adversely affect the establishment and effi-
cacy of mass-reared natural enemies (Kluge
and Caldwell, 1992). Infected hosts may
show no physical symptoms; however,
infected tissues appear white in some hosts.
Infected hosts may have soft bodies or bod-
ies that dry to a scale that yields a milky
mass in water (Thomas, 1974).
Microsporidia-infected insects may also
exhibit behavioural changes, including
changes in temperature preference (Horton
and Moore, 1993).
Diseases caused by fungiClassification of fungi is largely based on the
morphology of reproductive structures and
associated processes. Many entomopatho-
genic fungi have wide host ranges (e.g.
Beauveria bassiana, Metarhizium anisopliae) but
different isolates may demonstrate host
specificity. Although fungi may infect all
stages of the host, they are infrequent
pathogens of pupae and rarely infect insect
eggs (Tanada and Kaya, 1993).
Most pathogenic fungi penetrate their
insect hosts through the cuticle or spiracles;
however, some penetrate through the gut.
Once in the host, pathogenic fungi prolifer-
ate and compete for soluble nutrients in the
haemocoel, where they release mycotoxins.
They then invade and digest tissues and
cause premature death of the host (Thomas,
1974; Tanada and Kaya, 1993).
Adhesion and germination of fungal
spores on the host cuticle are highly depen-
dent on relative humidity and temperature,
but light conditions and nutritional require-
ments are also important factors (Tanada and
Kaya, 1993). In soft-bodied insects, fungal
penetration can occur anywhere on the body.
On larger, more heavily sclerotized insects,
penetration often occurs at the thinner
arthrodial membranes that are characteristic
of joints. In many insects, early instars may
be more susceptible to fungal penetration
because the cuticle becomes thicker in suc-
cessive instars. Insects that have recently
moulted may also be more susceptible to
fungal pathogens (Bell, 1974; Thomas, 1974;
Tanada and Kaya, 1993).
In some cases, behavioural changes occur
prior to death. Symptoms may include loss
of coordination and body tremors, reproduc-
tive behaviour of castrated hosts and
changes in microhabitat preference. The lat-
ter include elevation-seeking behaviour (fun-
gal ‘summit disease’), movement to exposed
locations, changes in oviposition or foraging
sites and change in temperature preference
(Horton and Moore, 1993).
The most common and easily recognized
signs of fungal infection are the presence of
filamentous hyphae and characteristic repro-
ductive structures (fruiting structures,
spores), usually produced on the external
surface of the dead host (Thomas, 1974). The
body of the infected host may discolour. It
may become mummified, hard or cheese-like
and will not disintegrate in water (Thomas,
1974). Fungi are important pathogens of
Bombus(MacFarlane et al., 1995) and several
coccinellid beetles (Lipa et al., 1975).Diseases caused by nematodesEntomopathogenic nematodes parasitize
their hosts by directly penetrating through
the cuticle into the haemocoel or by entering
through natural openings, such as the spira-
cles, mouth or anus (Tanada and Kaya, 1993).
Nematode-infected insects generally show
few external symptoms. The expression of
external abnormalities in the host is depen-
dent on nematode number, host age and the
time of infection. Such anomalies may
include colour changes, distorted abdomens,
deformed wings or the formation of inter-
sexes (Tanada and Kaya, 1993). Internal mor-
phological and physiological changes as a
result of infection may result in host sterility
or death (Tanada and Kaya, 1993).
Behavioural changes that may occur after
nematode infection include changes in activ-
ity level and reproductive behaviour of cas-
trated hosts, whereby parasitized females
release nematodes instead of eggs (Horton
and Moore, 1993). Nematodes are uncommon
pathogens of natural enemies; however,
they are known to infect Coccinella
(Rhamhalinghan, 1992) and to sterilize
Bombusqueens (MacFarlane et al., 1995).138 S. Bjørnson and C. Schütte