Quantitative experiments on the parametric decay instability of near-acoustic plasma waves provide strong evidence that trapped particles reduce the instability threshold below fluid models. At low temperatures, the broad characteristics of the parametric instability are determined by the frequency detuning of the pump and daughter wave, and the wave-wave coupling strength, surprisingly consistent with cold fluid, three-wave theories. However, at higher temperatures, trapped particle effects dominate, and the pump wave becomes unstable at half the threshold pump wave amplitude with similar exponential growth rates as for a cold plasma.