  We present a Spitzer IRAC and MIPS survey of NGC 2451 A and B, two open
clusters in the 50-80 Myr age range. We complement these data with extensive
ground-based photometry and spectroscopy to identify the cluster members in the
Spitzer survey field. We find only two members with 8 micron excesses. The
incidence of excesses at 24 microns is much higher, 11 of 31 solar-like stars
and 1 of 7 early-type (A) stars. This work nearly completes the debris disk
surveys with Spitzer of clusters in the 30-130 Myr range. This range is of inte
rest because it is when large planetesimal collisions may have still been
relatively common (as indicated by the one that led to the formation of the
Moon during this period of the evolution of the Solar System). We review the
full set of surveys and find that there are only three possible cases out of
about 250 roughly solar-mass stars where very large excesses suggest that such
collisions have occurred recently.
