for parasitoid death if it is transmitted by
female parasitoids during oviposition
(Brooks, 1993). The effect of S. marcescensand
a second facultative pathogen, Pseudomonas
fluorescens, has been tested on Metaseiulus
occidentalis(Nesbitt) (Lighthart et al., 1988)
and Hippodamia convergensGuérin-Méneville
(James and Lighthart, 1992).
Rickettsiae are obligate intracellular
symbionts. Many are non-pathogenic and
occur as commensals or mutualists (Tanada
and Kaya, 1993). Entomopathogenic ricket-
tsiae (family Rickettsiaceae, tribe
Wolbachiae) belong to two recognized gen-
era. Members of the genus Rickettsiellaare
common pathogens, whereas those of the
genus Wolbachiaare seldom pathogenic in
the true sense but have evolved various
means of manipulating their hosts in order
to enhance their own transmission (see
Stouthamer et al., 1999). Rickettsiae have
complex developmental cycles and a diver-
sity of forms. These range from typical rod-,
coccoid and kidney-shaped structures to
spherical or giant forms.
Rickettsiellaspp. have been isolated from
many different insect orders. They generally
produce chronic infections in their hosts.
Following the ingestion of the small, dense,
bacterial rods by a susceptible host, the rick-
ettsiae pass through the gut into the haemo-
coel and infect the cells of other tissues and
organs, especially the fat body and gonads.
Rickettsiellaspp. generate a variety of struc-
tures, including bipyramidal crystalline
bodies. Most tissues become infected, with
the exception of the gut epithelium. Infected
cells usually lyse and release their contents
into the haemolymph. After the infected
host dies, infective stages are released into
the soil, where rod-shaped forms can sur-
vive for 1 or more years. Rickettsiellaspp.
that produce sublethal infections are usually
transovarially transmitted (Tanada and
Kaya, 1993). Infection may induce promi-
nent behavioural changes in the host,
including elevation-seeking behaviour and
changes in temperature preference (Horton
and Moore, 1993).
Wolbachia spp. are common cytoplasmic
symbionts of insects, crustaceans, mites and
filarial nematodes (see Stouthamer et al.,
1999). Wolbachiaspp. are rarely pathogenic
but may manipulate the host biology by
inducing parthenogenesis (whereby infected
females exclusively produce daughters),
feminization (whereby infected genetic
males reproduce as females), male-killing
(whereby infected male embryos die while
female embryos develop into infected
females), cytoplasmic incompatibility (uni-
directional in its simplest form: whereby the
crossing of an uninfected female and
infected male result in embryo mortality) or
by enhancing host fecundity (Stouthamer et
al., 1999).
Wolbachia spp. may be present in various
tissues but are predominantly present in
gonadal tissue (Stouthamer et al., 1999). The
symbionts are transmitted vertically
through the egg. Therefore, infected mothers
give rise to infected offspring. Phylogenetic
studies of Wolbachia indicate that horizontal
transmission must have taken place rather
frequently. An intraspecific horizontal trans-
fer of Wolbachia has recently been reported
in Trichogramma kaykai (Huigens et al., 2000).
Wolbachia-infected arthropods may be cured
by treatment with several antibiotics and by
rearing in elevated temperatures (Stouthamer
et al., 1999). As culturing Wolbachiaoutside
their hosts has been successful in only one
case, molecular techniques, such as the poly-
merase chain reaction (PCR), are used in
detecting Wolbachiainfections (Stouthamer
et al., 1993). However, a recent study demon-
strated that Wolbachia infections can be sim-
ply established, stably maintained and
stored in vitrousing standard tissue-culture
techniques (Dobson et al., 2002). These find-
ings will facilitate the development of a
Wolbachia stock centre and permit novel
approaches for the study of Wolbachia.
Wolbachiaspp. are known to infect a wide
array of natural enemies and are common
among parasitoids, including Aphytis,
Encarsia, Lysiphlebus, Muscidifurax, Nasonia
andTrichogramma. Wolbachia have also been
reported to infect predaceous natural ene-
mies, including Adalia, Phytoseiulus,
Neoseiulusand Metaseiulus (Stouthamer et al.,
1999). Although Wolbachiaspp. may affect
host reproduction, they may have little or no
measurable effect on host fitness (Zchori-136 S. Bjørnson and C. Schütte