we would not expect to see the right-handed
skew in wasp size that occurs in these results
(Fig. 17.2). The number of eggs potentially
available in a female’s ovaries (Bai et al.,
1992; Smith, 1996; Honda and Luck, 2001),
along with her walking speed (Honda and
Luck, 2001), is known to increase with wasp
size. These results suggest that the average
per capita efficiency of Trichogrammain an
augmentative biological control programme
could be enhanced two- to threefold if larger
wasps were used. However, the cost of pro-
ducing them must be less than two to three
times that of producing wasps from S.
cerealellaor E. kuhniellaeggs (e.g. Bai et al.,
1992). Marston and Ertle (1973) estimated
that the cost per wasp of producing larger
Trichogrammain the desired size range is
about 40 times that of producing them from
S. cerealellaeggs. While these costs can be
reduced substantially with recent improve-
ments in mass production and implementa-
tion, the production costs for the larger
wasps still seems to preclude their economic
production.
Can small size be compensated by release of
larger numbers of parasitoids?Perhaps, in some cases, releasing higher
rates of smaller wasps can compensate for
the lower encounter rates manifested by the
smaller wasps (Stinner et al., 1974). However,
the use of smaller Trichogrammacan have
consequences for the success of a biological
control programme other than encounter
rates. As has been widely documented, life-
time fecundity and the potential egg load,
i.e. the number of mature eggs present in the
ovaries, also increase with wasp size (e.g.
Waage and Ng, 1984; Bai et al., 1992;
Bourchier et al., 1993). This can be important
in the biological control of pests that lay their
eggs in masses or clusters (Honda and Luck,
2001). For example, in California, T. platneri
is a frequent constituent of the natural-
enemy complex associated with two spo-
radic avocado pests, the avocado leaf-roller,
Amorbia cuneanaWalshingham (Lepidoptera:
Tortricidae), and the omnivorous looper,
Sabulodes aegrotata (Gueneè) (Lepidoptera:
Geometridae) (Oatman et al., 1983). In evalu-
ating T. platneri’s potential as an augmenta-
tive biological control agent for these pests,
Honda and Luck (2001) found that, for T. pre-
tiosumreared on S. cerealellaor E. kuehniella,
several females must encounter an egg mass
or egg cluster if a substantial number of the
eggs in these masses or clusters are to be par-
asitized. An A. cuneanafemale lays about 30
eggs per egg mass, whereas an S. aegrotata
female lays about 14 eggs per egg cluster. T.
platnerireared on S. cerealella(i.e. a small
wasp) lays an average of 1.5 eggs per A.
cuneanaegg and parasitizes 16 of the 30 eggs
in an average A. cuneanaegg mass, whereas
wasps reared on T. ni eggs (a large wasp)
parasitize 26 of the 30 eggs in an average A.
cuneana egg mass. Similarly, a T. platneri
female reared on S. cerealella(a small wasp)
lays five eggs per S. aegrotataegg and para-
sitizes one or two of the 14 eggs in an aver-
age S. aegrotataegg cluster, whereas wasps
reared on T. nieggs (a large wasp) parasitize
five of the 14 eggs in average egg cluster.
Thus, large wasps are more effective in para-
sitizing the eggs of both moth species. But
note that, even with large wasps, only one-
third of the eggs in an average S. aegrotata
egg cluster are parasitized.
This disparity is even more pronounced
when we compare a small versus large
wasp’s investment (time) in parasitizing an
S. aegrotataversus an A. cuneanaegg: S. aegro-
tataeggs are more difficult for even large T.
platnerito parasitize. It takes a large wasp
about 54 min to drill, drum and lay five eggs,
the normal clutch size allocated to an S.
aegrotata egg; but only 28% of the first
attempts to parasitize an egg are successful.
Another 50% of the parasitoids are success-
ful on their second attempt, adding another
21 min to the time each wasp invests in para-
sitizing a host egg. The remaining 21% are
successful on their third attempt, but that
adds another 25 min to a wasp’s investment.
If we take the weighted average of the time a
large wasp invests in allocating each of its
five offspring to an S. aegrotataegg, it takes
about 15 min per wasp egg. In contrast, a
large T. platneriparasitizing an A. cuneana
egg mass is always successful on its first
attempt and it invests 5 min for each of the240 R.F. Luck and L.D. Forster