decreasing number of pesticides available,
with increasing costs per unit of volume for
chemical pesticides and implementation of
pesticide levies, as is currently taking place
in several countries, the aspect of relatively
high costs of natural enemies will disap-
pear. Also, effective techniques to mass-pro-
duce natural enemies on artificial diets are
often not available. Fewer than ten species
of natural enemies can be produced on arti-
ficial diets, but their field performance may
be poorer than that of natural enemies
reared on a host insect (Chapter 9).
Although mass production on artificial diets
may lead to reduction of costs, the risks of
changing natural-enemy effectiveness
should not be underrated (see below).
Another obstacle for mass production is the
lack of techniques to prevent selection pres-
sures leading to genetic deterioration of the
mass-produced organisms. Through such
deterioration, the natural enemy could lose
its effectiveness (Boller, 1972; Boller and
Chambers, 1977).
Cannibalism among predators may make
individual rearing (e.g. for Chrysopaspp.) or
rearing at relatively high prey densities (e.g.
for Amblyseiusand Phytoseiulusspp.) neces-
sary and will lead to high rearing costs.
Superparasitism with parasitoids has the
same effect. Rearing of parasitoids and
predators under ‘unnatural’ conditions on
‘unnatural’ hosts or prey or on artificial
media may change their reactions to natural-
host or host-plant cues as a result of missing
or improper preimaginal or imaginal condi-
tioning (Chapters 3 and 4). Rearing para-
sitoids on unnatural hosts may lead to
reduced vigour as a result of an inadequate
supply of nutrition (quantity or quality)
from the unnatural host; the same effect can
occur when the host is reared on an unnat-
ural diet, even if the host itself remains
apparently unaffected (Chapter 9).
Finally, the rearings can be infected by
pathogens (Chapter 10). One of the problems
often encountered in insect rearing is the
occurrence of pathogens and microbial cont-
aminants, leading to high mortality, reduced
fecundity, prolonged development, small
adults, wide fluctuations in the quality of
insects or direct pathological effects.
Goodwin (1984), Shapiro (1984), Sikorowski
(1984), Singh and Moore (1985), Bjørnson
and Schütte (Chapter 10) and Stouthamer
(Chapter 8) give information on the effects of
microorganisms on insect cultures and the
measures available to minimize or eliminate
the pathogens or contaminations. Further,
they discuss the recognition of diseases and
microorganisms in insect rearing and the
common sources of such microbial contami-
nants. The most common microbial contami-
nants encountered in insect rearing are fungi,
followed by bacteria, viruses, protozoa and
nematodes. The field-collected insects that
are used to start a laboratory colony are a
major source of microbial contaminants. The
second main source is the various dietary
ingredients. Disinfection of insects and
dietary ingredients is recommended to pre-
vent such contaminations. The causes of
microbial contamination are usually rapidly
found, but elimination of pathogens from
insect colonies is difficult (Bartlett, 1984a;
Chapter 10).Behavioural variation in natural enemiesThe variation and changes in behaviour of
natural enemies that can be caused by rear-
ing conditions are manifold. The main ques-
tion is whether erratic behaviour of natural
enemies can be prevented or cured. This
issue, together with a thorough theoretical
background, is discussed in Chapters 3 and- Most ecologists are aware that variability
in natural-enemy behaviour occurs fre-
quently. It is important to know how natural
enemies function in agroecosystems because
such understanding may help in designing
systems where natural enemies can play an
even more important role in inundative and
seasonal inoculative releases.
The core of natural-enemy behaviour,
host-habitat and host-location behaviour,
shows great variability, which often leads to
inconsistent results in biological control.
Most studies aimed at understanding such
variability have focused on extrinsic factors
as causes for any inconsistencies seen in for-
aging behaviour. Typically, however, forag-
ing behaviour remained irregular even when
12 J.C. van Lenteren