If you want to farm beetles for cash, here's what I usually do.
1. Go to your island after 7 pm.
2. You'll need to remove all of the shrubs and trees. Remove ONLY the palms on the east or west side of the island, leaving about 3 or 4 palm trees on one side of the island. Removing shrubs and regular trees prevent butterflies and other non-valuable beetles from spawning.
3. This leaves only the large beetles, those beach roaches (not sure what they're called), and hermit crabs available to spawn.
4. Starting from the top of the island run down the side that has no palm trees and chase off all of the trash bugs. Once you start to near the palm trees, walk along the beach watching the trees for big beetles. Once you reach the top of the island, just repeat.
5. Carry a fishing rod with you too, that way if you see a fin you can catch it. Ocean fish aren't worth keeping. You also use the rod to tell you when your storage is full. If you catch a valuable beetle, dump the rod, and then go put your stuff into your storage. Pick up the rod again when you go back outside.
6. Golden Beetles and Hercules Beetles are the most valuable. Goliath beetles (the red and black ones) are the least valuable big beetle. I don't even bother to catch them.
7. A full storage of beetles and sharks usually gets me between 350K to 500K per island trip.