Introduction
Optimizing the mass rearing of arthropods
for release into the field is an extremely com-
plex problem. Much of this complexity arises
from a fundamental evolutionary conflict
implicit in the mass-rearing process: the
ideal captive population is maximally
adapted to both the rearing conditions and
the field conditions into which it will be
released. In most cases, this ideal cannot be
achieved and the optimal solution is a com-
promise between efficient rearing and good
field performance. However, the details of
the compromise will depend upon the
genetic structure of the population. By
adopting a population-genetic approach, we
can avoid many of the pitfalls of establishing
and managing captive populations and
hopefully achieve close to the best solution.
There is an extensive literature on the
evolutionary problem of how organisms in
nature maintain simultaneous adaptation in
two (or more) environments. This is the
problem of adaptive plasticity (see Via et al.,
1995) and the evolutionary solution depends
upon the degree of genetic correlation
between high fitness in the alternative envi-
ronments. A high positive correlation means
that genotypes with high fitness in one envi-
ronment also have high fitness in the second.
However, a high negative correlation means
that genotypes with high fitness in one envi-
ronment have a low fitness in the other. It is
the existence of negative genetic correlations
that creates the evolutionary problem of
adaptive plasticity and what I shall refer to
as the ‘paradox of captive breeding’ –
improving performance in the rearing facil-
ity can result in decreased performance in
the field.
The paradox of captive breeding means
that the optimal solution is generally a com-
promise between quantity and quality.
Quantity is the productivity of the rearing
programme, measured by numbers reared
per unit time. Quality is the ability of cap-
tive-reared individuals to function as
intended after field release (see Chapter 1),
and can be measured relative to the field per-
formance of individuals from the natural
population. The optimal solution aims to
maximize the product (quantity) ×(quality),
and I shall argue that, in general, this most
effective strategy does not maximize either
quantity or quality.The Problem: the Trade-off Between
Quantity and Quality
Measuring the trade-offThe measurement of quantity, i.e. the success
of captive rearing, is a straightforward count
of the numbers of individuals produced
under the rearing protocol. Quantity is
expected to improve with domestication, i.e.
adaptation to the captive environment. This
improvement can be evaluated experimen-
tally by comparing the productivity of the
established captive population to a recently
wild-caught control population. The mea-
surement of quality, i.e. field performance, is
more complex and must be determined by
the specifics of the release programme.
While the goal is to control the numbers of
some specific pest, the specific agent used
may be a parasitoid, predator or, in the case
of the sterile-insect technique (SIT), a conspe-
cific male. However, at a minimum, the mea-
sure must integrate three components: first,
the ability of individuals to disperse from the
release site and find the target (hosts or prey
in the case of natural enemies, females in the
case of SIT); secondly, their ability to success-
fully interact with the target (parasitize the
hosts or eat the prey in the case of natural
enemies, or mate in the case of SIT); and,
thirdly, the ability of the released individu-
als to survive in the field and continue to
find their targets. Finally, if the project goal
is to establish a self-perpetuating population
of a natural enemy (classical biological con-
trol), then the ability of individuals to repro-
duce and to survive through unfavourable
seasons defines a fourth essential component
of quality.
It is important that the quality of the cap-
tive-reared population is measured relative
to a recently wild-caught control population.
However, in practice, it is unlikely that the
field performance of a captive strain could74 L. Nunney