The first step in a mass-rearing pro-
gramme is a trial to rear the natural enemy
on a natural host (the pest organism) in an
economical way. Most natural enemies are
reared in this way. However, several natural
enemies are not mass-reared on their natural
host because it is either too expensive or
undesirable due to the risk of infection with
the pest organism or concurrent infection
with other pests or diseases when natural
enemies are released on their natural sub-
strate. In these cases a search is made for an
opportunity to rear the natural enemy on an
alternative host (and often an alternative
host plant).
A subsequent step in making mass rear-
ing more economical is to change from a nat-
ural host medium (host plant) to an artificial
medium for rearing the host. Rearing insects
on artificial diets was developed earlier this
century and considerable progress has been
made recently. Rearing on artificial diets is
considerably cheaper as less expensively cli-
matized space is needed, but artificial rear-
ing may create serious quality problems,
which will be discussed later in this chapter.
Singh (1984) summarizes the historical
development, recent advances and future
prospects for insect diets as follows:
1.Some 750 species, mainly phytophagous
insects, can be reared successfully on (semi-)
artificial diets.
2.Only about two dozen species have been
successfully reared for several generations
on completely artificial diets.
3.Large-scale mass rearing on artificial
media has been developed for fewer than 20
species of insects.
4.Quality control is essential, as there can be
dietary effects on all critical performance
traits of the mass-reared insect and also on
the natural enemy produced on a host that
was mass-reared on an artificial medium.
5.Suitable bioassays are important for
answering the question ‘what is the ultimate
effect of the diet on the reared insect?’
A final step when trying to minimize rear-
ing costs is the search for ways to rear the
natural enemy on an artificial diet. This has
been attained for several ecto- and endopara-
sitoids (e.g. Trichogramma) and a few preda-
tors (e.g. Chrysoperla). The technology for
rearing natural enemies on diets is, however,
far less developed than that for rearing pest
species (Chapter 9; Grenier et al., 1994).
The rapid development of commercial
biological control based on mass-produced
natural enemies can be illustrated well with
data from Europe. About 150 species of nat-
ural enemies have been imported and
released into Europe during the 20th century
to control about 55 mite and insect pest
species. Until 1970 this mainly concerned
inoculative (classical) biological control. After
1970 many developments took place in
greenhouses and annual field crops, and
commercial biological control programmes
for c. 50 pest species were developed by
importing more than 60 species of natural
enemies. In addition, more than 40 endemic
species of natural enemies were employed in
commercial biological control. For all these
species, fine-tuned mass-production systems
had to be developed. Our experience with the
development of new biological control pro-
grammes has shown that dogmatism is use-
less when selecting natural enemies. This
contrasts with the approach of earlier biocon-
trol workers (see, for example, DeBach, 1964).
We have, for example, had excellent control
results by releasing endemic natural enemies
against exotic pests and vice versa: all combi-
nations are worth trying (Table 12.1).
The most important species of natural ene-
mies that are mass-reared in Europe are given
in Table 11.2 of Chapter 11, and an overview
of all available natural enemies is presented in
Table 11.1 of the same chapter. Although on-
farm production of natural enemies is possi-
ble, most growers purchase them from
commercial suppliers. Many of the mass-pro-
duction companies are, understandably, reluc-
tant to provide information on many aspects
of mass production. Our experience is that
most of the natural enemies produced for bio-
control in protected cultivation are reared on
their natural hosts (the pests) and host plants.
Rearing on purely artificial media (without
organic additives) is very rare, primarily
because this technology is insufficiently
developed for mass production and because
this way of production may lead to poor per-
formance of natural enemies when exposed to182 J.C. van Lenteren and M.G. Tommasini