The influence of an external electric field or temperature on holographic scattering from holographic polymer-dispersed liquid crystals (H-PDLCs) was investigated experimentally. The auto-generated parasitic gratings are affected by an externally applied electric field in a similar manner as the elementary grating. We find that the threshold voltage, switching-on and -off times are independent on the grating vector. On the contrary, the suppression level of the refractive-index modulation is proportional to the spatial frequency. Heating of the sample reduces the strength of parasitic gratings analogously to the case of an external electric field.