  We have investigated the mass accretion rate implied by published surface
abundances of Si and C in the white dwarf component of the 3.62 hr period
pre-cataclysmic binary and planet host candidate QS Vir (DA+M2-4). Diffusion
timescales for gravitational settling imply $\dot{M} \sim 10^{-16}M_\odot$
yr$^{-1}$ for the 1999 epoch of the observations, which is three orders of
magnitude lower than measured from a 2006 {\it XMM-Newton} observation. This is
the first time that large accretion rate variations have been seen in a
detached pre-CV. A third body in a 14 yr eccentric orbit suggested in a recent
eclipse timing study is too distant to perturb the central binary sufficiently
to influence accretion. A hypothetical coronal mass ejection just prior to the
{\it XMM-Newton} observation might explain the higher accretion rate, but the
implied size and frequency of such events appear too great. We suggest
accretion is most likely modulated by a magnetic cycle on the secondary acting
as a wind "accretion switch", a mechanism that can be tested by X-ray and
ultraviolet monitoring. If so, QS Vir and similar pre-CVs could provide
powerful insights into hitherto inscrutable cataclysmic variable and M dwarf
magnetospheres, and mass and angular momentum loss rates.
