about every nine generations. Such frequent
stock changes may be impractical in many
cases; however, as noted earlier, this time
period corresponds with the time it takes
many captive populations to come close to
their stable level of domestication. A satisfac-
tory compromise may be possible with less
frequent stock changes supplemented with
augmentation from natural populations.
Catching and adding individuals from
the wild may present little problem; how-
ever, this does not ensure that these new
individuals contribute to the strain.
Maladapted to the rearing environment,
they may reproduce poorly or not at all. It
may be necessary to hybridize the wild
genotypes with the laboratory strain
(Calkins, 1989), perhaps under semi-natural
conditions, before the new genes can be suc-
cessfully introduced. Haeger and O’Meara
(1970) showed that wild-caught female C.
nigripalpus(a mosquito) rarely bred in cap-
tivity; however, they could introduce wild
genetic material into the captive population
by crossing wild males with colony females.
Introductions should have a defined goal
in terms of some measurable character. There
should be a measurable change in the moni-
tored character following a successful supple-
mentation with wild-caught genotypes. For
example, Young et al.(1975) demonstrated
increased mating competitiveness in the field
of a maize earworm population augmented
by crossing to local wild-caught moths. In
this example, the character (field mating suc-
cess) was a direct measure of quality; how-
ever, the success of genetic introgression is
more conveniently monitored in the rearing
facility using a trait that shifts predictably in
response to selection for domestication.
Saul and McCombs (1995) argued against
the introduction of new genetic material into
established colonies, using the generally cor-
rect, but misguided, argument that such
introductions will reduce the fitness of indi-
viduals in a mass-rearing facility. In fact, this
is the purpose of such introductions: the goal
is to intentionally reduce fitness (i.e. quan-
tity) in order to gain quality and shift the
population closer to the point of maximum
effectiveness (see Fig. 6.1).
Colony improvementThe evolutionary trajectory from a natural
population to a domesticated one and then
potentially to an inbred one (Fig. 6.1) can be
modified, as noted earlier, by changing the
captive rearing conditions. It can also be
modified through selective breeding or
genetic engineering. This is a potentially use-
ful strategy whenever features of the release
programme suggest potential improvements
(Beckendorf and Hoy, 1985). For example,
temperature extremes were implicated in the
failure of the red-scale parasitoid Aphytis
lingnanensis to become established in the
inland areas of southern and central
California. White et al.(1970) successfully
selected A. lingnanensisfor increased toler-
ance to temperature extremes. However, the
effectiveness of this strategy was never
tested, because in the meantime a congener,
Aphytis melinus, became established in the
area. Selection of a complex trait, such as
temperature tolerance, with the goal of
adapting a population to novel features of
the release site is a strategy that has consid-
erable merit. Furthermore, it is unlikely that
complex traits will be amenable to genetic
engineering in the foreseeable future.
Heilmann et al.(1994) list a number of
genetically simple traits that they consider
potential candidates for genetic engineering.
This list includes such factors as sex ratio,
diapause control and pesticide resistance.
Both the elimination of diapause and pesti-
cide resistance have been the subject of tradi-
tional selection experiments (e.g. Herzog and
Phillips, 1974; Rosenheim and Hoy, 1988;
Hoy et al., 1989), but genetic engineering
may improve efficiency and success.
The techniques of genetic engineering
have been successfully applied to SIT eradi-
cation of medfly. An embryonic temperature-
sensitive lethal allele is used to destroy
female eggs (Franz and McInnis, 1995), so
that only sterilized males are released. Cayol
and Zarai (1999) showed that these flies were
effective in the field. However, this effective-
ness was probably far from its potential max-
imum. Even without irradiation and
shipment, fewer than 50% of the pupae pro-
duced males that were able to fly!82 L. Nunney