Laboratory plasma experiments show that, above a certain threshold field strength, a whistler-mode wave (WMW) may decay parametrically into a backward propagating WMW and a low frequency ion-acoustic wave. The required threshold field for a 2 kHz wave in the equatorial plane of the magnetosphere, just inside the plasmapause, is found to be well below 1 mVm-1. It is suggested that the resulting backward travelling WMW may be responsible for triggering precursors-rising frequency emissions which are occasionally observed to precede 2 (or more) hop whistlers.