In studying the non-linear evolution of non-resonant trapped-particle instabilities, it is desirable to exploit the fact that their frequencies are smaller than the bounce frequency by employing a kinetic equation for the bounce-averaged distribution function. Such an equation for trapped particles is derived in axisymmetric toroidal geometry by transforming the familiar drift-kinetic equation to toroidal co-ordinates and employing an annihilation operator. The "bounce-averaged drift-kinetic equation" facilitates the application of Dupree's strong turbulence formalism in toroidal geometry, under the assumption that turbulent scattering of particles out of the magnetic well is unimportant. On the basis of this formalism, the trapped-particle interchange mode is analysed. The saturation amplitude of the instability and the corresponding turbulent transport coefficients are obtained.