Effect of circling and background magic on bug quality
This table shows the influence of circling and background magic on the quality of bugs summoned. The "charged circle" had 2 Flaudia's Earmuffs (60gp) cast in it before summoning the bugs.
All summonings were performed while at full hitpoints, and without changes to magic skill bonuses. All shields were plague sized, so influence on swarm size could not be determined.