Control of the main African malaria vector
Anopheles gambiae (Diptera: Culicidae) continues to rely heavily on application of residual insecticides, either for indoor residual house spraying [
1] or bednet impregnation [
2]. These approaches have been highly effective in reducing malaria morbidity and mortality [
2], but associated problems regarding environmental pollution [
3,
4], acceptability and cost [
5,
6] and the now widespread and continuing development of resistance [
7‐
10] underscore the need for alternative strategies, such as vector control with biological agents [
1,
11,
12].
Entomopathogenic fungi are among the biological control agents used against insect pests. Interest in using the hyphomycete
Metarhizium anisopliae against adult African malaria vectors has recently increased [
13]. The fungus has proven to be highly virulent for this vector, both in the laboratory [
14] as well as in the field (Scholte
et al., in preparation). The principal method of contamination of the target insect population with the fungus is through application of conidia on indoor resting targets. However, in order to achieve the highest possible impact on the target population, it is desirable that contamination pathways other than the primary mode of contamination are utilised, for instance horizontal transmission. Horizontal transmission of pathogens within the same host/target species is called autodissemination, and this phenomenon has been suggested for biocontrol of several insect pests [
15,
16]. Successful transmission of
M. anisopliae by honeybees for infection of the pollen beetle
Meligethes aeneus [
17], of
Beauveria bassiana between adult flies of
Delia radicum [
18] and of
M. anisopliae and
B. bassiana between adult tsetse flies,
Glossina morsitans morsitans [
19] confirms the capability of insects to transmit fungi horizontally. Autodissemination of insecticidal biocontrol agents, such as insect-pathogenic fungi, provides an additional advantage over pesticides, as the impact on pest populations increases beyond direct contact. In several cases, autodissemination of entomopathogenic fungi within populations of insect pests, using attractant traps as the initial source of infection, has succeeded [
18,
20‐
22]. The strategy envisaged for the use of
M. anisopliae against adult
An. gambiae is that host-seeking females, and occasionally also males that rest indoors, will receive primary infections while resting indoors on fungus-impregnated resting targets. Under optimal circumstances, prior to death, this infection may be transmitted to conspecifics upon contact (e.g. during mating). These mosquitoes are, therefore, not infected through direct contact with fungus-impregnated materials, but indirectly, upon physical contact with infected counterparts. It is estimated that approximately half of newly hatched, virgin females take a blood meal before mating [
23,
24]. A female, with contaminated legs and mouthparts following the blood-feeding visit to a house containing fungus-impregnated resting targets, may contaminate male counterparts when she mates the following dusk period, thereby spreading the fungus through the population.