Fein et. al., 2000). Therefore, Wolbachiaspp.
are not always considered to be detrimental
to natural enemies that are used for biologi-
cal control (Chapter 8).
Diseases caused by protozoaMass-reared beneficial arthropods are
known hosts of several types of entomopath-
ogenic or symbiotic protozoa, including
eugregarines, neogregarines, microsporidia
and trypanosomes.
Eugregarines have been observed in
invertebrates for many years (Brooks, 1974)
and a large number have been described
from arthropods. However, few studies
regarding eugregarine pathogenicity and
host–parasite relationships have been pre-
sented (Brooks, 1974; Tanada and Kaya,
1993).
The life cycle of eugregarines can vary
greatly and is often complex (Tanada and
Kaya, 1993). Transmission of gregarines is
achieved per os(orally) when a susceptible
host ingests gregarine spores. Shortly after-
ward, sporozoites are released in the diges-
tive tract where they become partially
embedded in, and invade, the epithelial cells
of the intestinal wall. During the initial
phase of development, the parasite (tropho-
zoite) develops intracellularly and may
increase greatly in size. These eventually
emerge from the host cells, destroying them
in the process.
Trophozoites obtain nourishment in two
ways: through the host epithelial cells to
which they remain attached by a specialized
organelle (epimerite), or by the absorption of
fluids from the gut lumen after they detach
from these cells (Brooks, 1974; Tanada and
Kaya, 1993).
Most species of eugregarines are rela-
tively harmless parasites or commensals
that live in the intestinal tract or haemocoel
of their hosts. The absence of asexual repro-
duction (merogony) in their life cycle is con-
sidered to be a major factor for their
apparent lack of pathogenicity (Brooks,
1974; Tanada and Kaya, 1993). Eugregarines
have been reported in Bombus(MacFarlane
et al., 1995), Coccinellaand Hippodamia(Lipa,
1967; Lipa et al., 1975). Conversely, neogre-
garines are known to produce lethal infec-
tions in insects. These pathogens have been
reported in Bombus(Liu et al., 1975; Lipa and
Triggiani, 1996).
Microsporidia are spore-forming proto-
zoa that infect a wide range of hosts from
all major animal phyla. Fish and arthropods
are their most common hosts (Tanada and
Kaya, 1993). Microsporidia are obligate,
intracellular parasites that lack mitochon-
dria. These pathogens are dependent on
host cells for energy in the form of adeno-
sine triphosphate (Canning and Hollister,
1990), which they use for their own devel-
opment. Microsporidia can cause either
acute infections, resulting in the death of
the host, or chronic infections, which pro-
duce sublethal and debilitating disease
(Tanada and Kaya, 1993).
The microsporidian life cycle is complex
and consists of two phases. The vegetative
phase is responsible for the proliferation of
the pathogen, whereas the sporulation phase
results in the production of transmissible
spores. These are somewhat resistant to
unfavourable environmental conditions and
provide the pathogen with a means of dis-
persal (Maddox, 1973).
Microsporidia may invade the host tis-
sues when spores are ingested, when the
pathogen is transmitted from parent to prog-
eny or occasionally through wounds in the
integument (Tanada and Kaya, 1993).
Microsporidian spores are structurally
unique and contain a characteristic tube-like
polar filament through which an infective
stage (sporoplasm) is injected into an adja-
cent host cell. This begins the infective cycle
of the pathogen.
Over 50 cases of microsporidioses have
been reported in beneficial arthropods
(Bjørnson, 1998), including Bombus,
Coccinella, Cotesia, Encarsia, Muscidifurax
and Phytoseiulus. Microsporidia often cause
a significant reduction in host fecundity and
longevity, increased mortality, developmen-
tal problems (delayed development, defor-
mations, moulting problems) and reduced
adult emergence of infected parasitoids
(Geden et al., 1995; Bjørnson and Keddie,
1999). Therefore, microsporidia mayPathogens of Natural Enemies and Pollinators 137