Pattern BushFireRed&LeafGreen
"A dense thicket on the south side of Six Island. Off the beaten path and easy to skip, but worth exploring if you are thorough with the Sevii post-game."
PATTERN BUSH MAP
Pokemon Encounters
Heracross at 20% is the headliner, one of the strongest Bug-types you can catch in FRLG. Spinarak at 30% and Ledyba at 5% bring Johto variety. Caterpie and Weedle fill the gaps at low levels.
Ledyba
TRAINERS
Bug Catchers and Breeders fill the area with Bug and Grass-types near level 50. 12 fights total, but individual teams are small and go down fast.
Garret
Pattern Bush Guide
Pattern Bush on Six Island is the only location. It shows up at 20% encounter rate at levels 15-30, so you won't need to hunt long. Heracross is Bug/Fighting with 125 base Attack. One of the stronger Pokemon available in the post-game Sevii areas.
Islands One through Three unlock after you beat Blaine on Cinnabar Island. Bill takes you there as part of the story. Islands Four through Seven require beating the Elite Four first, then completing Celio's Ruby and Sapphire quest on One Island to get the Rainbow Pass.
You can physically reach all seven islands without it, but evolved Johto Pokemon only show up in the wild after you get the National Dex. That means 60 or more Pokedex entries plus completing Celio's quest. If you're only seeing Kanto species, that's why.