the same source material; it is also assumed
that it contained both unisexual and sexual
forms. In total, approximately 12,500 indi-
viduals were released but only very few
individuals were recovered: 3 years after the
last release two males and one female were
found, and 6 years after the last release
another two males and a single female were
found (Dysart, 1990).
Unisexual successfulEncarsia berlesei, a unisexual parasitoid of the
white peach scale, proved extremely efficient
in the control of this pest. The parasitoid was
released in several European countries
where the scale was a pest, and complete
control was attained (Clausen, 1978).
Biological control of the citriculus mealy
bug by the unisexual encyrtid Clausenia pur-
pureain Israel was very successful and no
other parasitoids were released. Similarly,
the unisexual parasitoid Tetrastichus asparagi
is a successful imported parasitoid of the
asparagus beetle in North America, where it
became established (Clausen, 1978).
In a shipment of parasitoids for the
lucerne weevil, a number of ichneumonids
(Biolysia tristis) were present that did not end
up in their intended locale (Utah) but were
diverted to Washington, DC. There they
were released and have since spread to many
of the states on the east coast and in the
Midwest, where they reduce the populations
of their host, the clover-leaf weevil (Hyperica
punctata). The unisexual sweet-clover-weevil
parasitoid Pygostolus falcatuswas established
successfully in Canada; however, hardly any
control was exerted by these parasitoids
(Clausen, 1978).
The unisexual Hexacola sp. nr. websteri
was imported together with several other
sexual species for the control of eye gnats;
although several parasitoids, including the
Hexacolasp. nr. websteri, were established,
none exerted substantial control of the pest
(Clausen, 1978). The unisexual wasp A.
pediaswas established in Canada on the spot-
ted tentiform leafminer from an initial collec-
tion of only two individuals and spread
rapidly in a large area (Laing and Heraty,
1981). Several unisexual lines of the South
American species Microctonus hyperodaewere
released for biological control of the weevil
Listronotus bonariensis. Several populations
were released, all originating from different
parts of South America. Successful establish-
ment took place and high parasitism rates
have been reported (Phillips et al., 1997;
Goldson et al., 1998).Case-studies: Unisexuals in Seasonal
Inoculative Biological Control
The best-known case of the use of unisexuals
in seasonal inoculative biological control is
the use of E. formosafor the biological control
of the greenhouse whitefly (T. vaporariorum).
This parasitoid is probably one of the most
applied biological control agents in green-
houses and its use and biology have been
extensively reviewed (see, for instance, van
Lenteren et al., 1997). No sexual forms of this
species are known, so no comparative work
has been done on the relative advantages of
either form. Other unisexual species, such as
Eretmocerus staufferiand Amitus bennetti, are
also being considered for inoculative biologi-
cal control of whiteflies in greenhouses
(Drost et al., 2000).Case-studies: Unisexuals in Inundative
Biological Control
The only controlled study that has been done
thus far to test the potential advantage of uni-
sexual forms over sexual forms of the same
species is the study of Silva et al.(2000). In
this study small greenhouses were used, in
which unisexual and sexual forms of the
same line were released, both of Trichogramma
deionand of Trichogramma cordubensis. The
sexual forms had been derived from the uni-
sexual forms by antibiotic treatment. In the
greenhouse, tomato plants were placed with
egg cards attached to them. Either a mixture
of unisexual and sexual wasps was released
or the different forms were released in adja-
cent greenhouses. The location and the num-
ber of the parasitized egg patches was
determined, as well as the mode of reproduc-104 R. Stouthamer