We present a systematic study of mid-infrared ( mid-IR ) emission from 141 nearby supernovae ( SNe ) observed with the InfraRed Array Camera ( IRAC ) on the Spitzer Space Telescope .
These SNe reside in one of the 190 galaxies within 20 Mpc drawn from the ongoing three-year SPIRITS program .
Both new SPIRITS observations and data from other programs available in the archive are used in this study .
We detect 8 Type Ia SNe and 36 core-collapse SNe .
All Type I SNe fade and become undetectable within 3 years of explosion .
About 22 \pm 11 % of Type II SNe continue to be detected at late-times with five events detected even two decades after discovery .
Dust luminosity , temperature , and mass are obtained by fitting the spectral energy distributions using photometry with IRAC bands 1 and 2 .
The dust mass estimate is a lower limit as the dust cloud could be optically thick or there could be cooler dust hiding at longer wavelengths .
The estimate also does not distinguish between pre-existing and newly produced dust .
We observe warm dust masses between 10 ^ { -2 } and 10 ^ { -6 } M _ { \odot } and dust temperatures from 200 K to 1280 K. We present detailed case studies of two extreme Type II-P SNe : SN 2011ja and SN 2014bi .
SN 2011ja was over-luminous ( [ 4.5 ] = - 15.6 mag ) at 900 days post-explosion accompanied by a monotonic growth of the dust mass .
This suggests either an episode of dust formation similar to SN 2004et and SN 2004dj , or an intensifying CSM interactions heating up pre-existing dust .
SN 2014bi showed a factor of 10 decrease in dust mass over one month suggesting either an episode of dust destruction or a fading source of dust heating .
A re-brightening in the mid-IR light curve of the Type Ib SN 2014C coinciding with a rise in the dust mass indicates either an episode of dust production perhaps via CSM interactions or more pre-existing dust getting heated up by the CSM interactions .
This observation adds to a small number of stripped-envelope SNe that have mid-IR excess as has been previously reported in the case of SN 2006jc .
The observed dust mass and the location of the CSM interactions suggest that the CSM shell around SN 2014C is originated from an LBV-like eruption roughly 100 years before the explosion .
We also report detections of SN 1974E , SN 1979C , SN 1980K , SN 1986J , and SN 1993J more than 20 years post-explosion .
The number of outlying SNe identified in this work demonstrates the power of late time mid-IR observations of a large sample of SNe to identify events with unusual evolution .