seemed to have spread, at least in some areas
of the USA, where the unisexual form pre-
vailed earlier (Stouthamer and Luck, 1991).
The introduction of both the unisexual
Aphytis yanonensisand the sexual Coccobius
fulvusfor the control of the arrowhead scale
Unaspis yanonensis was studied in detail
(Itioka et al., 1997). In this introduction, it
appeared that the control exerted by the sex-
ual C. flaviuswas much more substantial
than that of A. yanonensis. Only in the last
few dates of the study, when the scale popu-
lation density was substantially reduced did
the importance of A. yanonensisincrease. At
the lower densities of the host and therefore
of the wasps, the Allee effect may result in
the lower efficiency of the sexual form.
Apparently in this study this was not the
case, either because the wasp density did not
become that low or because the hypothesis is
not correct. A study covering the next few
years of this interaction showed that also in
the years after the last reported date the
importance of A. yanonensisdid not increase
substantially (T. Itioka, Nagoya, Japan,
January 2001, personal communication).
The control exerted by the unisexual
Aphytis chrysomphalion California red scale
in the first half of the 20th century in
California was considered variable and in
some cases satisfactory. The parasitoids had
spread throughout the red-scale-infested
areas of California. In 1947 the sexual species
Aphytis lingnanensis was introduced. This
species replaced A. chrysomphaliin a rela-
tively short time in all areas except in a few
small coastal areas (Clausen, 1978). For the
control of the California red scale, several
unisexual species have been released (E. per-
niciosi, A. chrysomphali, Habrolepis rouxi);
although they do play a minor part in its
control, in general the sexual species are
more important.
Another case where the initially success-
ful unisexual parasitoid was replaced by sex-
ual species is in the biological control of the
citrus blackfly in Mexico (Smith et al., 1964;
Flanders, 1969). In one of the most thorough
biocontrol efforts ever, H.D. Smith collected
large numbers of individuals of several
species of whitefly parasitoids in India and
Pakistan. These parasitoids were mass-
reared in infested orchards throughout
Mexico and subsequently distributed. Three
sexual species were imported: Encarsia opu-
lenta, Encarsia clypealisand Amytis hesperidum
and a single unisexual species Encarsia
smithi. Initially, E. smithiwas very successful
and a total of approximately 3,000,000 wasps
had been dispersed throughout the country.
In one grove where releases were carried
out, the results were spectacular and in a
short time the blackfly was controlled.
However, in most other areas the sexual par-
asitoids did better and replaced E. smithi
(Smith et al., 1964; Flanders, 1969). This was
not only because the other sexual Encarsia
species produced males hyperparasitically,
but also because the larvae of A. hesperidum
proved competitively superior to those of E.
smithi (Smith et al., 1964). In subsequent
infestations in Texas and Florida, only the
sexual species were released. An accidental
introduction into a rearing facility in Florida
of E. smithibrought to light the fact that this
species is most probably also a sexual
species, whose males closely resemble those
of E. opulenta (Nguyen and Sailer, 1987).
Since these wasps originated in Mexico,
either H.D. Smith wrongly classified the
species or the species consisted of two differ-
ent forms when released and the sexual form
became the contaminant in the mass rearing.
For the control of the olive scale, a large
number of different forms of Aphytis mac-
ulicornis were collected throughout the
world. Several were introduced; the unisex-
ual strain collected from Egypt was released
in 1949 and it became established. Later on, a
unisexual Spanish strain and sexual Persian
and Indian forms were released. The sexual
Persian strain soon established and was
apparently the most effective (Clausen, 1978).InconclusiveAeschlimann et al.(1989) tried to establish
Anaphes dianain Australia from material col-
lected around the Mediteranean consisting of
both unisexual and sexual forms
(Aeschlimann, 1990). The establishment in
Australia failed (Aeschlimann et al., 1989). In
North America A. dianawas released fromUse of Unisexual Wasps as Biocontrol Agents 103