P. foveolatusis also highly susceptible to
N. varivestis, particularly when it parasitizes
late-instar beetle larvae that are infected as
first instars. Under these circumstances, all
parasitoid progeny are infected and about
half are unable to emerge. Spores are present
in midgut epithelial cells, the ventral nerve
cord and adipose tissue. N. varivestis has no
effect on parasitoid development. However,
infected females produce fewer eggs and a
higher proportion of male progeny than
uninfected females. N. varivestisis vertically
transmitted and, although it is less virulent
than N. epilachnae, disease prevalence is close
to 100% for both species (Own and Brooks,
1986). Most infected adults appear normal;
only a small proportion (12%) has mal-
formed wings and greatly distended
abdomens (Own and Brooks, 1986). Because
most infected parasitoids appear normal,
microsporidia may remain undetected in P.
foveolatus colonies, ultimately having an
adverse effect on their efficacy as biological
control agents (Chapman and Hooker, 1992).
Trichogramma(Trichogrammatidae):
parasitoids of LepidopteraBACTERIA.Wolbachia-induced parthenogenesis
is a well-studied phenomenon in the genus
Trichogramma. Molecular evidence for the
presence of Wolbachiahas been found in at
least nine Trichogrammaspecies: T. brevicapil-
lumPinto and Platner, T. chilonis Ishii, T. cor-
dubensis Vargas and Cabello, T. deionPinto
and Oatman, T. kaykaiPinto and Stouthamer,
T. nubilaleErtle and Davis, T. oleae, T. pretio-
sumRiley andT. sibericumSokorina (Chapter
8; Stouthamer et al., 1993; cited in
Stouthamer, 1997, and van Meer, 1999). Four
completely parthenogenetic species (T. deion,
T. chilonis,T. platneriNagarkatti, and T. pre-
tiosum) have permanently reverted to pro-
ducing male and female progeny when
treated with antibiotics (tetracycline
hydrochloride, sulphamethoxazole or
rifampicin) or when reared at temperatures
above 30°C (Stouthamer et al., 1990). These
revertible lines carry microorganisms within
their eggs, whereas the eggs of non-revert-
ible lines or arrhenotokous (bisexual) lines
do not (Stouthamer and Werren, 1993).
In most known cases of Wolbachia-
induced thelytoky, only infected thelytokous
lines are known (for example, E. formosa and
M. uniraptor). However, in Trichogramma,
most species are infected at a low level
(Stouthamer, 1997) and uninfected arrheno-
tokous and infected thelytokous forms co-
occur and interbreed in mixed populations
in the field. Under such conditions, horizon-
tal transmission of Wolbachia may occur
between individuals that share the same
host. For example, infected T. kaykailarvae
transmit Wolbachia to conspecific uninfected
larvae, after which the transferred Wolbachia
may be vertically transmitted to their off-
spring (Huigens et al., 2000). However, T.
kaykaipopulations in the field have infection
levels of 6–26% (cited by Stouthamer et al.,
2001). In this case, the infection is kept at a
low level by a parasitic B chromosome that
causes females to produce only male off-
spring. The B chromosome destroys the
paternal chromosomes with the exception of
itself, thereby converting the diploid fertil-
ized egg into a male haploid egg
(Stouthamer et al., 2001).
Thelytokous and arrhenotokous con-
specifics co-occur in many field populations
of Trichogramma; therefore, one can choose
either form for use in biological control pro-
grammes. In the case of Trichogramma, the
question arises whether one form is more
effective for biological control than the other
(for a detailed discussion, see Chapter 8;
Aeschlimann, 1990; Stouthamer, 1993). For T.
cordubensisand T. deion, Wolbachia-infected
(thelytokous) lines were compared with
uninfected (arrhenotokous) lines (Silva et al.,
2000). Arrhenotokous lines have a higher
fecundity rate and greater dispersal than the-
lytokous lines under laboratory conditions.
However, dispersal is not affected or
reduced when arrhenotokous lines are
released in the greenhouse. Therefore, thely-
tokous lines have a greater potential for bio-
logical control, despite their low fecundity
(Silva et al., 2000).
Wolbachia symbionts of the A subdivision
are present in several strains of the arrheno-
tokous species of Trichogramma bourarachae
Pintureau and Babault (Vavre et al., 1999).
Strains infected with this type of Wolbachia154 S. Bjørnson and C. Schütte