Impact of exotic herbivores on native tritrophic interactions: A case study of the African cotton leafworm, Spodoptera littoralis

When exotic herbivores invade new environments, they have the potential to interfere with native predator-prey relationships. This interference can be indirect, via changes induced in native host plants, and may have cascading consequences at the community level.  Here we investigate the impact of the presence an exotic generalist insect herbivore, the African cotton leafworm Spodoptera littoralis, on the outcome of native tritrophic interactions between the plant Brassica rapa, the herbivore Pieris brassicae, and its parasitoid Cotesia glomerata.  

In olfactometer choice-tests, plants damaged by S. littoralis and plants damaged by both S. littoralis and P. brassicae were consistently less attractive to the parasitoid than plants damaged by P. brassicae alone. Chemical analyses revealed that the volatiles typically induced by S. littoralis dominated the headspace in case of dual herbivore infestation.

In behavioral observations in petri dishes, C. glomerata wasps mistakenly attacked S. littoralis larvae significantly more often when P. brassicae was present, and attacks on both herbivores were comparable in terms of time (attack duration) and energy investment (number of eggs injected).  Successful parasitism of S. littoralis was never observed, but larvae exposed to C. glomerata for 24 hours exhibited reduced survivorship, possibly as a result of failed parasitism attempts.

In tents with herbivore-infested B. rapa plants, the presence of S. littoralis did not have an effect on the parasitism rates of P. brassicae by C. glomerata, regardless of whether the two species were on the same or on different plants. Field experiments in newly invaded environments are necessary to evaluate the realized impact of S. littoralis