competitiveness in the laboratory was very
high; however, field performance had
declined by about fourfold (relative to gener-
ation 5). The Japanese researchers believed
that decreased flight ability and decreased
mating success at lek sites (where males con-
gregate to attract females) contributed to this
decline. Field experiments demonstrated that
dispersal distance of the captive-reared pop-
ulation was substantially less than that of
wild-caught male melon flies, and this was
confirmed in laboratory experiments on
flight duration (see Ito, 1988). The differences
between domesticated and wild stocks of
melon fly became apparent after the first
hour of flight, and Ito (1988) links this rela-
tively subtle distinction in performance to
the significant loss of quality. The ‘low-qual-
ity’ melon flies were generally able to fly for
more than an hour. Contrast this level of
flight ability with a criterion used for evalu-
ating the quality of captive-reared medfly –
the ability to successfully fly out of a 20 cm
tall, 9 cm diameter cylinder (Boller et al.,
1981). A similar criterion is still being used
(see Cayol and Zarai, 1999).
Since the quality of captive-reared popu-
lations declines over time, an important
practical issue concerns when a population
should be replaced. One useful approach is
to compare the old strain with a newly
established one, under field conditions.
Ahrens et al.(1976) used recapture as a mea-
sure of quality in their comparison of two
strains of sterilized screw-worm flies. The
older strain had a relative recapture rate of
0.49 and had a lower mean dispersal dis-
tance. As a result, the older strain was
phased out of production. The twofold dif-
ference in the performance of these two
strains illustrates how the field environment
can reveal gross inadequacies in a strain that
performs well under captive conditions. A
similar effect was observed with maize ear-
worm (Young et al., 1975). The field mating
performance of sterile males from a labora-
tory colony was significantly improved by
incorporating genetic material from a local
wild population.
As noted earlier, Bush and Neck (1976)
found evidence of directional selection
favouring allele 2 at the -GDH locus in cap-
tive populations of screw-worm flies.
Whitten (1980) provided evidence that this
genetic change adversely affected field per-
formance. Specifically, in a field release, he
compared prerelease and recaptured flies.
The frequency of allele 2 decreased from 0.80
to 0.68 (the natural population had a fre-
quency of 0.31). This shift in frequency sug-
gested poor survival in the field of
individuals carrying allele 2, even though
they are favoured under captive rearing.
Bush and Neck (1976) proposed that the con-
stant temperature of the rearing facility was
promoting the spread of allele 2 and Whitten
(1980) provided support for this view by
finding a significant association between the
-GDH heterozygosity of captured flies and
the prevailing temperature.Improving the Effectiveness of Mass
Rearing
Boller (1972) had the insight to suggest that
the first step in improving the effectiveness of
mass rearing is a psychological one: we
should stop thinking in terms of production
efficiency (cost per individual), and instead
think in terms of the cost to achieve a goal.
This goal-directed approach would lead us to
maximize effectiveness (the product of quan-
tity and quality), as diagrammed in Fig. 6.1.
While recognizing that the optimum strat-
egy is case-specific, there are some general
guidelines that can be used to help maintain
the quality of captive-reared populations.
These guidelines can be considered under
four general headings: colony founding;
colony maintenance; colony replacement;
and colony improvement.Colony foundingIt is important to avoid the detrimental
effects of inbreeding during the first few gen-
erations of a new captive population. These
detrimental effects include the random
increase in the frequency of deleterious alle-
les and the random loss of potentially benefi-
cial genetic variation. They can be avoided
by ensuring that the effective (i.e. genetic)Managing Captive Populations 79