known to consist of both forms. The best-
studied species is Trichogramma kaykaifrom
the Mojave Desert (Stouthamer and Kazmer,
1994; Pinto et al., 1997; Huigens et al., 2000;
Stouthamer et al., 2001). The distinction
between these two classes – fixed versus
mixed – is important in regard to the influ-
ence the Wolbachiamay have on the life-his-
tory characters of the infected wasps.
In fixed populations we expect that there
will be a selection for accommodation
between the bacteria and their wasp hosts.
The evolutionary interests of both host and
Wolbachiaare the same as long as the infec-
tion is only passed on from the mother to
her offspring. The wasp–Wolbachiacombina-
tion that produces the most daughters will
be selected for. In the case of infected indi-
viduals occurring in a population with sex-
ual individuals where gene flow between
the sexual and unisexual individuals still
occurs, the evolutionary interests of the
Wolbachiaand the nuclear genes of the wasp
are not the same (Stouthamer, 1997;
Stouthamer et al., 2001). Under such circum-
stances the optimal sex ratio for the nuclear
genes is a sex ratio involving at least some
males, whereas for the PI Wolbachiathe opti-
mal sex ratio is 100% female. This conflict in
evolutionary interest of these two genetic
elements can lead to an arms race between
these elements, which may have as a by-
product a reduced offspring production of
the infected females. Other differences also
exist between infected females from fixed
versus mixed populations. When females
from fixed populations are fed antibiotics,
the males they produce are often unable to
successfully mate with the females of their
own line (Zchori-Fein et al., 1992;
Stouthamer et al., 1994; Pijls et al., 1996;
Arakaki et al., 2000). The most likely cause of
this is an accumulation of mutations in
genes involved with sexual reproduction in
these infected lines. Such mutations are
assumed to accumulate because they are not
selected against any longer. Therefore, in
populations where the infection has been
fixed for a prolonged period, sexual repro-
duction is no longer possible and sexual
lines cannot be established from these
forms.
Besides the influence of microbial infec-
tion on parthenogenesis, there are also
many cases of unisexual reproduction
where this mode of reproduction is not sen-
sitive to antibiotic treatment and therefore
we assume that there is a genetic cause of
unisexual reproduction. Examples of such
unisexual reproduction are Venturia
canescens(Speicher et al., 1965; Beukeboom
and Pijnacker, 2000) and Trichogramma
cacoeciae(Stouthamer et al., 1990a).Unisexuals in Biocontrol
The incidence of unisexual reproduction
among wasps used in biological control
appears to be very high. In a sample of
wasps used for biological control the per-
centage of unisexuals was at least 15% (Luck
et al., 1999). The cause for this high propor-
tion of species carrying unisexual forms
may be: (i) the proportion is not extreme but
simply reflects the proportion of unisexual
forms in nature; or (ii) the proportion is high
because the establishment of a species in
quarantine, in mass rearing (Stouthamer and
Luck, 1991) and in the field is much more
successful for unisexual forms than for sex-
ual forms (Hung et al., 1988). If this latter
reason is correct, then the high proportion of
species that are unisexual in wasps used in
biological control is simply a reflection of
our inability to start cultures with only a few
sexual individuals.
In Table 8.1, I give an overview of the
cases of unisexual reproduction in para-
sitoid wasps. Most of these cases were dis-
covered when the wasps were studied for
biological control purposes. In some gen-
era, such as Aphytis, Encarsia and
Trichogramma, the number of unisexual
species is particularly high. This is most
probably due to the relative importance of
these groups in biological control and the
awareness of the workers in this field of
unisexual reproduction (DeBach, 1969). In
other cases, it appears that certain host
species seem to have a disproportionately
high number of unisexual wasps attacking
them; an example of this is weevils. TheUse of Unisexual Wasps as Biocontrol Agents 95