Control of Disease Vectors using Fungi - Biological and Environmental Control of Disease Vectors - page 72

5.2 Fungi for Mosquito Vector Control

Applying dry spores may also cause clumping
(Bukhari et al. , 2010), and may result in lower
persistence in the environment due to direct
contact with water, UV exposure and thermal
damage. All these characteristics may contribute
to high dosage requirements to achieve larval
control (Roberts and Panter, 1985). Formulating
spores in oil may increase infectivity, but may
still only reduce pupation by 50% or less
(Bukhari et al. , 2011).
These technical problems combined with
the demonstration of the ef ectiveness of Bacillus
thuringiensis var. israelensis and Bacillus
sphaericus against larvae let attention on fungal
larval control dwindle (Heimpel, 1967; Gill et
al. , 1992; Lacey, 2007). However, following the
ef ective use of commercial fungal products to
combat agricultural crop pests, interest in fungi
for vector control re-emerged, this time directed
against adult mosquitoes (Hajek and St Leger,
1994; Scholte et al. , 2004a). Models demonstrate
that a reduction in adult mosquito survivorship
can have a large impact on malaria transmission
by reducing the basic reproductive number of a
disease (Smith and McKenzie, 2004). The
ei cacy of the two main genera of Hyphomycete
fungi, Beauveria spp. and Metarhizium spp., have
been demonstrated in the laboratory against the
important malaria vectors An . stephensi , An .
gambiae s.s., An . arabiensis and An . funestus , not
only reducing survival (Scholte et al. , 2005), but
also limiting blood-feeding and fl ight (Scholte et
al. , 2006; Blanford et al. , 2011; George et al. ,
2011) and af ecting development of the malaria
parasite within the mosquito (Blanford et al. ,
2005; Fang et al. , 2011).

5.2.1 Larval mosquito control

The use of entomopathogenic fungi for larval
mosquito control is not a novel idea and pre-
dates concerns about insecticide resistance;
observations of Coelomomyces spp. parasitizing
larval mosquitoes were made as early as 1947
(Couch and Dodge, 1947). These studies focused
mainly on larval control, possibly due to the ease
of application of dry spores or simple
formulations of fungi to water bodies and the
potential to target high densities of mosquitoes
in confi ned areas. From the 1960s to date,
isolates from over 13 dif erent fungal genera
have been tested for controlling immature stages
of Anopheles , Stegomyia (formerly Aedes ), Culex ,
Ochlerotatus , Culiseta , Orthopodymia ,
Uranotaenia , Mansonia and Psorophora species of
mosquitoes in the laboratory and fi eld (Scholte
et al. , 2004a).
Despite some reports of success against
anopheline larvae (Clark et al. , 1968; Roberts,
1977; Geetha and Balaraman, 1999; Bukhari et
al. , 2010, 2011), results of spore application of
Beauveria and Metarhizium to water bodies have
been variable with some experiments resulting
in moderate or no impact on mosquito larval
survival (Clark et al. , 1968; Riba et al. , 1986;
Sandhu et al. , 1993; Geetha and Balaraman,
1999). Variation in the isolate used, the timing,
formulation and method of application and
genus and larval stage of the mosquitoes
exposed goes someway to explain these
contradicting results (Pinnock et al. , 1973;
Geetha and Balaraman, 1999). Control may be
inef ective if larvae are infected shortly before
moulting; they may survive if they shed the
infected integument before deeper invasion can
occur. Due to the hydrophobic nature of conidia,
applying dry spores can result in few being
submerged (Riba and Keita, 1984). Formulating
spores in sand, on maize-cob granules, clay dust
and in oil causes spores to sink and results in
greater mortalities in Stegomyia and Culex
(which are bottom feeders), but reduces the
impact on surface-feeding Anopheles mosquitoes
(Roberts, 1977; Riba and Keita, 1984; Alves et
al. , 2002a); therefore infection depends upon
matching spore distribution within the water
bodies to the feeding habits of the larvae.

5.2.2 Adult mosquito control

The fi rst successful infection of adult mosquitoes
with Hyphomycete fungi by exposing adults of
Cx . pipiens , Cx . tarsalis and St . aegypti to airborne
B . bassiana spores continually within jars
resulted in 96-100% mortalities within 5 days
(Clark et al. , 1968). Interest in fungi for adult
mosquito control was renewed following
research from the agricultural sector demon-
strating the ef ectiveness of oil formulations of
M . anisopliae applied as sprays to control locusts
and grasshoppers even in the extremely dry
conditions of the Sahel (Langewald et al. , 1997).