BH51, that's what I had to do with all of my beds except my herb garden the second year I lived here, and it did the trick. At the time the lettuce bed wasn't, period. It was a brick-topped cistern that I got tired of looking at and simply shoveled about 3-4 inches of bagged topsoil and maybe one of composted cow manure (with supplements of each and a bag of vermiculite or something similar) every year or two). It did well for about 6 years, and this year was the first time I had such a problem with that particular bed (it is a little short of 3'X4'). It is too late to do that this year (it wouldn't have worked this year, period. You need warmth and sunshine for it to work), but maybe I'll do lettuce somewhere else next year and give it a try. Assuming we actually have a summer where the temps get above 80. You can't sterilize soil if it doesn't get hot enough. BUT the big problem is that there is a cistern under there. My tomato garden is just to one side of it, and my herb to the other, with normal soil depths. The earwigs only attacked that specific spot, so I have to think they're living in the probable filth many, many feet below the cap to the old cistern. Hey, if it doesn't work, no biggie. I'm also thinking of digging all the soil out (not a huge chore) and putting down some weed cloth, and shovelling new soil over that. I hesitate to poison, it seems to me that lettuce must be the most susceptible to carry the stuff you put in the soil. We're debating which options to try. I suspect, given our past few winters and this cool summer, that our first heavy frost isn't too far off, so any decisions will have to wait for spring ... to minimally treat and plant, or to go all out and skip a year, replacing or sterilizing the soil. My husband (NOT the gardener in the family) wants to not blame the cistern, but similar plants that grew just inches outside of the cistern suffered minimal damage, whereas I wasn't even willing to bring in the greens this year, they were so infested AND so little was left to rescue. Husband tends to see something on TV and thinks he knows it. As in, "why don't we grow xyz in that spot instead of lettuce!" Reply, "Husband, dearest, that soil is only 3" deep, you cannot grow xyz there!" Over the years it has been perfect for greens of various kinds, hardier ones in the spring and fall (that can take a frost) and more delicate lettuce in between. So I'd like to treat it, even if it means digging it up and starting over with some kind of barrier between the bricks and the soil.
Whew! This is my first year of gardening in this heat zone. My husband and I built two raised gardens 12" high, 4 ft wide and 12 and 16 ft long. We filled them with layers of steer manure, hay, newspaper, a little blood meal and soft phosphate and covered it with black plastic to "cook" in the sun over the summer. I'm planting this week. Strawberries, spinach, acorn squash, tomatoes, broccoli, cucumbers, radishes, carrots, leeks, sugar snap peas, pole beans, sunflowers and marigolds are the plan. I'd be grateful for any suggestions. 
