Control of Disease Vectors using Fungi - Biological and Environmental Control of Disease Vectors - page 70

agricultural products globally, which have been
registered for indoor and outdoor use (Stevenson,
2008).

period during which insect immune responses
limit fungal growth and proliferation. This has
been demonstrated using quantitative real-time
PCR techniques, where B. bassiana and M.
anisopliae were shown to experience several days
of little or no nuclear replication, followed by a
sudden onset of division (Bell et al. , 2009). This
growth suppression and subsequent proliferation
may be associated with the interplay with the
host defence. Hyphal bodies, or blastospores,
multiply by fi ssion, bud and extend growth tubes
within the haemocoel. They form mycelia, which
then invade tissues and organs (Fig. 5.3).
Normally, insect death occurs 3-15 days
after spore contact (Gillespie and Claydon,
1989). Death is attributed to a combination of
physical damage upon entry (with subsequent
water loss and/or resultant infections from
wounds), release of fungal toxins, nutrient
depletion, and general obstruction and damage
of tissues (Samuels et al. , 1988; Hajek and St
Leger, 1994; Mohanty et al. , 2008). The
proliferating mycelia can restrict haemocoel
circulation and tissue necrosis and loss of organ
function may follow (Ferron, 1981; Gillespie
and Claydon, 1989; Goettel and Inglis, 1997;
Fuguet and Vey, 2004). Host death ends the
parasitic phase of fungal growth. Subsequently,
these mycelia grow saprophytically, releasing
antibacterial compounds to suppress competitor
growth. Finally, the mycelia break through the
cuticle to the exterior and grow across the outer
surface (Fig. 5.4). Under suitable conditions, the
emergent hyphae produce conidiophores from
which conidia develop (Luz and Fargues, 1998;
Arthurs and Thomas, 2001). Timings may vary
with ambient conditions; under optimal
conditions spores may form within 2-3 days
following emergence from the insect body. At
low humidity, cadavers can remain intact
without sporulation for several months until
appropriate conditions occur (Gillespie and
Claydon, 1989; Goettel and Inglis, 1997).
Advances in molecular techniques are
allowing more of the proteome to be described
during the various stages of fungal development
(Barros et al. , 2010) and growth within the
insect can be monitored using DNA detection
techniques (Bell et al. , 2009). However, the
processes resulting in insect death are complex.
Fungal metabolites that allow invasion, such as
cuticle-degrading enzymes, and toxins that may

5.1.2 Fungal pathogenesis

Fungi invade insects directly through the cuticle,
and this invasion route may be more ef ective
than ingestion; feeding Phlebotomus papatasi and
Lutzomyia longipalpis sandfl y adults on sugar
solutions with B. bassiana had no ef ect on their
survival, whereas cuticle contact caused
mortalities of 100% within 3 days (Warburg,
1991). This property allows for application of a
fungal biopesticide to surfaces where mosquitoes
may make contact, for example whilst resting
during the day or after blood meals.
Fungal invasion involves many biochemical,
cellular and structural processes to allow it to
breach the insect cuticle, invade the haemocoel
and fi nally colonize the body of the host (Hajek
and St Leger, 1994; Clarkson and Charnley,
1996). Successful invasion requires both
breakdown of the insect cuticle and the ability to
overcome host defence responses. The intricacies
of the invasion process are often unique to the
specifi c host-isolate interaction but involve
similar sequential steps.
The initial stage of invasion occurs when
spores (conidia), the infective stage of both
Beauveria and Metarhizium fungi, contact the
cuticle, swell and germinate to form a structure
known as an appresorium. Germination occurs
under appropriate environmental conditions
(pH, light, temperature and humidity) (Samson
et al. , 1988; Luz and Fargues, 1998, 1999) and
requires recognition of a susceptible host, using
chemical and topographical cues, and enzymatic
breakdown of the cuticle. An infection peg
develops to invade the cuticle using mechanical
invasion and degrading enzymes, such as
lipases, esterases, proteases and chitinases
(Ferron, 1981; Ferron et al. , 1991; Boucias and
Pendland, 1998). The invasive peg grows
through the upper and lower insect cuticle
directly or by forming lateral plates that may
cause fractures, further aiding invasion (Hajek
and St Leger, 1994).
The invasion of the cuticle takes approxi-
mately 12-48 h (Boucias and Pendland, 1998).
Once the cuticle is crossed, there may be a latent