introduced host population is almost exter-
minated, the natural enemies invade the sur-
rounding crop (van Steenis, 1995).
The moment of introductionIn many cases, the natural enemies are
released when the pest organism has been
observed, although it is not unusual to apply
‘blind releases’ when sampling of the pest is
difficult (e.g. whiteflies) or when pest popu-
lations develop very quickly, such as those of
aphids and thrips. When pest generations
are not yet overlapping early in the growing
season, proper timing of the release(s) is
essential so that the beneficials are available
when the preferred host stages are present.
Determining the dosage, the distribution
and the frequency of the releases is a very
difficult problem, which is encountered in
both inundative- and seasonal inoculative-
release programmes. Release ratios are not
critical in inundative-release programmes as
long as it is possible to release a (super)abun-
dance of natural enemies. This, however,
may be limited by the cost of mass produc-
tion. In seasonal inoculative programmes
release ratios are more critical: if too few
beneficials are released, effective control will
be obtained after the pest has caused eco-
nomic damage; if too many are released,
there is a risk of exterminating the pest and
thus eventually also the natural enemy. This
is a practical problem in small tunnels and
greenhouses. In the latter situation resur-
gence of the pest is likely and a serious
threat. In these seasonal innoculative-release
programmes, the release ratios are usually
determined by trial and error, but the first
simulation programmes are appearing for a
more scientific estimate of release rates
(number of releases, spacing between release
points and timing of releases; see, for exam-
ple van Roermund (1995) for seasonal inocu-
lative releases and Sueverkropp (1997) for
inundative releases).Conclusions
Mass production of natural enemies has
undergone a very fast development during
the past three decades: the numbers produced
have greatly increased, the spectrum of
species available has widened dramatically
and mass-production methods have clearly
evolved. Developments in the area of mass
production, quality control, storage, shipment
and release of natural enemies have decreased
production costs and led to better product
quality, but much more can be done.
Innovations in long-term storage (e.g. through
diapause), shipment and release methods may
lead to a further increase in natural-enemy
quality, with a concurrent reduction in costs of
biological control, thereby making it easier
and more economical to apply.188 J.C. van Lenteren and M.G. Tommasini
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