  Spitzer Space Observatory IRAC and MIPS photometric observations are
presented for 20 white dwarfs with T < 20,000 K and metal-contaminated
photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG
1457-086, while the remaining targets fail to reveal mid-infrared excess
typical of dust disks, including a number of heavily polluted stars. Extending
previous studies, over 50% of all single white dwarfs with implied metal
accretion rates dM/dt > 3e8 g/s display a warm infrared excess from orbiting
dust; the likely result of a tidally-destroyed minor planet. This benchmark
accretion rate lies between the dust production rates of 1e6 g/s in the solar
system zodiacal cloud and 1e10 g/s often inferred for debris disks at main
sequence A-type stars. It is estimated that between 1% and 3% of all single
white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar
dust, signifying an underlying population of minor planets.
