6 Managing Captive Populations for Release:

a Population-genetic Perspective

L. Nunney

Department of Biology, University of California, Riverside, CA 92521, USA

© CAB International 2003. Quality Control and Production of Biological Control Agents:
Theory and Testing Procedures (ed. J.C. van Lenteren) 73

Abstract

The success of biological control, particularly augmentative biological control, depends upon the effec-
tive mass rearing of natural enemies. However, developing the best rearing strategy is complicated by
the ‘paradox of captive breeding’: increasing quantity generally decreases quality. Quantity is the num-
ber of individuals produced per unit time, and it is easily measured. Quality is the ability of captive-
reared individuals to function as intended in the field, and can be measured as field success relative to
the success of individuals from a natural population. Such field measurements are almost always diffi-
cult and expensive. Unfortunately, quantity and quality are usually negatively correlated, since genetic
adaptation to the rearing environment often adversely affects adaptation to the field environment.
Here I review examples of adaptation to captive rearing and of the trade-off with field performance.
Given this trade-off, the optimum management strategy is a compromise that can only be defined after
extensive field tests. However, we can identify some general factors that are likely to influence the out-
come of a breeding programme. A large, genetically variable founding population from a geographical
region climatically similar to the release site maximizes the chance of adaptation of the control agent to
the release site. While a large founding population minimizes the immediate risk of inbreeding, it may
include a few genotypes preadapted to the captive rearing conditions. The success of these few geno-
types can result in a genetic bottleneck. The problem can be minimized by temporarily maintaining
many small breeding units, so that the reproductive success of a lot of individuals is ensured during
the initial phase of domestication. However, any genetically variable population will adapt to its new
artificial conditions, and the breeding facility should be designed to minimize selection for characteris-
tics known to reduce field performance. Even so, field-adapted genotypes must be incorporated on a
regular basis, either by monitored addition to the captive population or by establishing a completely
new population. Alternatively, adaptation to the captive environment can be avoided by maintaining a
large number of inbred (isofemale) lines. This approach, combined with prerelease crosses, can be very
effective at maintaining quality. These considerations highlight an important problem associated with
using genetically manipulated stocks. We must be careful that the potential benefits of genetic engi-
neering are not squandered by incorporating beneficial genetic changes into laboratory-adapted stocks
that are ill suited to field release.