Spider mites are not covered well with the usual insecticides and you haven't stated if you are an Organic grower or not.
Insecticidal soap is a very good choice for this problem however it normally takes several applications to get the job done.
The down side with this is if your berries are still blooming, because Insecticidal soap is not discriminating it will get beneficial insects too.
Before you spray look for natural beneficial insects like lace wings, lady Beatles, if present I'd say let nature take care of it for you.
That said we all have our personal tolerance level for pest and disease damage, being an organic grower means having a high threshold for damage.

Randy
Yamhill County Master Gardener