Pathogenicity of five east African entomopathogenic fungi against adult Anopheles gambiae s.s. mosquitoes (Diptera, Culicidae) (2003)

Five different insect-pathogenic fungi, isolated in West Kenya, were screened for their pathogenicity to adult Anopheles gambiae Giles sensu stricto mosquitoes. The tested entomopathogens belong to the Hyphomycetes (Deuteromycota): Beauveria bassiana (Vuillemin), a Fusarium sp. (Link) and three different strains of Metarhizium anisopliae Metsch. (Sorokin). Infection percentages ranged from 46.5 ± 3.6% (M. anisopliae soil sample) to 88.7 ± 3.3% (M. anisopliae IC30). LT50 values ranged from 3.39 ± 0.37 (M. anisopliae IC30) to 5.87 ± 0.77 days (M. anisopliae soil sample) and were all significantly lower (p=0.003) than the control group (8.80 ± 0.65 days). Our results indicate that the IC30 strain of M. anisopliae is the most pathogenic of the fungi tested in this study, and therefore considered the most appropriate fungus to be used for development of biological control strategies for adult African malaria vectors. Malaria is considered the most important mosquito-borne disease in the world, affecting millions of people, especially in sub-Sahara Africa, where 90% of the annual death toll from this disease, estimated at 0.5-3 million cases, occurs (WHO, 2000). A public health crisis due to resistance against affordable drugs (Trape et al., 2002) and commonly used pyrethroid insecticides (Zaim & Guillet, 2002) is looming and calls for an expansion of the limited arsenal of environmentally safe vector control methods that can be used against Anopheles gambiae Giles s.l. (Diptera: Culicidae), Africa's most important malaria vector. Considering that a female mosquito from the time of first infection needs to visit the house environment at least three times for a blood meal before transmission of malaria occurs, it has been argued that control should focus on adults in or near houses rather than on immature stages (MacDonald, 1957). The two techniques mostly applied for adult mosquito control are dependent on the use of residual insecticides either on bednets or on walls/roofs of houses, thus targeting the female mosquito either before or after she takes a blood meal. Though both methods have delivered major successes in curbing malaria morbidity and mortality (Lengeler, 1998), they do have fairly major limitations both technically (Zaim & Guillet, 2002), environmentally (WWF, 1998; WHO, 1999) and in terms of acceptance/cost (Karanja et al., 1999). New methods that target adult females when visiting the house environment, preferably without the use of insecticides, may thus be worthwhile to develop. Insect pathogens, like fungi, have been evaluated against disease vectors like tsetse flies (Glossinidae; Maniania, 1998) and the search for effective species against mosquito vectors is ongoing. However, only few fungi have been found on adult stages of mosquitoes in nature. Most of them belong to the order Entomophthorales (Zygomycota) (Roberts, 1974; Humber, 1997) like Entomophthora culicis (Braun) (Kramer, 1983) and Conidiobolus coronatus (Cost.) Kev. (Low & Kennel, 1972), although these species have not been reported from Africa. Moreover, these fungi are hard to culture in vitro and are therefore not considered practical for mosquito control. A few Deuteromycetes have been found on adult mosquitoes (Sur et al., 1999), but it is generally assumed that fungal infections in the field other than Entomophthorales on adult mosquitoes are rare. No reports have been made of fungal infections in adult An. gambiae in the field. This does not imply, however, that adult mosquitoes, including An. gambiae, are not susceptible to certain fungi. In laboratory experiments, adults of Culex tarsalis Coquillett, Cx. pipiens L., An. albimanus Wiedemann, Aedes aegypti L., Ae. sierrensis Ludlow, and Ae. nigromaculis Ludlow, have been