Control of Disease Vectors using Fungi - Biological and Environmental Control of Disease Vectors - page 77

procedures are therefore required to select
isolates that meet criteria for successful fi eld
deployment, such as high germination levels,
optimal spore persistence and suitable virulence
against the target species under operating
environmental conditions using appropriate
doses and application methods (Darbro et al. ,
2011).

an oil formulation of an isolate of B . bassiana
sprayed on to clay tiles resulted in 100%
mortality within 5 days (Blanford et al. , 2011), a
faster kill than was observed when sprayed on to
cardboard material. Mosquitoes contacting
treated clay tiles within the fi rst two to three
feeding cycles would not survive the full duration
of the extrinsic incubation period.
As with chemical insecticides, most
experiments with fungi have shown dose-
dependence; mortality increases as spore
concentrations rise. Concentrations of spores
below a certain threshold will not result in any
lethal or sub-lethal ef ects (Stevenson, 2008)
and, above a certain concentration, saturation
occurs when no further increases in infection
levels nor rate of death are evident (Mnyone et
al. , 2009a; Darbro et al. , 2011). Increasing
coverage by producing high concentrations of
spores, however, can be costly and so the
saturation thresholds need to be established.
This may vary depending on the isolate used
(Blanford et al. , 2005; Stevenson, 2008), the
insect targeted and the formulation and
application technique. The majority of studies
on malaria mosquitoes have used concentrations
in the region of 10 10 spores ml −1 . One option to
reduce the costs of employing high spore
concentrations widely is to apply spores to a
point source to which mosquitoes are attracted.
Point sources, such as odour-baited fungal traps,
have been investigated (Lwetoijera et al ., 2010)
and research continues to develop such traps
(B.J. Knols, personal communication).
How fungi are formulated can also af ect
spore availability, viability and virulence, as
well as ease of application. Oil-based formu-
lations aid application of fungal spores within
houses. Walls, eaves, resting targets, screens,
curtains and bed nets can be impregnated. The
oils promote spore viability, thereby increasing
spore coverage, and allow germination even in
areas with low humidity (Bateman et al. , 1993;
Lomer et al. , 2001). Studies using B . bassiana
and M . anisopliae showed that the use of
mineral oil formulations like Shellsol T are
more ei cient in killing adult An . gambiae s.s.
than dry conidia, using aqueous carriers or
thicker organic oils like Ondina (Farenhorst et
al. , 2011). On dry materials, the lighter and
more volatile Shellsol oil allows for ease of
spraying. As it is more volatile, it evaporates

5.3.2 What is the best mode of delivery?

Factors that infl uence delivery

The appropriate method of delivering fungi will
depend on the mode of action of the specifi c
isolate being deployed, the stage of the mosquito
life cycle that is being targeted and which
mosquito behaviours are being exploited.
Delivery methods must ensure high coverage
and persistence of spores.

Mode of action and maximizing coverage
Maximizing the number of insects that come
into contact with a fungal biopesticide will
ensure optimal ef ectiveness. Furthermore,
depending on the expected mode of action,
exposure early in life may be vital to have an
ef ect on transmission. For example, if the main
ef ect of the fungi deployed is to kill, and this
takes up to 2 weeks, it is essential to infect
mosquitoes soon after emergence. If coverage
levels of the fungal biopesticide are high enough,
vectors are likely to contact fungi before or
during their fi rst feeding cycle (which is also the
fi rst opportunity to contract the disease
pathogen), so death of the vector should occur
before it becomes infectious and the pathogen
can be transmitted. However, if one or more
feeding cycles are completed before the vector
contacts spores, and survival, feeding and
dispersal are unaf ected for up to 2 weeks, then
the disease transmission potential still remains.
Increasing coverage may be possible by
selecting more virulent fungal isolates or by
enhancing the ef ective dose a mosquito receives
through increasing the concentration applied,
encouraging longer exposure, treating more
surfaces, or applying fungi in a manner that
optimizes spore availability. For example,
exposure of An . stephensi female mosquitoes to