Utilizing associational resistance for biocontrol: impacted by temperature, supported by indirect defence

Associational herbivore resistance is potentiated by neighbouring heterogenic plant species that impact a focal plant's attraction to herbivores or the damage that they cause. One mechanism to confer associational resistance is believed to be exposure to neighbour-emitted volatiles, the receivers of which range from intra- and interspecific neighbour plants to higher-trophic-level insects. In previous studies the passive adsorption of neighbour-emitted semivolatiles has been reported, but little is known regarding the mechanisms and ecological consequences on the receiver plant and its associated biota. To utilize volatile-based associational resistance for agricultural applications, it is imperative to know its effectiveness under varying diurnal temperatures and whether herbivore natural enemies, providing biological control, are impacted. Mimicking varying diurnal temperatures in a laboratory set-up, we assessed how the tritrophic model system Brassica oleracea var. italica (broccoli)-Plutella xylostella (crucifer specialist herbivore)-Cotesia vestalis (endoparasitoid of P. xylostella) is influenced by exposure to the natural semivolatile emitter plant Rhododendron tomentosum Harmaja.

Rhododendron tomentosum-exposed B. oleracea was less susceptible to P. xylostella oviposition at both night-time (12°C) and day-time (22°C) temperatures and less favoured and damaged by P. xylostella larvae at 12°C. Exposure did not interfere with indirect defence, i.e. attraction of the natural enemy C. vestalis on host-damaged, R. tomentosum-exposed B. oleracea under 22°C, while there was a reduction in attraction (marginal preference towards host-damaged B. oleracea) under 12°C.

The ability of R. tomentosum exposure to render associational resistance against an agriculturally important Brassica herbivore P. xylostella without severely compromising the specialist parasitoid C. vestalis host location encourages further studies on the potential of using this naturally abundant plant for biocontrol. The generality of our finding on temperature as a potential regulating mechanism for the efficacy of semivolatile emitter-based associational resistance towards specialist pest larval damage should be further studied in natural and agricultural associations. Our study emphasizes the need to develop techniques to compare volatiles at the leaf versus air interface and associate their appearance and ecological role with times of activity and level of specialisation of herbivores and their natural enemies.