I've never fed mine, and to agree with Dunc they catch, at times, a huge amount, so extra would be quite superfluous. When they're small they seem to struggle on for a year or two, remaining small until they're able to catch larger insects, at which time they're off the blocks like a lamprey on an ice rink.
Never tried feeding whilst small, but if I get round to pollinating this year (something I've planned but failed to facilitate for the past couple of years, though followers of mine will know that last year went somewhat contrary to plan), I'll try something.
Regards
Nigel