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