  We explore the use of galaxy bispectrum induced by the nonlinear
gravitational evolution as a possible probe to test general scalar-tensor
theories with second-order equations of motion. We find that time dependence of
the leading second-order kernel is approximately characterized by one
parameter, the second-order index, which is expected to trace the higher-order
growth history of the Universe. We show that our new parameter can
significantly carry new information about the non-linear growth of structure.
We forecast future constraints on the second-order index as well as the
equation-of-state parameter and the growth index.
