Insect Control: Biological and Synthetic Agents - Index of

11: Entomopathogenic Fungi and their Role in Regulation of Insect Populations 397
Some entomopathogenic fungi have been recorded
from as limited a host range as a single species of
insect. In the Entomophthorales, several species are
known from only one host, but it is difficult to know
if this is a true reflection of host range or the result of
limited study. Few species have been cultured and
even fewer have been bioassayed against a number
of insects to determine potential host range.
Zoophthora radicans (Entomophthorales) has been
described from over 80 insect species from Diptera,
Coleoptera, Lepidoptera, and Homoptera, but there
is clear indication that strains are restricted, generally,
to some species in a single insect class (Milner
and Mahon, 1985). The individual strains are generally
more pathogenic to the insects more closely
related to the original host, but there are many
exceptions.
Other groups of entomopathogenic fungi are limited
in host range. Coelomomyces opifexi, for example,
is known from only two mosquito species
and a copepod intermediator (Glare and Milner,
1991). The many species of Cordyceps represent
both broad and narrow host range species (e.g.,
Kobayasi, 1941), but here, again, information is
limited.
In many cases, entomopathogenic fungi will kill
certain species only in special situations. M. anisopliae
is rarely recorded naturally as a pathogen of
mosquitoes, but in the laboratory, many strains are
highly pathogenic to mosquito larvae (Daoust and
Roberts, 1982), indicating that other factors than
susceptibility are important in occurrence of disease
in nature. In some cases, the fungus can only kill
weakened or stressed hosts. Behavioral avoidance
can also lead to nonsusceptibility in the field, when
laboratory bioassays indicate susceptibility. For example,
Aspergillus flavus can be isolated from many
wasp (Vespula spp.) nests in New Zealand and is
highly pathogenic in the laboratory to Vespula
(Glare et al., 1996), but the hygienic behavior of
wasps is such that infections in healthy nests are
not seen. Zoophthora phalloides is a pathogen of
aphids, but shows distinct preferences in the field for
some species. Remaudiére et al. (1981) recorded
72% of Myzus ascalonicus infected with Z. phalloides
on a single bush, but only 6% of Myzus
ornatus on the same bush. Conversely, Pandora
neoaphidis on the same bush infected 71% of
M. ornatus and only 1% of M. ascalonicus. The
difference in susceptibility is likely due in part to
the mode of infection by the two fungal species, as
much as to possible differences in resistance among
the aphid species. Zoophthora phalloides infects via
the sessile capilliconidia, making it more effective
against mobile hosts, while P. neoaphidis uses
forcibly discharged primary and secondary conidia,
which are more effective when they land directly on
a host. Unsurprisingly, M. ascalonicus is a more
mobile aphid than M. ornatus.
These results highlight the difference between laboratory
and field susceptibility. If results from laboratory
assays are to be used to predict activity in the
field, pertinent environmental and exposure parameters
must be incorporated as much as possible in
the bioassay design (Butt and Goettel, 2000). For
instance, in the laboratory, most bioassays do not
allow for avoidance of the pathogen through
biological, ecological, or behavioral methods. In
the field, an insect may never come in contact with
sufficient inoculum to succumb to infection. An
example is caterpillars and E. maimaiga, a pathogen
of gypsy moth. Hajek et al. (1996) examined the
field incidence of caterpillars infected with E. maimaiga
where high gypsy moth infections were occurring.
They found only two individual caterpillars
(1 of 318 Malacosoma disstria and 1 of 96 Catocala
ilia) of a total of 1511 larvae from 52 species
belonging to seven lepidopteran families infected
with Entomophaga maimaiga. In the laboratory,
more species were found to be susceptible and high
percentages of the few species found infected in
the field were infected in the laboratory. Despite
E. maimaiga being a pathogen of the Lymantriidae
in the laboratory, species other than gypsy moth
are unlikely to be infected in the field unless they
spend periods of time near the leaf litter where
the fungus is sporulating (Hajek et al., 2000).
Similarly, differential infections of mosquito hosts
can be linked to position in the water profile, as
bottom feeding species are more likely to come in
contact with settling inoculum than surface feeders
(Sweeney, 1981).
In most entomopathogenic fungi, there is differential
virulence towards life stages of insects that are
susceptible. Unlike bacterial, viral, and protozoan
pathogens, most fungi directly penetrate the cuticle
and do not need to be ingested. Therefore, entomopathogenic
fungi have the potential to be active
against nonfeeding stages such as pupae. Aquatic
species, such as Lagenidium and Coelomomyces,
rarely attack adult stages, although they can persist
in them (see Section 11.3.4). Hyphomycetes such as
Metarhizium and Beauveria often infect both adult
and larval stages. Keller and others have used the
ability of Beauveria brongniartii to infect both larval
and adult stages of the European cockchafer,
Melolontha melolontha, to develop biopesticide
strategies based on spraying adults in order
to transfer inoculum to the larvae in the soil (Keller
et al., 1989). Most fungi, even if they attack all