tion of the female that had found the patch.
This experiment showed that females of both
forms were equally capable of finding host
patches, but that the sexual females para-
sitized more hosts per patch. Overall, the
conclusion was that, under these circum-
stances, the use of unisexual wasps is more
economic even when the hosts are found in
patches. When the hosts are solitary, the use
of the unisexual wasps becomes even more
economically practical (Silva et al., 2000).
Discussion
While in theory the use of unisexual wasps
in biocontrol should result in advantages, in
classical biological control this thesis has not
been rigorously tested. In a number of cases,
the unisexual form established when
released in a new area, but whether the sex-
ual form, had it been released in sufficient
numbers, would have done worse remains
untested (Hung et al., 1988). The two studies
where both sexual and unisexual forms of
the same species were released do not give a
clear result either. In the case of E. perniciosi
the unisexual form was replaced by the sex-
ual form (Neuffer, 1990), while in the case of
Anagyrusthe species either did not establish
or established in such low frequency that no
conclusion can be drawn (Aeschlimann et al.,
1989; Dysart, 1990). In the biological control
effort against citrus red scale, the initial
established unisexual species was competi-
tively displaced by the sexual species, A.
lingnanensis(Clausen, 1978). While for the
control of the arrowhead scale the unisexual
A. yanoneniscoexists together with the sexual
Coccobius(Itioka et al., 1997), of these two
species the sexual species appears to be the
more effective parasitoid, both at high host
densities and at low host densities. The pre-
dicted advantage of the unisexual form at
low densities did not materialize. All in all,
the present literature on classical biological
control releases cannot be used to make
statements about the superiority of unisexual
forms over sexual forms. However, few stud-
ies are done that directly test this thesis and,
in many cases, unisexual forms are effective
classical biological control agents.
In seasonal inoculative biological control,
a single extremely effective unisexual bio-
logical control agent stands out: E. formosa.
In the genus Encarsiasexual species gener-
ally have a curious mode of reproduction, in
which males develop as hyperparasitoids of
female larvae, sometimes of conspecific
female larvae (Walter, 1983). This mode of
reproduction is called heteronomous hyper-
parasitism. The presence of only a het-
eronomous hyperparasitic species may
hamper its own population growth rate and
effectiveness, although in classical biological
control such heteronomous species are often
very effective. However, if the main biocon-
trol agent is a unisexual species the presence
of a heteronomous species may influence the
growth rate of the unisexual species,
because sexual males are produced on uni-
sexual female larvae (Vet and van Lenteren,
1981; Pedata and Hunter, 1996; Hunter and
Kelly, 1998).
No large-scale use is made of unisexual
lines in inundative biological control. Most
inundative biological control programmes
use species of the genus Trichogramma. In this
genus large numbers of unisexual lines are
known (Stouthamer, 1997), but no unisexual
form is known in the species most com-
monly used in inundative programmes:
Trichogramma brassicae. In other Trichogramma
species used for biological control, unisexual
forms are known and are sometimes applied
in biological control programmes. For
instance, T. cacoeciaeis used in the USA and
Europe for the control of codling moth
(Dolphin et al., 1972; Hassan and Rost, 1993),
Trichogrammanr. sibericum is used for the
control of cranberry pests (Li et al., 1994), a
unisexual form of Trichogramma pintoiis used
in China (Wang and Zhang, 1988), and
Trichogramma chilonismay be used in Taiwan
(Chen et al., 1992). However, among other
commercially used species, unisexual forms
exist but are not used on a large scale: these
species include Trichogramma platneri,
Trichogramma pretiosum and Trichogramma
evanescens (Stouthamer, 1997). The most
likely reason for the lack of application of
these unisexuals is that they are only known
in academic institutions and are unknown in
the insectary industry.Use of Unisexual Wasps as Biocontrol Agents 105