Insect Control: Biological and Synthetic Agents - Index of

418 11: Entomopathogenic Fungi and their Role in Regulation of Insect Populations
Another example of an establishment in a
completely new environment was the release of an
Israeli isolate of Z. radicans to control the clover
aphid Therioaphis trifolii in Australia. The experiments
were limited in time and the area studied was
small, yet it was shown that dispersal of the fungus
up to at least 65 m took place within 2 months of
release (Milner et al., 1982). Another isolate of this
same species was released in Illinois in 1984 to
control the potato leafhopper, Empoasca fabae
(McGuire et al., 1987a). Epizootics of Z. radicans
then occurred in E. fabae and contributed to regulation
of the population, and there was evidence to
suggest that one of the isolates might have become
established (McGuire et al., 1987b).
Neozygites floridana from South America has
been considered for classical biological control
of the cassava green mite (Mononychellus tanajore)
in Africa (Elliot et al., 2000). The data on dispersal
in plots in Brazil indicated that the fungus would
be capable of spread under African conditions
and contribute to long-term control of the pest.
However, it was concluded that N. floridana
would not be capable of providing adequate
control of the mite if used as the sole control
agent. Releases of the Brazilian isolate were
made in Benin in 1999 (Hountondji et al., 2002).
Postrelease monitoring revealed that the introduced
isolate had become established, with infection
prevalences of up to 36%, 48 weeks after initial
introduction.
The use of fungi for classical biological control is
at the present time a rather difficult issue. First, one
must find a potential agent that does not already
occur in the chosen environment. Then, laboratory
studies are warranted to document that the proposed
agent has potential for classical biological
control. Finally, the authorities must be convinced
that the agent for release and establishment poses no
harm to the native fauna, and this is difficult, but
not impossible, to accomplish (Hajek and Goettel,
2000; Goettel and Hajek, 2001). Today, the latter
is difficult to achieve, due to the present attitude
in society that more or less demands zero risk to
biodiversity.
11.9. Role in Conservation Biocontrol
Conservation biological control (sometimes also
called habitat manipulation) is defined as
Modification of the environment or existing practices
to protect and enhance specific natural enemies or
other organisms to reduce the effect of pests
Eilenberg et al. (2001)
It is probably the most attractive among all strategies
for biological control, yet the least studied.
No agents are released, but the farming systems and
practice are modified in order to stimulate the naturally
occurring predators, parasitoids, and insect
pathogens, to regulate the pest insect populations
below the damage threshold (Barbosa, 1998). Sufficient
regulation by entomophthoralean fungi and
other natural enemies may actually already exist
for the vast number of crop herbivorous insect and
mite species, which never do any damage because of
low densities. The conservation strategy also fits
well into modern sustainable agriculture, where
actions to make direct use of the naturally occurring
enemies of pests are taken into consideration,
including increasing biodiversity and supporting a
positive interaction between the crop, the flora,
and fauna within the field and field margins, and
landscape elements (Gurr and Wratten, 2000;
Landis et al., 2000). Fungi from the order Entomophthorales
fit very well into this strategy (Pell
et al., 2001). They have the ability to establish natural
epizootics depending on the host insect and a
number of biotic and abiotic factors. A number of
entomophthoralean species exist in each field, forest,
private garden, greenhouse, stable and nature
area. Certainly, this diversity of species (and isolates
from each species) could be explored and exploited
in conservation biological control.
Each system should be analyzed adequately.
Studies on the epizootiology of N. fresenii in the
cotton aphid, Aphis gossypii, in Arkansas have
demonstrated that it is possible to monitor and
to some extent predict the epizootic development
of the fungus in the target insect population
(Hollingsworth et al., 1995; Steinkraus et al.,
1996; Pell et al., 2001). The monitoring of the
pest insect can therefore be supplemented with
knowledge on the symptoms of fungal disease in
the host, which would allow extension officers to
modify their recommendations of intervention;
e.g., if the pest population is high, but fungus
infected, no pesticide treatment would be required
as the pest population would soon succumb to the
disease.
Habitat manipulation and modification of existing
practices are often proposed as means to promote
infections caused by entomopathogenic fungi (Pell
et al., 2001): more hedges, less spraying with
chemicals (e.g., fungicides), and irrigation are all,
at least theoretically, supportive to development of
natural fungal epizootics among pest insects. It was