maintenance, it was possible to improve the
performance of the strain and achieve the
efficiency limit of at least 75% parasitisation in
the field.
A thorough analysis of the production
system and the performance requirements of T.
brassicae under the maize growing conditions in
Switzerland led to the discovery of important
traits which are crucial for a high efficacy. Since
attributes like locomotory activity, host
acceptance, host suitability and temperature
tolerance were negatively affected by the former
rearing system, we developed a new production
unit. At the same time, risk evaluations of other
deteriorations in the strain were performed and
methods for measuring single traits and the
field performance were developed.
In recent years, the production system of T.
brassicaewas changed from a short period
production with a high daily output to a long
period production with a low daily output.
Improvements of the long-term storage of the
parasitoids (diapause) have prolonged the
mass-production period from 2 to 9 months per
year...
The total production system consists of
three subunits, namely the European corn
borer rearing unit as natural host of the wasps,
the Ephestia kuehniellaunit as factitious (mass-rearing) host and the T. brassicae unit. Each of
these subunits has its own quality control
system... Quality control procedures in the
host units concentrate on: the rearing diet
quality (origin, storage, grind, mixture), egg
hatch, larval and pupal weight per unit rearing
diet, egg production, egg sterilisation
efficiency, temperature and humidity
regulation, ventilation control to prevent health
hazards by insect scales and sanitation
procedures to avoid insect diseases.
Eggs of the European corn borer are used to
produce the sting stock of T. brassicae. This
population is permanently reared under semi-
field conditions, i.e. in a field insectary in
summer and in a greenhouse with fluctuating
temperatures in winter; corn borer egg masses
are attached to corn plants and the adults of
the parental generation emerge 2 to 4 m away
from the plants so that the egg masses must be
reached by flight. A portion of the sting stock is
regularly used to develop the strain on eggs of
E. kuehniella. From our experience we know
that the performance decreases with an
increasing number of generations reared on the
factitious host. Therefore, we recommend
production of no more than six to seven
generations before releasing the parasitoids for
biological control purposes...266 J.C. van Lenteren et al.
Strain improvementEfficiency
limit1001975 1980 1985 1990
Experiments Commercial application
YearPercentage parasitism806040200Fig. 19.1.Percentage parasitism of Ostrinia nubilaliseggs by Trichogramma brassicae in maize from 1975
to 1990 in Switzerland (after Bigler, 1994).