1.5 eggs it allocates to an A. cuneanaegg. The
difference in the time that a small T. platneri
invests for each egg it lays in these two host
species is even greater. A small wasp can
expect to invest 20 min for each of the five
eggs it lays in an S. aegrotataegg, whereas it
still invests the same 5 min for each of the 1.5
eggs it lays in an A. cuneanaegg.
Effect of host quality on acceptance and
oviposition by parasitoidThis disparity in the value of the two hosts is
also reflected in the percentage of a wasp’s
egg load that is expended on each host
species (Honda and Luck, 2001). A large T.
platneriretains 39% of its egg load when it
finishes parasitizing an S. aegrotataegg clus-
ter, whereas it retains 24% of its egg load
when it finishes parasitizing an A. cuneana
egg mass. This difference in egg-load reten-
tion is even more pronounced in the smaller
T. platneri. The small wasp retains 77% of its
egg load if it is exposed to an S. aegrotataegg
cluster, whereas it retains 24% of its egg load
if it is exposed to an A. cuneanaegg mass.
This occurs even though the size of a wasp
offspring arising from an average clutch allo-
cated to an egg of either host species is the
same and the female offspring show the same
relationship between their size and their egg
loads. Thus, from the parental wasp’s per-
spective, there appears to be little difference
in reproductive value among the wasp off-
spring produced from these two hosts other
than the time that she must invest in obtain-
ing them, i.e. in parasitizing each host
species. Honda and Luck (2001) interpreted
these results as suggesting that parasitizing
S. aegrotataeggs was less valuable per unit
time than parasitizing A. cuneanaeggs. Thus,
after allocating a few clutches to an S. aegro-
tataegg cluster, a female T. platnerileaves the
host egg cluster because it is more likely to
obtain better returns for her searching time if
she seeks hosts other than S. aegrotata(see
Stephens and Krebs, 1986). Honda and Luck
(2001) also concluded that it is unlikely that
augmentative releases of T. platneriwould
suppress S. aegrotatato subeconomic densi-
ties on its own.
Honda and Luck (2001) viewed the pat-
tern of egg retention as an index reflecting
the reticence that a Trichogramma species
manifested in exploiting a host species. It is
related to the reproductive value that a
female obtains in investing its time in
exploiting a particular host. This reticence
was revealed using a modification of the
direct observational techniques first outlined
by van Dijken et al.(1986) to evaluate three
strains of Trichogrammaspecies for use as
augmentative agents against several pests of
cabbage in The Netherlands. This evaluation
technique was also used by van Bergeijk et
al.(1989) to determine why Trichogramma
brassicaeBezdenko (= maidis Pentureau et
Voegele) reared on A.(= E.) kuehniellafor
more than a few generations became less
effective in suppressing the Swiss popula-
tions of European cornborer in 1980. Bigler
(1986) had developed a mass-release pro-
gramme against this borer, but in one year,
1980, the programme was ineffective (see
Chapter 19 for more details). By changing
the rearing method for T. brassicae, the qual-
ity of the parasitoid was dramatically
improved. Subsequently, a parasitoid line
was established that was continuously
reared on the European cornborer under
semi-natural conditions in a greenhouse.
This line supplied the wasps that were mass-
produced on A. kuehniellaeggs for the five
generations required to obtain sufficient
numbers for release (van Bergeijk et al.,
1989). Van Bergeijk et al.(1989) used behav-
ioural observations to determine why the
control failure occurred. They found that the
number of encounters, drills and parasitiza-
tions of Ostrinia nubilalis Hübner
(Lepidoptera: Pyralidae) eggs or egg masses
declined with the number of generations that
T. brassicaehad been reared on A. kuehniella
eggs. T. brassicaereared on O. nubilalisreadily
accepted A. kuehniella eggs, whereas only
30% of those reared on A. kuehniellaeggs for
12 or more generations accepted O. nubilalis
eggs. Moreover, of those that accepted, i.e.
oviposited in, this host, only 35% of them
yielded wasp offspring. Van Bergeijk et al.
(1989) interpreted their results as arising
from two processes. First, because food for
the wasp offspring was limited in A.Behavioural Approaches for Quality Control 241