Discussing bugs is difficult subject since everywhere I have lived has provided a unique cast of insects and other pests to deal with. Aphids are the only insect that are seemingly omnipresent. This is the only pest that has led me to exasperation during the seedling stage.
These insects can cause major damage in just a couple days. They breed quickly and aren’t always seen in the early stages. I have no cure – but they can be knocked back with pyrethrin sprays and insecticidal soap but they have to be applied twice per week without fail. Occasionally I’ve had to resort to Malathion when the others stop being effective. The key point is to be vigilant and watch the plants carefully. Also, aphids attack the weakest seedlings first so watch them with extra care.
Another control method that I use on little seedlings is to bring them back in for eight hours or so and stuff them back into my germination heat chamber which is kept at 86 degrees F. Aphids hate that. In fact it dries them up into shriveled sticky dust. I can’t do this once the plants are bigger because they won’t fit in the tray slots anymore.