including Muscidifuraxspp., Nasonia vitripen-
nis (Walker), Pachycrepoideus vindemiae
(Rondani), Spalangia cameroni Perkins, and
Spalangia endiusWalker (Dry et al., 1999).
Host specificity of microsporidia that infect
pteromalid parasitoids is not known.
Microsporidia are found in wild parasitoids
collected on dairy farms; however, disease
prevalence is often greater on farms where
commercially reared parasitoids have been
previously released (Becnel and Geden, 1994;
Dry et al., 1999). The microsporidium Nosema
muscidifuraciswas described from M. raptor
collected from dairy farms in central New
York. Developmental stages of the pathogen
are detected in the midgut epithelium,
Malpighian tubules, ovaries and fat body of
infected larvae and adults. Two morphologi-
cally distinct spores are found in larvae,
pupae and adults. One of these is thought to
be responsible for autoinfection (reinfection
of the same host) and another for pathogen
transmission from one host to the next. A
third spore type, found in eggs of M. raptor,
is thought to either initiate infection at eclo-
sion or to transmit the pathogen to a new
host when infected eggs are cannibalized
(Becnel and Geden, 1994).
Horizontal transmission of N. muscidifuracis
occurs when uninfected parasitoid immatures
cannibalize infected immatures in superpara-
sitized pupal hosts or when uninfected adults
feed on infected parasitoid immatures within
host puparia. N. muscidifuracis infects the
oocytes of female parasitoids, resulting in ver-
tical transmission that is 100% efficient (Geden
et al., 1995). Parasitoid crowding, which leads
to superparasitism, can greatly amplify the
prevalence of this microsporidium in mass
rearings (Geden et al., 1995).
Infected M. raptortake longer to complete
development, live half as long and produce
one-tenth as many progeny as uninfected
parasitoids (Zchori-Fein et al., 1992; Geden et
al., 1995). Therefore, microsporidia may
cause a dramatic reduction in the efficacy of
M. raptoras a biological control agent.
Treatment of infected adult parasitoids
with fumagillin has no effect on the disease;
however, disease prevalence is reduced
when fly puparia that contain parasitoid
eggs are immersed in a 47°C water-bath for
30–60 min. The most practical way to elimi-
nate N. muscidifuracisfrom M. raptorcolonies
with a 100% disease rate is to use heat to
reduce disease prevalence and then isolate
uninfected females and use them to start
uninfected colonies (Geden et al., 1995).
The presence of microsporidia in M. rap-
torraises questions regarding the quality of
mass-produced parasitoids used for biologi-
cal control. The presence of disease may also
explain some of the inconsistencies reported
for M. raptor life-history traits (Zchori-Fein
et al., 1992).Nasonia(Pteromalidae): parasitoids of flies
BACTERIA.Nasonia vitripennis is known to
have a female-biased sex ratio, resulting
from a low (20%) hatch rate of male eggs
(cited by Balas et al., 1996). This son-killer
trait is caused by the bacterium
Arsenophonus nasoniae(Gherna et al., 1991).
The bacterial cells are non-motile, non-
spore-forming, long rods, which are occa-
sionally filamentous in young cultures
(Gherna et al., 1991). A. nasoniaeinfects sev-
eral tissues, including the brain, fat body,
muscles, eyes, haemocytes and reproductive
tract (Huger et al., 1985). Infections originate
in the larval midgut, suggesting that infec-
tion takes place per os(Huger et al., 1985).
Bacteria are apparently transferred to fly
pupae during parasitoid oviposition. A.
nasoniaeis maternally transmitted; horizon-
tal transmission occurs when progeny from
an uninfected female develop within a host
parasitized by an infected female parasitoid
(Werren et al., 1986). A. nasoniaehas also
been reported in field-collected Nasonia
longicornisDarling, a sibling species of N.
vitripennis(Balas et al., 1996). Over 2 years,
the son-killer trait was expressed in 5.4 and
7.8% of natural Nasoniapopulations, respec-
tively (Balas et al., 1996).
Wolbachia-induced cytoplasmic incompat-
ibility is a well-documented phenomenon in
Nasoniaspecies. Wolbachiaspp. are found in
Nasonia giraultiDarling, N. longicornisand N.
vitripennis(Breeuwer et al., 1992; Werren et
al., 1995). Cytoplasmic incompatibility is typ-
ically unidirectional: crosses between
infected males and uninfected females yield152 S. Bjørnson and C. Schütte