Parametric instabilities during the ion Bernstein wave (IBW) plasma heating experiment on PBX-M are investigated both theoretically and experimentally. The theoretical work shows that the RF power threshold of the instabilities is very low when the plasma density near the antenna satisfies the condition 2ωpi < ω0 << ωpe. The threshold becomes very high, >>1 MW, and is determined by convective losses due to plasma inhomogeneity when the plasma density near the antenna is sufficiently high that 0.5 < ωpi/ω0 < 1. According to the theory, the parametric instability activity should increase as the plasma is moved away from the antenna, creating a low density electron plasma wave 'gap' region. To test this hypothesis on PBX-M, the plasma position was deliberately varied while monitoring this activity. Under normal IBW operating conditions, very little parametric instability activity was observed ⩽50 dB below the pump wave (ω = ωRF). However, when the plasma edge was moved away from the antenna by about 2 cm, the parametric instability activity increased greatly, exceeding 20 dB of the pump wave. This result shows that the observed parametric instability activity can be explained in terms of the plasma inhomogeneity convective model. It was also demonstrated that, by controlling the plasma position with respect to the antenna, parametric instability activity can be controlled at a low level