Oviposition preference and performance of Plutella xylostella are modulated by natural enemies, larval odours and immune status

Insect herbivores frequently must balance host plant quality and the risk
of attack by their natural enemies when making oviposition decisions. Yet,
which factor is more important remains unresolved in plant-insect
ecology. Here, we report the oviposition preference and larval
performance of the brassicaceous specialist Plutella xylostella,
in the context of plant quality (cabbage Brassica oleracea, vs.
mustard B. juncea) and associated natural
enemies. Despite the greater larval weight and adult life-span on
cabbage, ovipositing females strongly preferred mustard. Both the egg
parasitoid Trichogrammatoidea bactrae and the larval
ectoparasitoid Bracon brevicornis are more likely to
attack P. xylostella that feed on cabbage; thus, mustard
represents enemy-reduced space from these two
parasitoids. However, larval diet had no impact on the
parasitization rate of specialist Cotesia vestalis. Feeding on
mustard improved larval immunity. The total hemocytes number, diversity
and phenoloxidase activity was higher on mustard-fed larvae that increased
their survival against the entomopathogen, Bacillus
thuringiensis. Interestingly, host plants altered the larval body
odour profile. Mustard fed larvae emitted allyl isothiocyanate (AITC) and
butyl isothiocyanate (BITC) while cabbage fed larvae emitted dimethyl
disulphide (DMDS) and dimethyl trisulphide (DMTS) that served as
short-range cues for larval parasitoids. For B. brevicornis, host
body odour guided oviposition choice was crucial as their fitness was
affected by the host larval diet. Although, C.
vestalis showed a clear preference towards volatiles emitted by
mustard fed larvae, their fitness was unaltered. Taken together,
our results illustrate that P. xylostella prefers to lay
eggs on mustard plants providing enemy-reduced space from some, but not
all, natural enemies.