Bluestar 18 is every 7/8 mins Mon-Sat daytimes. They're not taking the attack lying down though...
Click the graphics for more details from each camp. Not just boasting about the buggy spaces, Bluestar have cut their fares. Slightly. A Southampton Dayrider is normally £3.50, but one will be available for the 18 only at £3. A FirstDay City is £3.20, although despite the name, it doesn't quite cover the whole city, just the pink zone of this map.
Bluestar are offering a special weekly ticket just for the 18 too, priced at £11. A Bluestar Southampton weekly ticket is already £11 if you buy it on The Key or on their mobile app, but rises to £13 if you want to buy a paper ticket (other than on route 18). FirstWeek Millbrook at £8.50 is First's answer to that, only valid between Millbrook and town. FirstWeek Southampton will set you back £17 but also includes Hedge End, which is excluded from Bluestar's Southampton zone tickets.
I can't not point out a typing error on Bluestar's rushed web page announcing the new deal for Millbrook:
The red box is mine of course. They might well need to appoint a Bluetsar to oversee this battle. Sweets? That's fighting dirty surely! How on Earth can First compete with that? If there's an obesity epidemic in Millbrook, you know who to blame :-)
Outside the city, the New Forest Tour from Bluestar and More gets going again this weekend with more routes than before. The fares are premium but include travel to and from the Forest on Bluestar 6 and More 112, although you might be hard pressed to find buses running at sensible times to make a day trip of it from Hythe on the 112. Probably best to get Bluestar 8 or 9 to Totton and change there to the 6 instead.