High levels of abiotic noise in volatile organic compounds released by a desert perennial: implications for the evolution and ecology of airborne chemical communication

Plants release airborne volatile organic compounds (VOCs) in response to abiotic and biotic stimuli, including herbivory. These chemicals are used by insect parasitoids as sources of information that aid in finding hosts. It is unclear how biotic and abiotic factors interact to affect blend composition and the ability of insects to interpret signals. Here, we present a novel stimulus-space model, and use it to examine patterns of VOC emission. In field experiments, we manipulated herbivory levels and collected VOCs in a population of wild Datura wrightii, while simultaneously measuring key abiotic factors. We mirrored field experiments under controlled conditions in the lab, and used both sets of data to test predictions made by our proposed model. VOC blends were structured mainly by variation in abiotic factors, not herbivory. However, linear discriminant analysis showed that it is possible to distinguish different herbivory levels. We show that most compounds produced by D. wrightii are invariant, or respond solely to environmental variation or herbivory. Our results suggest that blend composition may be under selection for noise reduction, to maximize responses from potential receivers, and that abiotic variation can act as potentially strong sources of noise in chemical communication displays.