may undergo when introduced into the labo-
ratory are given in Table 1.2.
Variability in performance traits is usually
abundantly present in natural populations
(Prakash, 1973) and can remain large even in
inbred populations (Yamazaki, 1972). But
differences between field and laboratory
environments will result in differences in
variability. When natural-enemy cultures are
started, part of the ‘open population’ from
the field, where gene migration can occur
and environmental diversity is large, is
brought into the laboratory and becomes a
‘closed population’. Thereafter, all future
genetic changes act on the limited genetic
variation present in the original founders
(Bartlett, 1984b, 1985; Chapters 6 and 7). The
size of the founder population will directly
affect how much variation will be retained
from the native gene pool. Although there is
no agreement on the size of founder popula-
tions needed for starting a mass production,
a minimum number of 1000 individuals is
suggested (Bartlett, 1985). Founder popula-
tions for commercial cultures of a number of
natural enemies were, however, much
smaller, sometimes fewer than 20 individuals
(for examples, see van Lenteren and Woets,
1988). Fitness characteristics appropriate for
the field environment will be different to
those for the laboratory. These environments
will place different values on the ability to
diapause or to locate hosts/prey or mates.
Such laboratory selection forces may pro-
duce a genetic revolution (Mayr, 1970) and
new, balanced gene systems will be selected
for (Lopez-Fanjul and Hill, 1973).
One of the methods often suggested to
correct for genetic revolutions is the regular
introduction of wild individuals from the
field. But, if the rearing conditions remain
the same in the laboratory, the introduced
wild individuals will be subjected to the
same process of genetic selection.
Furthermore, if a genetic differentiation has
developed between laboratory and field
populations this may lead to genetic isola-
tion (Oliver, 1972) and usually the labora-
tory-selected population will take over. Also,
positive correlations have been found
between the incompatibility of such races
and the differences between the environ-
ments (laboratory, field) where the races
occur (e.g. Jaenson, 1978; Jansson, 1978), and
for the length of time that the two popula-
tions have been isolated. Given these
processes, introduction of native individuals
to mass-rearing colonies is likely to be useless
if incompatibility between field and laboratory
populations is complete. If one wants to intro-
duce wild genes, it should be done regularly
and from the start of a laboratory rearingNeed for Quality Control of Biocontrol Agents 9Table 1.2.Factors influencing changes in field populations after introduction into the
laboratory.1.Laboratory populations are kept at constant environments with stable abiotic factors
(light, temperature, wind, humidity) and constant biotic factors (food, no predation or para-
sitism). There is no selection to overcome unexpected stresses. The result is a change of
the criteria that determine fitness and a modification of the whole genetic system (Lerner,
1958)
2.There is no interspecific competition in laboratory populations, resulting in a possible
change in genetic variability (Lerner, 1958)
3.Laboratory conditions are made suitable for the average, sometimes even for the
poorest, genotype. No choice of environment is possible as all individuals are confined to
the same environment. The result is a possible decrease in genetic variability (Lerner,
1958)
4.Density-dependent behaviours (e.g. searching efficiency) may be affected in laboratory
situations (Bartlett, 1984b)
5.Mate-selection processes may be changed because unmated or previously mated
females will have restricted means of escape (Bartlett, 1984b)
6.Dispersal characteristics, specifically adult flight behaviour and larval dispersal, may be
severely restricted by laboratory conditions (Bush et al., 1976)