 2008Albernaz et al 2009;Santos et al 2009). The filter paper that is used for oviposition in the laboratory also proved to be a useful substrate for tests on the ovicidal effects of diverse other entomopathogenic fungi in the laboratory (Luz et al 2007).…”

Biological Control of Dengue Vectors

“…Work targeting A. aegypti eggs has also been carried out. Both M. anisopliae and B. bassiana have proved to be ovicidal (Luz, et al 2007), but high levels of humidity were required (Luz, et al 2008). …”

Survival of anopheline eggs and their susceptibility to infection with Metarhizium anisopliae and Beauveria bassiana under laboratory conditions

“…While this has not been previously investigated for anopheline species, B. bassiana, M. anisopliae, and other fungi can infect aedine eggs (Luz et al 2007(Luz et al , 2008Albernaz et al 2009;Santos et al 2009) and also reduce the eclosion of larvae in the laboratory (Luz et al 2007(Luz et al , 2008. For water-formulated M. anisopliae to invade through Aedes aegypti (Diptera: Culicidae) egg shells and to prevent larval eclosion, both high humidity and a 10-day exposure period to this fungus were required ).…”

“…Unlike the eggs of aedine species that can survive for several months when not in contact with water (Sota and Mogi 1992; Silva and Silva 1999;Luz et al 2008), anopheline eggs are not adapted for long-term survival (Gray and Bradley 2005). The dynamics of anopheline egg dormancy are not clearly understood, but under both laboratory and field conditions it has been recorded that A. gambiae (Diptera: Culicidae) eggs that, for any reason, are not in direct contact with a water surface survive from 2 to 14 days after oviposition (Beier et al 1990;Minakawa et al 2001;Shililu et al 2004;Yaro et al 2006).…”