Hi Loretta and George,
Oh dear that sounds bad and you are not alone! Its an epic year for lettuce root aphid and I have never seen damage like it. Normally I lose a few older plants in August but this week I suddenly have a whole block of 140 plants keeled over, just like that, faster than usual.
Yes its bad for business, to put it mildly, the best remedy I know is to grow endive instead, but not all customers love that. I know no way of keeping root aphid out, except perhaps fine mesh but that might increase mildew and slow growth a bit.
