0851996884.pdf

the tomato hornworm, M. sexta. These are
the species of moths whose eggs are used by
T. pretiosumin this cropping system (Oatman
and Platner, 1971, 1978).
After collecting them in the field, the eggs
were returned to the laboratory and individ-
ually isolated in gelatin capsules for wasp
emergence. Each emerged wasp was then
mounted on a microscope slide and its hind
tibia length was measured as an index of its
size. The frequency distribution of wasp
sizes estimated the size distribution of wasps
emerging and searching for hosts in the
tomato field. To obtain an estimate of the size
of wasps finding hosts in the field, these
workers glued 15–35 irradiated T. nieggs on
a card. When the sampled generation was
due to emerge in the field (6–8 days after the
initial sample), 400 egg cards with T. nihosts

were attached to tomato plants in the field

and the cards were inspected daily for

ovipositing wasps. Trichogramma that

appeared on these cards were collected as

adults and returned to the laboratory and

their hind tibia lengths were measured. The

size distribution of female wasps emerging

from the field-collected eggs was then com-

pared with that of wasps appearing at the

egg cards in each of 2 years. If a female’s size

was unrelated to her ability to locate hosts in

the field, we would expect the size distribu-

tion of females emerging from field-collected

eggs and those encountering hosts in the

field to be the same. They were not. The size

of females appearing at the egg cards was

significantly larger than that of females

emerging from the field-collected eggs, sug-

gesting that the smaller females were less

238 R.F. Luck and L.D. Forster

A

B

C

After selection D

Before selection

25

20

15

10

5

0

28

21

14

7

0

28

21

14

7

0

95 123 151 179 207 235

Hind tibia length (μm) Hind tibia length (μm)

90 120 150 180 210 240

1/16

1/8

1/4

1/2

1

2

Males

Females

Females – 1990

Per cent of observations

Relative post-dispersal fitness

Fig. 17.1.The effect of wasp size on their ability to find hosts and mates in the field. (a)–(c) show the
frequency distribution of Trichogramma pretiosumRiley emerging from field-collected eggs, i.e. ‘before
selection’ for dispersal ability (light grey bars) and sizes of adults dispersing to egg cards placed in the field
after wasp emergence, i.e. ‘after selection’ (dark grey bars). Hind tibia lengths were used as an index of
wasp size: (a) data for females in 1988, (b) for females in 1990, and (c) for males in 1990. (d) Shows the
data expressed as the relative success of wasps in locating hosts or mates as a function of wasp size.
(Redrawn from Kazmer and Luck, 1995.)