Control of Disease Vectors using Fungi - Biological and Environmental Control of Disease Vectors - page 84

Improving spore persistence 5.3.4 Do fungal biopesticides have any

negative impacts against non-target
organisms?

Isolates show natural variability in tolerance to
temperature, humidity and UV radiation
(Fargues et al. , 1996; Morley-Davies et al. ,
1996), so selection of suitable isolates with
improved persistence under operating conditions
is possible. Out of ten fungal isolates, three B .
bassiana strains demonstrated more than 50%
viability after 14 weeks (Darbro and Thomas,
2009). Isolates may undergo repeated selection
to enhance their persistence. More recently,
isolates of B . bassiana with high virulence
against adult St . aegypti (including an isolate
that is available commercially, IMI 391510)
were shown in vitro to have half-lives of more
than 150 days (Darbro et al. , 2011). UV-B
tolerance and speed of germination also varied
with fungal growth substrate and nutritional
environment (Rangel et al. , 2004), indicating
the potential of altering culturing techniques to
produce more persistent isolates.
Use of a biopesticide that consists of a living
organism will mean there will be limits to the
duration of residual activity. The use of dif erent
formulations and application techniques may
improve viability. As described above, formu-
lating spores in oil permits fungal application at
suboptimal humidities compared to dry spore
powder (e.g. application of M . anisopliae spores
in oil for application in desert conditions;
Bateman et al. , 1993), and can increase
tolerance of spores to UV radiation (Bateman et
al. , 1993; Hunt et al. , 1994; Alves et al. , 1998;
Lomer et al. , 2001). The application of spores
indoors for endophagic and endophilic insects
may also reduce exposure to extreme
temperatures, aridity and UV. The material used
for delivering the spores may also af ect
persistence; spore viability on cotton cloth was
shown to be greater than on polyester netting
(Farenhorst et al. , 2011). Mnyone et al . (2010)
showed that mortality of An . gambiae s.s. adults
exposed to B . bassiana and M . anisopliae- treated
mud tiles could remain as high as 82% for 14
days whereas the decline in conidial viability on
polyester netting was much more rapid.
Formulation of products or use in the agro-
chemical, pharmaceutical and food industry is
highly advanced, so the use of similar
technologies could create formulations to
promote spore viability beyond what has been
demonstrated in the laboratory and fi eld to date.

Concerns have been voiced about fi eld
application of Hyphomycete fungi that have
wide host ranges (Hutchinson and Cunningham,
2005; Kanzok and Jacobs-Lorena, 2006), but
while the host range of fungal 'species' may be
large, individual isolates can be extremely host-
specifi c (Thomas et al. , 2005). Therefore, the
impacts on non-target organisms (NTOs) have to
be analysed for each isolate. Toxicity of dry
spores of M . anisopliae has been demonstrated
against shrimps, backswimmers and frog
embryos (Genthner et al. , 1997; Lahr et al. ,
2001), but the dosage applied far exceeded that
intended for mosquito control. Other studies
have not shown any adverse responses in
mammals exposed to dry spores and conidial
suspensions of M . anisopliae or B . bassiana orally,
or through ocular, subcutaneous or intra-
peritoneal injections (e.g. Shadduck et al. , 1982;
Zimmermann, 1993, 2007; Toriello et al. ,
2006).
Extracts of M . anisopliae have given allergy-
type responses in mice similar to those that are
associated with asthma (Ward et al. , 1998,
2000; Instanes et al. , 2005) and potential
allergens have also been identifi ed in B . bassiana
(Westwood et al. , 2005). While no allergic
responses were reported in wide-scale use of M .
anisopliae in Brazil against spittle bugs (Roberts
and Panter, 1985), more recent studies have
demonstrated allergies in factories where high
quantities of spores were produced (Kaufman
and Bellas, reported in Westwood et al. , 2006). It
appears that large numbers of airborne spores
have the potential to prompt allergic reactions.
The opportunistic nature and lack of
adaptation to grow in human tissues of
Hyphomycetes, which cannot grow at human
body temperatures, make them unlikely to cause
systemic human infection. There are some
reports of B . bassiana and M . anisopliae isolated
from patients with eye damage (Sachs et al. ,
1985; Low et al. , 1997; Kisla et al. , 2000; Jani et
al. , 2001; Tu and Park, 2007), and immune-
compromised patients (Marsh et al. , 2008) with
leukaemia (Burgner et al. , 1998; Tucker et al. ,
2004; Osorio et al. , 2007) or on immuno-
suppressant drugs (Revankar et al. , 1999; Henke