On the island, I think the easiest place to find them is in the lower south cave. Go in with a longneck or a shotgun, turn left at the first intersection, kill the boas and spiders I. The first chamber, near the end of it there’s usually 1-3 beetles. Tame, and carry back to base w/ Petra. Easy fertilizer.
They can destroy wooden structures. I was in the east underwatercave for deep SEA loot and Had a 1x1 wooden House with a bed. I had No weapon and so a lvl 200 dungbeatle Killed me nearly, Destroyer my bed and my foundation not what i expected.
On Ragnorak you can find them in the volcano biome beside the highlands . Very easy tame just use a couple of spoiled meats or dung. They have to be on wander to produce fertilizer, i keep mine in my greenhouse.