Last year I had a big problem with cockchafer (or some other bug, maybe Japanese beetle) grubs in my potted plants.
I wonder if covering the soil with mulch or gravel can prevent bugs from getting into the pots and laying eggs there. Does anyone have experience with that?
The First step would be to get rid of the grubs (cockchafer grubs live 3-4 years in the soil). I guess you already did that by replacing the soil.
I don't know about mulch, but insect nets definitely help. You can put them on your pots like a tent. For cockchafers, you only need to keep them up the month or two they are flying and probably only from late afternoon till morning if you don't like the look of them and are willing to put in the work. It also helps to keep lights off your plants at night. Too much light attracts the beetles (they fly at dusk mostly) and gets you pots on their egg laying radar.
For stronger measures there are biological pesticides for cockchafer grubs based on the fungus Beauveria Brongniartii which should be safe for everything but the grubs. You should verify it's cockchafers first though since the fungus is pretty specific.
Prevention would be the preferred method though since there are easier ways to get the grubs out of pots than insecticides.
