  There is growing interest in functional, adaptive devices built from
colloidal subunits of micron size or smaller. A colloidal material with dynamic
mechanical properties could facilitate such microrobotic machines. Here we
study via computer simulation how active interstitial particles in small
quantities can be used to modify the bulk mechanical properties of a colloidal
crystal. Passive interstitial particles are known to pin dislocations in
metals, thereby increasing resistance to plastic deformation. We extend this
tactic by employing anisotropic active interstitials that travel
super-diffusively and bind strongly to stacking faults associated with partial
dislocations. We find that: 1) interstitials that are effective at reducing
plasticity compromise between strong binding to stacking faults and high
mobility in the crystal bulk. 2) Reorientation of active interstitials in the
crystal depends upon rotational transitions between high-symmetry crystal
directions. 3) The addition of certain active interstitial shapes at
concentrations as low as $60$ per million host particles ($0.006\%$) can create
a shear threshold for dislocation migration.
