Aphids & Death in the Garden

Things change so fast in the garden, even on a balcony garden. I didn't think I'd have many problems with insects since I'm up in a high rise, but I've discovered Aphids on my lettuce, and they have crept onto my tomato plants too... Here's a look at them:

Here's what one of the aphids on my lettuce looks like up close:

They appeared and reproduced so quickly! As a solution, I had some Safer's Trounce (an organic insecticide) around so I sprayed the lettuce with it a few days ago when they first appeared in droves. It's killed a lot of the aphids, but now the lettuce doesn't look anywhere near as healthy (check out photo below). I will likely have to cut down this crop and start over if I want more lettuce. This is disappointing but just another part of having a garden - another challenge to overcome and learn from.
I am not sure where the aphids come from; is it the soil (are they in it all along waiting for ideal conditions and temperatures?)? do they fly up to my balcony? did I bring them back to my balcony after a bike ride? -- it's just very strange to me that they suddenly appear with such a vengeance.
My peas have also fallen subject to the wrath of an insect infestation. Some kind of fruitish fly has taken over its pot and the peas have pretty much died. It may not have been getting enough sun, leaving the soil too damp leading to the fly problem. If I have them in direct sun though, they just fry. I struggled with peas last year too, so perhaps they're just not suited for the balcony.