On route 214 between Pastoria and Veilstone there is a Beauty(she's the one with the yellow hair)standing next to a patch of grass. Battle her and register Wormadam in your Pokedex. But if you want to catch one get some honey and put it on the trees and eventually you should get a female Burmy. My advice is to try to get your Burmy to evolve it inside so it can become a Bug/Steel type which makes it only weak against fire
three ways.1. get it from Cyrus when you beat him.2 win the lottery(match all the numbers so I suggest you trade) 3 Get it from Emerald and use the cloning glitch in emerald.
I think you have to talk to Dawn in jubilife so she says that Dialga Roams in Spear Pillar and you can get pasr the Grunt I think. Or you took wrong place but hope I helped coz this is all I remember of playing it