Potential Advantages of Unisexuals
As summarized by Stouthamer (1993), the
advantages of unisexual wasps in biocon-
trol are: (i) unisexual wasps have a poten-
tially higher rate of increase than sexual
wasps; (ii) unisexual wasps are cheaper to
produce, all the wasps reared in mass rear-
ing are females and only females are effec-
tive in biological control; (iii) unisexual
forms should be easier to establish in classi-
cal biocontrol projects because they do not
suffer from the Allee effect, i.e. a shortage of
mating partners, which may limit the
growth rate of sexual forms when wasp
density is low; and (iv) for the same reason
unisexual wasps may be able to reduce the
host density to lower levels than the sexu-
als, since low wasp densities may cause a
reduction in the ability of females to find
mates and therefore to produce daughters
for the next generation.
Do unisexual wasps indeed have a higher rate
of increase than sexual forms?
This will depend entirely on the number of
female offspring produced per unisexual
female versus per sexual female. Little is
known about the number of daughters pro-
duced by comparable sexual and unisexual
females. In the case of Wolbachia-induced
parthenogenesis, the relative offspring pro-
duction of unisexual (infected) females dif-
fers from that of the sexual females. This
has been studied extensively in
Trichogrammaspecies, where unisexual lines
could be cured of their infection and ren-
dered sexual (Stouthamer et al., 1990a).
When unisexual and sexual forms of the
same line are compared, the offspring pro-
duction of the sexual form is generally
much higher when the unisexual form orig-
inated from a population where both sexu-
als and unisexuals co-occurred (mixed
populations), while, if these comparisons
were made using Trichogrammafrom popu-
lations where the infection has gone to fixa-
tion, no significant difference in offspring
production could be found. In general, it
appears that the influence of the infection
on offspring production is much higher in
those cases where the infected and unin-
fected wasps occur together (i.e. mixed
populations) (van Meer, 1999). Similarly,
there appears to be hardly any negative
influence of the Wolbachia infection in
species such as E. formosaand Muscidifurax
uniraptor(Stouthamer et al., 1994).
These comparisons have been made in
the laboratory using conditions where the
wasps were given a surplus of hosts. In the
field, the situation may be entirely different.
Even if the unisexual forms are capable of
producing fewer offspring than the sexual
forms in the laboratory, this may not be very
important in the field. The number of hosts
that a wasp encounters determines the num-
ber of offspring produced and, as long as
this number is below the maximal egg pro-
duction of the unisexual line, all hosts
encountered by both forms will be para-
sitized (Stouthamer and Luck, 1993). Even
when the hosts are more numerous than the
maximum egg production (Mu) of the uni-
sexual line, the number of daughters pro-
duced by a unisexual female will be higher
until the number of hosts encountered
reaches the threshold T. If we define the sex
ratio produced by sexual females as S,
expressed as the fraction of daughters in the
offspring, Tcan be derived as follows:Mu= T×S, T= Mu/S
In the range of host densities of Muto T,
the sexual form will kill more hosts per
female than the unisexual form and yet the
growth rate of the unisexual population will
be higher than that of the sexual popula-
tion. These three zones of host-encounter
rates (Mu, Mu–T, T) are useful values
for making predictions about the relative
usefulness of releasing unisexual versus
sexual wasps for biocontrol (Fig. 8.2). As
long as the host density is and remains such
that the number encountered per female is
larger than T, then it is more useful to
release the sexual form. It will both have a
faster rate of population growth and kill a
higher number of hosts than the unisexual
form. In the range of host densities between
Muand T, the sexual form will kill a higher
fraction of the host population but the rate100 R. Stouthamer