  We propose that neutral, circumpulsar debris entering the light cylinder can
account for many time-dependent pulsar phenomena that are otherwise difficult
to explain. Neutral material avoids propeller ejection and injects sufficient
charges -- after heating, evaporation, and ionization -- to alter current flows
and pair-production and thus trigger, detune, or extinguish coherent emission.
Relevant phenomena, with time scales from seconds to months, include nulls,
rotating radio transients (RRATs), rapid changes in pulse profile (``mode
changes''), variable subpulse drift rates, quasi-periodic bursts from B1931+24,
and torque variations. Over the 10 Myr lifetime of a canonical pulsar with
trillion-gauss surface magnetic field, less than a millionth of an Earth mass
of material is needed to modulate the Goldreich-Julian current by 100%.
Circumpulsar material originates from metal-rich, supernova fallback gas that
aggregates into asteroids. Debris disks can inject sufficient material on time
scales of interest, yet be too tenuous to form large planets detectable in
pulse timing data. Asteroid migration results from collisions and the
radiation-driven Yarkovsky and Poynting-Robertson effects. For B1931+24, an
asteroid in a $\sim 40$~day elliptical orbit pollutes the magnetosphere
stochastically through collisions with other debris. Injection is less likely
for hot, young and highly magnetized pulsars or millisecond pulsars that
pre-ionize any debris material well outside their small magnetospheres.
Injection effects will therefore be most prominent in long-period, cooler
pulsars, consistent with the distribution of relevant objects in perid and
period derivative. A pulsar's spin history and its radiation-beam orientation
may influence whether it displays nulling, RRATs and other effects.
