cause of unisexual reproduction in many
Hymenoptera (Stouthamer, 1997), and initial
experiments have shown that in some cases
inter- and intraspecific transfers of these
bacteria are possible (Chapter 9; Grenier et al.,
1998; Huigens et al., 2000).
Two papers published in the early 1990s
discussed the use of sexual versus unisexual
lines in biocontrol. The first paper, by
Aeschlimann (1990), suggested initially
releasing unisexual forms, because they may
be easier to establish. Subsequently sexual
forms could be released to introduce genetic
variation in the population. The generality of
that idea was questioned by Stouthamer
(1993), who argued that the sequence in
which these two forms should be released
depends on: (i) the type of biological control
the release is intended for; and (ii) the den-
sity of the hosts that are to be controlled.
In the following sections, I shall give an
overview of the knowledge that we have
gained about unisexual reproduction over
the last 10 years and discuss work done
specifically to test the merits of using either a
sexual form or a unisexual form for biologi-
cal control.
Causes of Unisexual Reproduction
Two classes of causes are known for unisex-
ual reproduction in Hymenoptera: (i) micro-
bial infection; and (ii) other genetic
mechanisms that allow unfertilized eggs to
develop into females.
Over the last 15 years many species have
been discovered that are infected with PI
Wolbachia(Stouthamer et al., 1990b, 1993;
Stouthamer, 1997). These bacteria allow
infected females to produce daughters from
both fertilized and unfertilized eggs. In
many species where PI-Wolbachiainfection is
known, the infection has gone to fixation and
all individuals in the ‘fixed’ population are
infected females (Stouthamer, 1997). An
example is the biocontrol icon Encarsia for-
mosa(Zchori-Fein et al., 1992; van Meer et al.,
1995). In a number of other species the infec-
tion with PI Wolbachia is restricted to a
smaller part of the population and both
infected and uninfected individuals co-occur
and gene flow still takes place between these
two subpopulations (‘mixed populations’)
(Stouthamer and Kazmer, 1994). Only wasps
in genus Trichogramma populations are94 R. Stouthamer
Generations123456789101112131415Relative number of unisexual females in population 01002003004005006007008009001000Fig. 8.1.Relative size of unisexual population (= number of unisexual females/number of sexual females in
generation n) if at generation 1 equal numbers of sexual and unisexual wasps are released. The sexual wasps
produce offspring sex ratio (% females) of either 50%____or of 70% ............... No other differences
are assumed between the sexual and unisexual form; populations in exponential growth phase.