It is shown that a large-amplitude field-aligned whistler wave can excite nonducted whistler sidebands and nonresonant low-frequency electrostatic collisional gradient drift modes. The effects of the ponderomotive force as well as the heating nonlinearities are included in the study of the modulational instabilities. For this case it is found that the heating nonlinearity dominates over the ponderomotive force effects, provided that the doppler-shifted frequency of the collisional modes is smaller than the electron collision frequency. Our investigation can be relevant in understanding why enhanced density irregularities appear in the presence of electromagnetic heater waves in the lower part of the ionosphere.