Insect Control: Biological and Synthetic Agents - Index of

11: Entomopathogenic Fungi and their Role in Regulation of Insect Populations 409
(see Section 11.2.5). These same techniques are useful
when applied to monitoring fungal isolates in the
environment. For example, Enkerli et al. (2001) used
10 microsatellite markers to characterize B. brongniartii
and could use these markers to demonstrate
that strains applied in Switzerland in 1983 were still
active over 8 years later. Specific identification of B.
brongniartii, based on polymerase chain reaction
(PCR) primers designed to group 1 intron insertions
in the 28s rRNA gene, was used to identify and
monitor an introduced strain for control of the scarab
pest, Hoplochelus marginalis, in the Reunion
Islands (Neuvéglise et al., 1997). Other techniques,
such as chromosomal length polymorphisms,
RAPDs, and RFLP have all been used to assist monitoring
strains in the environment. Molecular
approaches to phylogeography of insect pathogens
are also developing. Using a molecular genetics approach,
Bidochka et al. (2001) found that genotypes
of M. anisopliae in Canada were more influenced by
habitat than host.
An extension of the use of molecular techniques for
distinguishing strains important in biological control
is the insertion of sequences (tags) or genes which can
be used to specifically assist monitoring. For example,
insertion of a green fluorescent protein gene was used
to allow detection and tracking of a genetically
modified M. anisopliae (Hu and St. Leger, 2002).
Media based detection methods, such as selective
agars, can determine the number of viable propagules
in the environment, but generally cannot
allow separation between applied and background
strains. Molecular approaches can allow separation
of strains, but while these molecular approaches are
extremely powerful, most techniques do not provide
information on the activity (viability) of the
propagules detected. Even though fungi can be detected,
it is not possible to say whether the detected
fungus is able to infect hosts. Use of Galleria or
other sentinel insects, such as in the soil baiting
method (Zimmermann, 1986), can demonstrate virulence,
but gives no quantification of propagules.
A combination of methods is required to monitor
applied fungi in the environment, to provide information
on the number of viable propagules of the
applied fungus present over time.
11.5. Epizootiology and Its Role in
Suppressing Pest Populations
Entomopathogenic fungi are well known for their
ability to rapidly decimate insect outbreaks through
spectacular epizootics, and it is principally this
property that has spurred interest in the use of
fungi for pest management. The term ‘‘epizootic’’
is used to describe a situation where the proportion
of infected individuals is high or at least higher than
the enzootic condition, i.e., the disease condition
that is usually of low prevalence and constantly present
in a host population (Tanada and Kaya, 1993).
Prevalence is defined as the number or proportion of
infected individuals at a given point in time (Fuxa
and Tanada, 1987). Natural epizootics caused
by fungi are an important factor in regulating pest
insect populations, often alleviating the need for
other interventions, such as application of chemical
pesticides. Here, some examples of fungi with a role
in suppressing pest populations in epigeal, soil, and
aquatic environments are provided.
11.5.1. Epigeal Environment
Epizootics often occur within the epigeal environment.
Infection may spread rapidly directly from
infected insects to uninfected insects, from spores
on leaves to insects feeding on the leaves, and also
via spores present in the air.
High prevalence is often found among aphids
(e.g., Feng et al., 1991; Hollingsworth et al., 1995;
Hatting et al., 2000). Hollingsworth et al. (1995)
monitored the fungus N. fresenii in populations of
Aphis gossypii in cotton in Arkansas, USA. Epizootics
were common and prevalences could reach
90%. Such prevalences were detrimental to the
aphid populations, which actually began to decline
when the prevalence reached 15%. Also, aphids
living in cold environments suffer from fungus infections.
Nielsen et al. (2001) sampled Elatobium
abietinum on sitka spruce in Iceland. Both Entomophthora
planchoniana and N. fresenii were
present in the aphids and were responsible for epizootics
during the autumn, when daily mean temperatures
were below 15 C. However, the effect
on the host population was not documented during
this study.
Another group of insects often found infected
with fungal pathogens are dipterans. Steinkraus
et al. (1993) studied the prevalence of Entomophthora
muscae in populations of Musca domestica
and found prevalences of up to 75% towards the
end of the season. In this case, as in other cases, the
epizootic builds up during the season. In studies of
M. domestica and Entomophthora schizophorae,
Six and Mullens (1996) documented that maximum
daily temperatures higher than 26–28 C correlated
with low prevalences.
Entomopthoralean fungi from the Entomophaga
grylli species complex are important pathogens of
acridids worldwide (Carruthers et al., 1997). They
commonly cause epizootics, particularly following
periods of high humidity or rain. Such epizootics