0851996884.pdf

of increase of the sexual form will be lower
than that of the unisexuals. Finally, below
Mu, the unisexuals and sexuals will cause
the same number of hosts to be killed per
female but the population growth rate of
the unisexuals will be higher (1/Stimes as
high per generation). If we assume that the
wasp population is in an exponential
growth phase, the criteria can be derived to
determine the relative number of hosts
killed over time in this tract; in the simplest
case, the relative number of sexual females
present of either form is given by (SV/Mu)n.
The number of hosts killed per female
equals V/Mu; therefore the relative fraction
of hosts killed by the sexual forms in gener-
ation nequals:
(SV/Mu)n×V/Mu= SnV1 + n/Mu1 + n.

Are unisexuals cheaper to produce?

A major part of the cost of producing para-

sitoids for biological control is the cost of

producing hosts. When unisexual wasps are

used, all hosts result in female parasitoids

and no hosts are wasted in the production of

males. This should result in a reduction in

production cost per female. While in species

with a female-biased sex ratio the difference

in production costs is not very large, in

those species with sex ratios close to 50% the

difference can be substantial. In addition,

even in species that normally have a female-

biased sex ratio, the sex ratio in mass rearing

is often male-biased (Heimpel and

Lundgren, 2000). Particularly for species

used in inundative biological control, these

Use of Unisexual Wasps as Biocontrol Agents 101

Host density expressed as number of hosts parasitized

Relative population growth rate

0

1

2

3

Mu T

Relative host-kill rate

Unisexuals: relative population growth rate and host-kill rate

Sexuals: relative population growth rate

Sexuals: relative host-kill rate

Fig. 8.2.Relative population growth rate expressed as number of daughters per mother and the relative
host-kill rate expressed as the relative number of hosts killed per mother of a unisexual form and a sexual
form. The unisexual form produces 100% daughters but can only parasitize Mu(maximum egg production
of the unisexual female) hosts, while the sexual form can parasitize Thosts (maximum egg production of
sexual host). Sex ratio of sexual form is assumed to be 50% females.