Herbivore-induced plant volatiles and tritrophic interactions across spatial scales

Abstract

Herbivore-induced plant volatiles (HIPVs) are an important cue used in herbivore location by carnivorous arthropods such as parasitoids. The effects of plant volatiles on parasitoids have been well characterised at small spatial scales, but little research has been done on their effects at larger spatial scales. The spatial matrix of volatiles ('volatile mosaic') within which parasitoids locate their hosts is dynamic and heterogeneous. It is shaped by the spatial pattern of HIPV-emitting plants, the concentration, chemical composition and breakdown of the emitted HIPV blends, and by environmental factors such as wind, turbulence and vegetation that affect transport and mixing of odour plumes. The volatile mosaic may be exploited differentially by different parasitoid species, in relation to species traits such as sensory ability to perceive volatiles and the physical ability to move towards the source. Understanding how HIPVs influence parasitoids at larger spatial scales is crucial for our understanding of tritrophic interactions and sustainable pest management in agriculture. However, there is a large gap in our knowledge on how volatiles influence the process of host location by parasitoids at the landscape scale. Future studies should bridge the gap between the chemical and behavioural ecology of tritrophic interactions and landscape ecology.

Keywords: herbivore-induced plant volatiles (HIPVs); host location by parasitoids; landscape ecology; spatial scales; tritrophic interactions; volatile mosaic.

Figure 1

Herbivore‐induced plant volatiles (HIPVs) consist of chemicals from different chemical classes. Examples are provided for several different classes of compounds that can be found in HIPV blends.

Figure 2

The sender (plant) emits herbivore‐induced volatiles, which disperse as plumes in the environment as a result of air movement. Physical barriers such as vegetation further modulate the movement pattern of the volatile plume. With increasing distance from the source, the plume becomes more fragmented as a result of degradation, by reactions with other compounds in the atmosphere and dilution as airflow spreads the plume. Depending on the distance from the source, and traits of the receiver (insect parasitoid), the receiver may be able to follow the odour plume to the source.

Figure 3

Herbivore‐induced plant volatiles (HIPVs) on multiple spatial scales. (a) A plant can respond to herbivory with the production of HIPVs. The composition of these volatile blends is affected by many on‐plant factors, such as herbivore identity, herbivore feeding guild, herbivore community and plant species or traits. Parasitoid responses may vary with variation in HIPV blends. (b) In nature, the plant is part of a larger community of plants and their associated herbivores. Therefore, parasitoids search for their hosts in patches where a large variety of odour plumes shapes the information on presence of the host, representing a dynamic volatile mosaic. (c) At the landscape scale, different habitats can present different volatile mosaics and distance between these habitats becomes important. Landscape structure as determined by openness of the vegetation and plant diversity, as well as weather conditions such as wind direction, affect how far odour plumes travel. The ability of the parasitoid to perceive HIPVs emanating from patches further away and to move across these habitats influences movement patterns and resulting population distribution.