Insect Control: Biological and Synthetic Agents - Index of

11: Entomopathogenic Fungi and their Role in Regulation of Insect Populations 399
immunoincompetent child, but none of these have
been implicated from the commercial use of specific
strains (Goettel et al., 2001; Vestergaard et al.,
2003). The most recently reported case concerns
the deep tissue infection of an immunosuppressed
female by a Beauveria sp. (Henke et al., 2002). The
isolate in question was unable to grow in vitro at
37 C, which indicates that the inability to grow at
this temperature in vitro is insufficient to rule out
possible infection and growth in mammalian tissues.
Interestingly, this isolate was more virulent against
a test insect, the Colorado potato beetle, as compared
to a well-known entomopathogenic strain of
B. bassiana.
Entomopathogenic fungi produce an array of
metabolites (see Section 11.3.3.1 below), many of
which are toxic or carcinogenic to vertebrates
(Strasser et al., 2000; Vey et al., 2001). However,
any hazard to vertebrates would require exposure
to significant levels of these metabolites. Current evidence
suggests that, unless infected insects are actively
pursued and ingested (e.g., insect cadavers infected
with B. bassiana are widely consumed in China for
medicinal purposes), the risk to vertebrates should
be minimal. To date, no detrimental effects have
been noted in vertebrates fed insect cadavers. For
instance, ring-necked pheasant chicks given feed coated
with M. anisopliae var. acridum (as M. flavoviride)
spores or infected grasshoppers for 5 days showed
no significant changes in weight, growth rate, behavior,
or mortality rate as compared to controls fed
noninoculated food or noninfected grasshoppers
(Smits et al., 1999). Strasser et al. (2000) speculate
that entomopathogenic fungi should pose no obvious
risk to humans because toxin quantities produced
in vivo are usually far lower than those
produced in vitro and, therefore, toxin levels should
never rise to harmful levels in the environment.
Fungal epizootics may deplete an important food
source for certain vertebrates. However, at the same
time, depletion of a pest is the desired effect in the
use of an entomopathogenic fungus in microbial
control. The possible effects on vertebrates due to
the depletion of a food resource as a result of fungal
epizootics, either natural or induced, are poorly
documented. However, for the most part, depletion
of the target host is the desired effect and can
be more or less regulated in an integrated pest management
program where fungi are used in large
amounts (also called inundation). However, special
attention must be paid if exotic fungi are to be used
in the classical sense of biocontrol (Goettel and
Hajek, 2001).
Stringent regulatory requirements address vertebrate
safety issues. Before any entomopathogenic
fungus can be registered as a microbial control
agent, it must first undergo stringent testing for
potential harmful effects on vertebrates, including
mammals (see Laird et al., 1990; Siegel, 1997). In
principle, no harmful agent should ever make it onto
the market. However, with the increasing development
of immunosuppressive therapy in medicine,
the increase in iatrogenic factors and nosocomial
diseases, and the advent of new infectious diseases
such as AIDS, the list of opportunistic fungi causing
deep mycoses is increasing (Chabasse, 1994), and
this includes some entomopathogens. Entomopathogens
must be carefully scrutinized, but at the
same time, regulations must not become unjustly
stringent so as to significantly deter registration of
safe and useful products.
11.3.3. Mode of Action and Host Reactions
Entomopathogenic fungi kill the host by a variety of
means, from starvation through multiplication in the
host, to production of toxins. Given the pathogenic
habit, it is hardly surprising that entomopathogenic
fungi produce extracellular enzymes and toxins.
Entomopathogenic fungi produce a variety of chitinases
and proteases, which aid penetration of the
host physical defenses.
The main barrier to infection of insects by entomopathogenic
fungi is the cuticle. For the majority
of fungi, the route of infection is directly through the
cuticle, not after ingestion. The fungus, therefore,
needs enzymatic and/or physical means to penetrate
this thick, multilayered shell. The process of infection
starts with the spore contacting the cuticle of a
host. In many cases, the conidia are adhesive to the
cuticle, or secrete adhesive mucus as the conidium
swells during pregermination (Hajek and St. Leger,
1994). Capilliconidia, the infective sessile spores of
some Zoophthora, have a drop of adhesive on the end
of the spore to assist attachment to passing insects
(Glare et al., 1985). Zoospores of Lagenidium encyst
on the host surface, ensuring attachment (Kerwin
and Petersen, 1997).
The exact mechanisms used by each fungus for
penetration of the host cuticle may differ, but there
are general processes and structures involved.
Metarhizium anisopliae, for example, develops a
number of specific structures to assist the anchoring
and penetration of germ tubes, which arise from
conidia (St. Leger, 1993). Appressoria and infection
pegs assist the penetration of germ tubes. Once
the penetrative tube has passed through the layers
of the cuticle and epidermis, the fungus can proliferate
in the hemocoel. For some hyphomycete fungi,
such as M. anisopliae, this is initially performed by
blastospores, while for some Entomophthorales,