pest control in containers?

[katihannah]

I have some lettuce and trailing tomato plants in the same container, growing on an outside windowsill. I noticed this morning that some of the lettuce leaves have tiny reddish flies on them, and tiny white flecks (?eggs, larvae?) in the soil immediately under the plant. Any idea what they might be? And are they harmful? Obviously I don't want to spray them with pesticide as I don't fancy eating a load of chemicals with my salad, but is there any way of getting rid of them?

Thanx
Katie