Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Herbivore- or elicitor-induced volatile release varies with time and treatment, applied amount of elicitor (Halitschke et al. 2000), and the herbivore damage level (Gouinguene et al.
Category: