Biological Control of Insect Pests: Southeast Asian Prospects - EcoPort

4.16 Trichoplusia ni 347
when 2nd or 3rd instars are parasitised. Larval development is then
completed by the end of the 5th host larval instar (Browning and Oatman
1984). As a result of overlapping life cycles, two or more of these 4 species
could inhabit a host larvae simultaneously, unless a species was able to
discriminate between parasitised and unparasitised hosts.
As many as 10 adult Voria ruralis may emerge from a single host larva.
When limited numbers of hosts are present, a female lays more than one egg
on each larva. Excess eggs may result in premature mortality of the host
larva and any immature parasitoids already within it. External oviposition
probably prevents the ovipositing female from receiving sensory
information about the presence of parasite eggs or larvae already within the
host; and V. ruralis females will continue to oviposit as long as the host larva
reacts with any movement.
Copidosoma truncatellum parasitises host eggs of all ages following
antennal drumming and ovipositor insertion. Females are apparently able to
discriminate between unparasitised eggs and those parasitised by other
C. truncatellum females. Microgaster brassicae females insert their
ovipositors in all host larvae whether or not parasitised by Copidosoma
truncatellum, although they are weakly deterred from doing so in larvae
already parasitised by other M. brassicae females or by Hyposoter exiguae.
Parasitoid eggs were deposited in 91% of hosts previously unparasitised,
whereas those already parasitised by M. brassicae or H. exiguae showed
oviposition levels of 10 and 53% respectively, indicating a response to
sensory information after insertion of the ovipositor. When M. brassicae
oviposited in larvae already parasitised by C. truncatellum, the latter
emerged from 77.5% of the larvae, whereas M. brassicae emerged from only
12.5%. However, when M. brassicae oviposited in larvae containing the
slower-developing Hyposoter exiguae, the latter emerged from 16.7% of
larvae and M. brassicae from 76.7%. Hyposoter exiguae showed little
discrimination between unparasitised larvae and those parasitised by any of
the other 3 species, although ovipositor insertion did not result in additional
eggs in larvae already containing H. exiguae. High levels of parasitisation by
H. exiguae occurred in host larvae already parasitised by C. truncatellum,
possibly due to the delayed development of the latter. Nevertheless, only
C. truncatellum emerged from such larvae. Further details of these and other
interactions under laboratory conditions are given in the valuable paper by
Browning and Oatman (1984). It is interesting that the parasitoid complex
attacking T. ni on cotton in the field shows little change from season to
season in species composition and relative importance. However, this does
not enable a simple decision to be made on what impact there would be on
abundance of T. ni (or on plant damage sustained) if one or more of the