0851996884.pdf

production costs may form an important
reason for choosing the unisexual form.

Are unisexual forms easier to establish in

classical biocontrol projects?

Few papers directly address this issue. There
are some indirect indications, summarized in
the paper by Hung et al.(1988). They discuss
a list of species where the mode of reproduc-
tion in the native area of the species is sexual,
while in the area where they are released for
biological control the mode of reproduction is
unisexual. The most likely explanation is that
in the native area the population consists of a
mixture of both sexual and unisexual forms
and in the release area the unisexuals are bet-
ter able to colonize. Statistics on the mode of
reproduction and the success of colonization
are not available. However, Hopper and
Roush (1993) show how important Allee
effects in mate finding may be for the success
of biological control. Some striking examples
exist of the establishment of unisexual forms
in biocontrol; for instance, Laing and Heraty
(1981) report the successful establishment of
Apanteles pedias after placing only two
females in sleeve cages in the field.

Are unisexuals able to suppress the pest to a

lower density than sexuals?

The same Allee effect as mentioned above
would allow the unisexual to suppress the
host density to a lower level than the sexual.
Very low host densities and therefore low
wasp densities would make mate finding
difficult, but unisexuals would not suffer
from such a drawback. This may not be very
important because, if low host densities (and
therefore low wasp densities) cause wasps to
be unable to encounter each other, it may
also be extremely difficult to find hosts. No
evidence exists for this hypothesis.

Disadvantages of Unisexuals

Often the lack of sexual reproduction is seen
as a dead end, because the wasps would be

unable to adjust to changes in the environ-

ment and over time mutations in the genome

of the wasps are assumed to accumulate

(Muller’s ratchet). This may indeed be a

problem, but the time-scale in which these

problems may manifest themselves is proba-

bly substantial. For instance, all of our

knowledge of E. formosa indicates that

parthenogenesis has been present in that

species for a long time. Sexual reproduction

is no longer possible between males and

females of this species and no sexual popula-

tions are known. This species has been

reared for biological control more or less

continuously since the 1930s and shows no

signs of losing its effectiveness.

A potential disadvantage of the unisexual

reproduction of the parasitoid may be the

evolution of resistance against the parasitoid

by its sexual host species. In the case of E.

formosa, we tried to find evidence for such

resistance in its host, the greenhouse white-

fly, Trialeurodes vaporariorum. Several factors

made it unlikely that we would have to fear

this development of resistance, the main rea-

son being the method the insectaries use to

grow the E. formosa. The whiteflies that are

used to start the host population in the next

generation are those that were not para-

sitized or that survived parasitization in the

wasp rearing. The rearing method that the

insectaries apply is the perfect experiment to

determine if resistance is evolving. No clear

evidence for the evolution of resistance to

the whiteflies has been found.

Case-studies: Unisexuals in Classical

Biological Control

Sexuals more successful than unisexuals

For several years, Neuffer (1962, 1964a, b)

tried to establish the unisexual form of

Encarsia perniciosifor the biological control of

San José scale in Germany. This was without

success. He subsequently established cul-

tures of the sexual form of E. perniciosi

(Neuffer, 1966, 1968, 1969, 1975, 1981, 1990),

and was able both to establish the cultures

and to control the San José scale. At the same

time, the sexual form of E. perniciosi also

102 R. Stouthamer