Hi anaclaire and welcome to LCT!! We went on the same cruise last June. Just to let you know about our experience, we didn't have any mosquito problems. BUT, we also didn't go into the forests or do a lot of "outdoorsy" activities. It was very warm in the port towns when we were there - high 80's and sunny. Take a pair of shorts just in case!!
We did the Sitka tour with the Russian dancers, totem pole park, and the Raptor center - no bugs there! (You can walk through the woods at the totem pole park so there might be bugs there this year.) The whale watching with Captain Larry - no bugs. And the steam train in Skagway - no bugs. Can't tell you about Ketchikan - missed that port.
Of course if you are doing totally different things, I can't speak about what mosquitoes you might run into. Also, there were no mosquito reports last year to speak of.
Have a great time!!