There are particularly abundant crops of hefty blackfly and greenfly this year. Vigilance is your best tool in the fight against these sap-sucking insects. Check new growth, flower buds and undersides of leaves for groups of the creatures, and be suspicious where you see ants travelling up and down the stems of plants. Believe it or not, ants "farm" aphids to feed on the sticky secretions, or "honeydew" that they exude: where you see ants on plants, you are likely to find aphids also.
If you choose not to use insecticides, hose away the offending pests, or rub them off with your fingers (or a tissue if you're squeamish), or (if it will not damage the growth or habit of the plant) nip out the worst-affected parts. Continue to check the afflicted plants every few days until you have controlled the infestation.
Meanwhile, when harvesting your first lettuces, don't pull the plant up by the roots. Instead cut it with a sharp knife just above ground level, the "stump" will produce another crop of leaves within six weeks or so.