0851996884.pdf

drogenase (ADH) loci declined from an ini-
tial frequency of 0.6–0.7 to close to 0.2.
Strong selection for adaptation to the cap-
tive-rearing environment can be expected to
reduce the genetic variability of populations.
Bush and Neck (1976) and Loukas et al.(1985)
noted that the genetic (allozyme) variability
of captive colonies decreased over time, and
Miyatake and Yamagishi (1999) found that
the heritability of larval development time in
captive melon fly declined over time until it
was not statistically different from zero.

Changes in Field Performance

There is no question that captive rearing
results in a cascade of genetic changes as a
population adapts to its new environment.
But this leaves open the question of the
extent to which these changes reduce field
performance. As noted earlier, measuring
field performance is almost always difficult
and understandably many researchers have
used simple laboratory tests to infer effective-
ness in the field (see, for example, Cohen,
2000). However, data from the Japanese
melon-fly SIT programme argue strongly
against this approach.

The melon fly was successfully eradi-

cated from the Japanese islands of Kume-

zima in 1977 and Miyako-zima in 1987. This

success came after several failed attempts in

other parts of the world to use SIT to eradi-

cate fruit flies. Ito (1988) reviewed the

Japanese project and concluded that, con-

trary to prevailing practice, a relatively low

sterile : wild-fly ratio is sufficient to achieve

success provided that the harmful effects of

domestication can be avoided. In particular,

he emphasized that, although the negative

effects of irradiation have always been a con-

cern in SIT, mass rearing can cause a much

greater reduction in the mating competitive-

ness of released males. This is a very impor-

tant point. It suggests that the prevailing

emphasis on quantity is misplaced. A failure

to maintain quality can drive up the cost of

biological control and can make successful

control unlikely. Calkins and Ashley (1989)

stressed this same point for medfly SIT.

Using estimates from three medfly stocks,

they calculated the dramatic increase in costs

incurred when quality is compromised.

A decline in the field mating competitive-

ness of mass-reared melon flies became

apparent after about 15 generations. No such

decline was apparent under laboratory con-

ditions (Fig. 6.2). At generation 18, mating

78 L. Nunney

Laboratory

Field

5101520

Generations of mass rearing

0

0.5

1.0

Competitiveness

Fig. 6.2.The decline in the field mating competitiveness of sterile, captive-reared male melon flies, as a
function of their time in mass production. Also shown are the results of laboratory trials at generations 16
and 17. The release programme on Kume-zima, Japan, started after generation 5. (Figure from Ito, 1988.)