Breeuwer and Jacobs (1996) detected
Wolbachiain a commercial population of P.
persimilisfrom The Netherlands. The effects
of Wolbachiaon the efficacy of P. persimilis
have not been established.
In a microscopic study of the digestive
tract of P. persimilis, bacteria-like entities
detected in the gut lumen were thought to
have entered the digestive tract during feed-
ing (Arutunyan, 1985). However, these bacte-
ria bear a marked similarity to the
birefringent dumb-bell-shaped crystals that
are frequently observed in the caecae and
rectum of some phytoseiid mites (see
Unidentified disease, below; Schütte et al.,
1995; Bjørnson et al., 1997).
PROTOZOA.Three distinct species of
microsporidia have been reported from P.
persimilis from three commercial sources
(North America, Europe and Israel; Bjørnson
and Keddie, 2000). Microsporidia are not
restricted to specific tissues, and spores are
found in muscle fibres, the super- and suboe-
sophageal ganglia, ovaries, eggs, cells under-
lying the cuticle and cells lining the caecal
lumen and Malpighian tubules. Early devel-
opment of all three microsporidia occurs in
cells of the lyrate organ. The lyrate organ
occupies a significant portion of the body
and is thought to be involved in oogenesis or
embryogenesis. Each microsporidium occu-
pies a specific site within these cells.
Infection of the lyrate organ may be neces-
sary for the efficient vertical transmission of
microsporidia in P. persimilis(vertical trans-
mission is 100%). Males do not contribute to
infection of the progeny.
Microsporidium phytoseiulidoes not infect
two-spotted spider mites (T. urticae).
Therefore, prey mites do not contribute to
pathogen transmission among P. persimilis
mites. Horizontal transmission of
microsporidia does not readily occur when
uninfected adult predators are placed in
proximity to infected P. persimilis females or
solutions of microsporidian spores.
Horizontal transmission is low (about 15%)
when uninfected immatures develop in
proximity to infected adult and immature
mites (Bjørnson and Keddie, 2001). Although
microsporidia are horizontally and vertically
transmitted, little is known regarding the
mechanisms of transmission.
Microsporidia-infected P. persimilis do not
exhibit any obvious external symptoms;
therefore, routine monitoring is necessary to
detect microsporidia when disease preva-
lence is low (Bjørnson and Keddie, 1999).
Microsporidia-infected P. persimilisproduce
fewer eggs than uninfected predators and the
longevity and prey consumption of infected
females are reduced (Bjørnson and Keddie,
1999). Decreases in fecundity, longevity and
prey consumption of microsporidian-infected
mites may have a profound effect on the per-
formance of P. persimiliswhen released on
crops for pest control.
To further complicate matters, micro-
sporidia may alter the sex ratio of P. persim-
ilis. Adult females infected with M.
phytoseiuli produce fewer female progeny
than those produced by uninfected females
(Bjørnson and Keddie, 1999). This may affect
the intrinsic rate of increase of the popula-
tion and have an adverse effect on the estab-
lishment of mite colonies.UNIDENTIFIED DISEASE.Birefringent dumb-bell-
shaped crystals have been observed in P. per-
similisfrom several sources (Schütte et al.,
1995; Bjørnson et al., 1997, 2000). Excessive
crystal formation is associated with white
discoloration of the opisthosoma. Symptoms
appear as two white stripes down the lateral
sides of the body in the region of the
Malpighian tubules or as a U-shaped discol-
oration in the distal opisthosoma. A rectal
plug may be observed when symptoms are
more pronounced. Rectal plugs often disrupt
normal excretion and may cause the affected
individual to become stuck to the leaf sur-
face (Bjørnson et al., 1997). The frequent
occurrence of a prominent white dot in the
opisthosoma of P. persimilis is correlated to
reduced fecundity in mites examined follow-
ing shipment (Bjørnson et al., 2000). Crystals
are observed in immature and adult P. per-
similis(Bjørnson et al., 1997); therefore, non-
excessive crystal formation is probably a
normal physiological process (Schütte et al.,
1995; Bjørnson et al., 1997). An examination
of P. persimilis from 14 commercial and
research sources showed that there is no cor-142 S. Bjørnson and C. Schütte