Unfortunately, the next stage in seed starting is always, at least for me, aphids. It starts with a few seedlings looking poorly, maybe with white mottling on their leaves, maybe with shiny sticky stuff on their leaves. If I look closely and under the leaves, I will see the tiny, soft green aphids, sucking the life out of my plants.
Fortunately, aphids are easily killed using a soap solution that clogs their spiracles (breathing pores) and suffocates them. I use the product made by the Safer company, buying it in bulk and mixing it up in a spray bottle as needed. I last bought an $8 bottle 3 years ago and have yet to use half of it. The year I didn't spray I lost my peppers and basil, but the aphids pretty much left the tomatoes alone.
The aphids hang out on the underside of the leaves, so I spray gently and run my hands over the leaves to make sure all parts get wet. The bottle says not to use the soap on jade plants, but aphids don't seem to like jade plants anyway.
That's not one of my photos above. My camera is acting up again but this time I can't fix it with a pencil eraser. the photo came from Plantedoktoren.