Entomopathogenic nematodes show dif-
ferential susceptibilities to nematophagous
fungi; therefore, knowledge of the types of
fungi that are present in the soil may be of
assistance when selecting the appropriate
entomopathogenic nematode for controlling a
particular pest (Kaya and Koppenhöfer, 1996).
Predatory mites (Acari: Phytoseiidae)Mass-reared phytoseiid mites may harbour
Wolbachia and microsporidia (for recent
reviews, see Poinar and Poinar, 1998; van der
Geest et al., 2000). Moreover, several cases of
unidentified diseases have been reported.
Metaseiulus: predators of mitesBACTERIA.Metaseiulus occidentalis is host to
unidentified microorganisms and Wolbachia
endosymbionts. In some cases, M. occidentalis
exhibits cream to pink rectal plugs that
extrude from the anus and occasionally
cause mites to become firmly attached to
their substrate (Hess and Hoy, 1982). Rectal
plug formation in M. occidentalisis common
in older females and is associated with
motor disfunction, reduced oviposition and
eventual death. Affected immatures and
males rarely exhibit rectal plugs. In some
cases, the immatures and females become
pale, thin and translucent.
Hess and Hoy (1982) described two
unidentified microorganisms in M.
occidentalis. One of these was found in the
midgut, Malpighian tubules and epidermis
but not in the ovarian or nervous tissues. This
microorganism was present in all mites in
varying numbers and was not detrimental to
M. occidentalis. The second microorganism is
rickettsia-like and was present in the majority
(two-thirds) of asymptomatic and sympto-
matic mites. These rickettsia-like organisms
were observed in the haemocoel and
Malpighian tubules. Their presence within
ovarian tissue and on egg surfaces suggests
that they are transovarially transmitted. These
microorganisms are thought to cause pathol-
ogy in some circumstances, particularly when
M. occidentalisis reared under crowded labo-
ratory conditions (Hess and Hoy, 1982).
By using molecular methods (PCR with
Wolbachia-specific primers), Wolbachia
endosymbionts were detected in eight of
nine laboratory populations of M. occidentalis
and in four laboratory populations of
Tetranychus urticaeKoch that served as food
for M. occidentalis (Johanowicz and Hoy,
1996). Therefore, it is probable that the rick-
ettsia-like microorganisms described by Hess
and Hoy (1982) are Wolbachia (for a discus-
sion, see van der Geest et al., 2000).
In M. occidentalis, Wolbachia spp. cause
non-reciprocal reproductive incompatibilities
between infected males and uninfected
females. Uninfected females crossed with
infected males produce few eggs and no
female progeny. Many of the eggs that are
produced are shrivelled (Johanowicz and
Hoy, 1998b). The mechanisms by which
Wolbachiaspp. cause reproductive incompati-
bilities in M. occidentalis are unknown.
However, Wolbachia infection seems to be
associated with fitness costs, as the number
of female progeny is lower in infected con-
trol crosses than in uninfected control
crosses. These fitness costs may have pre-
vented the rapid spread of Wolbachia in three
laboratory populations of M. occidentalis
(Johanowicz and Hoy, 1998a). Wolbachiaspp.
are eliminated from M. occidentalis when
they are reared at an elevated temperature
(33°C) (Johanowicz and Hoy, 1998a,b).
Lighthart et al.(1988) tested the effect of
several stress factors on the susceptibility of
M. occidentalisto the weak bacterial pathogen
Serratia marcescens. A high preinoculation
temperature pulse in relatively uncrowded
conditions was most effective in enhancing
susceptibility, higher mortality being the
only disease symptom. Remarkably, starva-
tion did not have such an effect.Neoseiulus(formerlyAmblyseius): predators
of mites and thripsVIRUSES.Unidentified, non-occluded virus-
like particles were observed in the yolk of
developing eggs inside Neoseiulus cucumeris
(Oudemans) females. The effects of these
virus-like particles on predator efficacy are
not known (Bjørnson et al., 1997).140 S. Bjørnson and C. Schütte