You need an Event Listener for a World getting loaded.
Try it like this:
ExampleMod Class:
@EventHandler
public void init(FMLInitializationEvent event){
MinecraftForge.EVENT_BUS.register(new WorldEventListener());
}
WorldEventListener Class:
@SubscribeEvent
public void worldLoaded(WorldEvent.Load event){
event.world.setBlock(0, 70, 0, Blocks.cactus, 1, 2);
// every time a world gets loaded this method is gonna get called
}