0851996884.pdf

able to find hosts in the field than their larger
counterparts (Fig. 17.1A, B). Thus, it
appeared that the larger wasps had an
advantage in finding more hosts, which
probably translates to the production of
more offspring.
Assuming that the behaviour involved in
allocating offspring number to a host is heri-
table, the results of this field experiment
imply that the larger females left more prog-
eny in future generations – that is, on aver-
age they parasitized more hosts in the field.
Kazmer and Luck (1995) tested this idea by
using these size distributions to compare the
reproductive potential, i.e. fitness, of small
versus large females (i.e. their ability to
encounter hosts). A female’s relative fitness
increased with female size until a threshold
value for hind tibia length of 170 m, after
which it levelled off (Fig. 17.1D). This
implies that females with a hind tibia length
larger than 170 m do not have a reproduc-

tive advantage over those that have a hind

tibia 170 m long. A similar comparison was

made for males emerging from field hosts

with those appearing at small cages contain-

ing virgin female T. pretiosumin the field.

Again, large males were more successful at

encountering virgin females in the field up to

a point (Fig. 17.1C). Several other studies

with different Trichogrammaspecies (Bennett

and Hoffmann, 1998) or with different para-

sitoid species have shown similar results

(Visser, 1994; West et al., 1996).

In general, the hind tibia lengths of

female T. pretiosumemerging from hosts par-

asitized in the field, i.e. those hosts listed by

Pinto (1998), suggest that the size range of

females emerging from S. cerealellaand E.

kuehniella, i.e. 120–140 m, lie in the bottom

quartile of the size range for T. pretiosum

females emerging from their field hosts (Bai

et al., 1992; Fig. 17.2). Furthermore, if the size

of the emerging offspring were unimportant,

Behavioural Approaches for Quality Control 239

T. pretiosum

n = 153

25

20

15

10

5

0

0.110.120.130.140.150.160.170.180.190.200.210.220.230.240.25

Hind tibia length (mm)

Frequency per cent

Fig. 17.2.The size distribution of Trichogramma pretiosumRiley emerging from the eggs of various
lepidopteran species, including Argraulis vanillae(L.) (Nymphalidae); Vanessasp. (Nymphalidae);
Helicoverpa(=Heliothis)zea(Boddie) (Noctuidae); Spodoptera exigua(Hübner) (Noctuidae); Trichoplusia ni
(Hübner) (Noctuidae); Manduca sexta(L.) (Sphingidae); Hesperidae; unknown Lepidoptera. (Redrawn from
Baiet al., 1992.)