Insect Control: Biological and Synthetic Agents - Index of

11: Entomopathogenic Fungi and their Role in Regulation of Insect Populations 401
locust regardless of toxin production. Furthermore,
in vivo passage through a host or repeated subculture
on artificial medium had a variable effect on
toxin production; virulence and toxicogenic activity
of one isolate was dependent on the mycological
media that the inoculum was produced on, whereas
virulence and toxicogenic activity of another isolate
was greatly increased after two passages through
the host.
In some cases, toxins are suspected, but not conclusively
demonstrated. Some of the lower fungi,
such as Coelomycidium, Coelomomyces, and the
Entomophthorales, may possess only weak toxins,
if any at all. It is more likely that they overcome
their hosts by utilizing the nutrients and invading
vital tissues (Roberts, 1981). Culture filtrates from
entomophthoralean fungi injected into greater wax
moth larvae (Galleria) resulted in blackening similar
to that found in fully infected larvae, suggesting
that toxins were active, but none were identified
(Roberts, 1981). A short-lived cell lytic factor is
thought to be responsible for death in lepidopteran
hosts infected with E. aulicae (Milne et al., 1994).
Antibiotics are produced by entomopathogenic
fungi in order to exclude saprophytes and resident
microbes that compete for nutrients in the cadavers.
Oosporein, a red-colored dibenzoquinone, is produced
by strains of Beauveria spp. and has antiviral
and antibacterial properties. Oosporein was found
to inhibit the herpes simplex virus-1 DNA polymerase
(Terry et al., 1992) and is active against
Gram-positive, but not Gram-negative, bacteria (references
in Vey et al., 2001). Beauvericin has shown
antibiotic activity against bacteria (Ovchinnikov
et al., 1971), and destruxin E has antivirus activity
against nucleopolyhedrovirus (Quiot et al., 1980).
The antibiotic phomalactone has been described
from H. thompsonii var. synnematos (Krasnoff and
Gupta, 1994) and was inhibitory to the entomopathogenic
fungi Beauveria, Tolypocladium, and
Metarhizium. Members within the Cordyceps also
produce a number of metabolites that may be weak
toxins or antibiotics. Cordyceps infected caterpillars
are used as a traditional medicine in parts of
Asia, and this may be partly based on the production
of cordycepin, a weak antibiotic, by Cordyceps
Zabra et al. (1996) reported that metabolites from
Pandora neoaphidis had antibacterial activity.
11.3.3.2. Behavioral responses There have been
relatively few detailed studies on the behavioral
responses by hosts as a result of infection by entomopathogenic
fungi. In the early stages of infection,
in many cases, there are no noticeable symptoms;
however, several days prior to death, symptoms
become evident and include reduced feeding, activity,
and coordination. Other responses include
increased feeding, behavioral fever, altered mating
or oviposition preferences, and positive or negative
photo- or geotropism. Most responses are seemingly
adaptations that favor either the host or the pathogen,
while others may be the result of depletion of
the host’s nutritional reserves and the process of
dying.
Reduced feeding has been reported in several
insects. Examples include grasshoppers and locusts
infected with M. anisopliae var. acridum (Prior et al.,
1992; Thomas et al., 1997; Arthurs and Thomas,
2000), gypsy moth larvae infected with E. maimaiga
(Hajek, 1989), Plutella xylostella larvae infected
with Z. radicans (Furlong et al., 1997), and larvae
of the Colorado potato beetle infected with B. bassiana
(Fargues et al., 1994). In contrast, no change
in consumption of food has been reported in some
insects, such as Plathypena scabra (Lepidoptera,
Noctuidae) infected with Nomuraea rileyi
(Thorvilson et al., 1985) and Cerotoma arcuata
(Coleoptera; Chrysomelidae) infected with B. bassiana
(Lord et al., 1987). Increased feeding has been
reported in Lygus hesperus (Noma and Strickler,
2000) and Colorado potato beetles infected with
B. bassiana (Fargues et al., 1994). With the Colorado
potato beetle, however, phagostimulation occurred
only within the first 24 h after inoculation.
Thereafter, there was no significant effect on consumption
until day 2, and after that, consumption
decreased significantly, resulting in an overall
reduction in food consumption.
Intuitively, one could surmise that increased
feeding during early stages of infection might favor
either the host or pathogen. The pathogen might be
favored if such increased feeding increases resources
provided to it, although spore production of E. maimaiga
in starved postinoculated gypsy moth larvae
equaled production on fed larvae, suggesting, at
least for this system, that larval feeding during the
period of pathogen incubation within the host is not
necessary for fungal development (Hajek, 1989).
However, it may favor the host if such feeding provides
more resources to fight off the pathogen. Most
probably, decreased feeding just prior to death is
a result of an overall shutting down of metabolic
functions as the host nears death.
Some insects respond to fungal infection by altering
their thermoregulatory behavior and achieving
a higher than normal body temperature by basking
in the sun or orienting on warmer surfaces. This
is called ‘‘behavioral fever’’ and has been shown
to either significantly slow the progress of infection
or at times even eliminate the disease. This