Several studies have shown that plants are able to even
detect insect egg deposition (the earliest stage of herbivory) and respond by activating direct and indirect
defences early before larvae hatch and cause damage
by feeding. This ‘early herbivove alert reduces the lag period for arrival of natural enemies.
Parasitism of eggs by egg parasitoids reduces larval emergence
and performance. Furthermore, emission of volatile
cues attractive to larval parasitoids following egg
deposition means that they are recruited in anticipation
of larval emergence and parasitism can start before the
larvae can cause much damage to the plant. Parasitoids
are under selection pressure to respond to such cues and
even distinguish between mechanical damage and those
induced by the presence of their hosts as this enhances
their foraging efficiency eventually improving their ecological
fitness. Attraction of parasitoid wasps can improve
biological control in open field environments, as already
shown with intercrops [11]. Such indirect defence has
perhaps been neglected as a potential trait for breeding
increased crop resistance to insect pests. Breeding may
have favoured ‘bottom-up’ rather than ‘top-down’ insect
resistance [9]. In this paper we discuss genetic variation
in crop responses to insect attack and strategies for crop
genetic improvement to enhance such traits. To illustrate
the concept we use our recent studies in maize with the
stemborer Chilo partellus and its parasitoid Cotesia sesamiae
because we have now found that insect egg induced
volatile emission does exist in commercial hybrids meaning
that there is a prospect of introgressing this trait into
improved varieties.