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using precisely the same set of external stim-
uli. Two types of adaptive variation have
been distinguished in the foraging behaviour
of natural enemies: genetically fixed differ-
ences and phenotypic plasticity. In order to
understand erratic behaviour and to be able
to manipulate such variation, biological con-
trol researchers need to know the origins and
width of variation (Chapters 3 and 4).
Foraging behaviour can also be strongly
influenced by the physiological condition of
the natural enemy. Natural enemies face
varying situations in meeting their food,
mating, reproductive and safety require-
ments. The presence of strong chemical,
visual or auditory cues, cues related to the
presence of enemies of the natural enemy
and (temporary) egg depletion can all reduce
or disrupt the response to cues used to find
hosts. For example, hunger may result in
increased foraging for food and decreased
attention to hosts. In that case, the reaction to
food and host cues will be different from
when the natural enemy is well fed
(Chapters 3, 4 and 5).
The sources of intrinsic variation in forag-
ing behaviour (genetic, phenotypic and those
related to the physiological state) are not
mutually exclusive but overlap extensively,
even within a single individual. The even-
tual foraging effectiveness of a natural
enemy is determined by how well the nat-
ural enemy’s net intrinsic condition is
matched with the foraging environment in
which it operates.

Managing variability in behaviour of natural

enemies

In order to be efficient as biological control
agents, natural enemies must be able to
effectively locate and attack a host and stay
in a host-infested area until most hosts are
attacked. Efficiency as a biological control
agent is used here in the anthropocentric
sense (i.e. our purposes for pest control),
which does not necessarily mean efficiency
from a natural-selection viewpoint. Manage-
ment of natural-enemy variation is particu-
larly important when species are
mass-produced in the laboratory, especially

if rearing is done in factitious hosts (Chapter

9). Such laboratory rearings remove natural

enemies from the context of natural selection

and expose them to artificial selection for

traits that are useless in the field (van

Lenteren, 1986a). In addition to the genetic

component, associative learning may lead to

many more changes in behavioural reactions

(Chapters 3 and 4).

Managing genetic qualities

Successful predation or parasitism of a target

host in a confined situation does not guaran-

tee that released individuals will be suitable

for that host under field conditions. When

selecting among strains of natural enemies,

we need to ensure that the traits of the nat-

ural enemies are appropriately matched with

the targeted use situations in the field.

Natural-enemy populations should perform

well on the target crop and under the specific

climate conditions.

Managing phenotypic qualities

Without care, insectary environments lead to

agents with weak or distorted responses. If

we understand the sources and mechanism

of natural-enemy learning, we can, in theory,

provide the appropriate level of experience

to correct such defects before releasing the

natural enemies. Also, prerelease exposure to

important stimuli can help improve the

responses of natural enemies through asso-

ciative learning, leading to reduction in

escape response and increased arrestment in

target areas.

Managing physical and

physiological qualities

Natural enemies should be released in a

physiological state in which they are most

responsive to herbivore or plant stimuli and

will not be hindered in their responses by

deprivations that interfere with host search-

ing. Thus, adult parasitoids should be well

fed (honey or sugar source available in mass

rearing; Chapter 5), have had opportunities

to mate and have had their preoviposition

period before releases are made.

Need for Quality Control of Biocontrol Agents 13