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:Previous studies have examined direct and indirect effects of Bt toxins in model tri-trophic systems with cruciferous plants, the herbivore Plutella xylostella (diamondback moth), and the parasitoid Cotesia plutellae (Chilcutt and Tabashnik 1997a, b; 1999a, b; Schuler et al. 1999, 2003).
Category: