Control of Disease Vectors using Fungi - Biological and Environmental Control of Disease Vectors - page 83

exit traps fi tted to the hut. Mosquitoes caught
from these traps showed high levels of fungal
infection and 95% mortality after 14 days
compared to 30% mortality of mosquitoes
caught in the control station (Lwetoijera et al. ,
2010). This device of ers an opportunity to
infect mosquitoes outdoors away from human
contact. Models have investigated where
such traps could be located so as not to increase
exposure to mosquito bites within a homestead
(Sumaye et al. , 2012), but further research is
required to establish the number of stations
required for optimal coverage, which would
then demonstrate impacts on whole popu-
lations.

when exposed to freshly treated materials,
mortality decreased under fi eld conditions and
conidial viability fell to 30% after 20 days.
Fungicide agents used by the manufacturer to
treat the raw materials may explain such rapid
reductions in spore viability. However, this would
still necessitate regular retreatment or use of
fungicide-free materials - avenues that are now
being explored.
Spore persistence is primarily af ected by
environmental conditions such as temperature,
exposure to sunlight and relative humidity
(Ignof o, 1992). Extremes in temperature can
kill some fungal species (Alves et al. , 2002a),
reduce growth and prevent sporulation. For
example, colonies of B . bassiana kept at 40°C for
more than 12 h cease to grow (Inglis et al. ,
1996). However, temperatures in this range are
rarely encountered in the fi eld.
The specifi c thermal optima of fungi can
vary depending on the isolate chosen, how it is
cultured, the dose applied and the insect targeted
(Thomas and Jenkins, 1997; Darbro et al. ,
2011). Exposure to UV has detrimental ef ects
on many isolates of Beauveria and Metarhizium ,
inactivating conidia or delaying germination
(Zimmermann, 1993; Hunt et al. , 1994; Fargues
et al. , 1996; Moore et al. , 1996; Morley-Davies et
al. , 1996; Braga et al. , 2001). Optimum conidial
germination occurs between 90 and 100% RH
(Gillespie and Claydon, 1989) and high levels of
humidity are required to result in marked
mortality of Stegomyia mosquitoes and
triatomine bugs following exposure to isolates of
Beauveria and Metarhizium (Luz et al. , 1998; Luz
and Fargues, 1999; Fargues and Luz, 2000).
Storing conidia at more than 90% RH at room
temperature can maintain viability of M .
anisopliae spores for several months, whereas
storage in airtight containers at stable humidities
and temperatures below 19°C can provide
conidial shelf lives of up to 1 year (Roberts and
Panter, 1985). When investigating ef ects of
environmental conditions and selecting
appropriate isolates, it is important to decide
how this will be measured. In vitro germination
and growth of fungi may not translate into
virulence when tested in vivo (Darbro et al. ,
2011). In selecting and testing isolates initially
within the laboratory, conditions that closely
resemble those in the fi eld must be employed.

5.3.3 How can spore viability be
promoted?

Factors affecting spore longevity

The persistence of a fungal biopesticide after
application to surfaces is crucial in determining
its success. Chemicals such as pyrethroids used
in current indoor residual spray programmes
are thought to have a residual activity of from 3
to 6 months (WHOPES, 2009), but in practice
many countries only have the resources to spray
houses once a year (PMI, 2012). Therefore,
selecting isolates with long half-lives and
developing formulations and application
methods that promote their viability in the
external environment is desirable, unless
frequent maintenance or retreatment can be
made cost-ef ective. While conidial viability of
stored suspensions can remain high for long
periods of time, many studies have shown low
infective spore persistence after application. This
has been highlighted as a particular challenge
for sustained vector control (Enserink, 2005;
Kanzok and Jacobs-Lorena, 2006). Spores of M .
anisopliae and B . bassiana lost infectivity to An .
gambiae s.s. after 28 days in laboratory tests
(Mnyone et al. , 2009a). In fi eld trials using
treated black cloth, infection rates of anophelines
dropped from 95% 1 day after impregnation to
63% after 3 weeks (Scholte et al. , 2005). Howard
et al. (2011) showed that whereas polyester
netting treated with B . bassiana oil-formulations
resulted in elevated mortality of An . gambiae s.s.