Does Sevin wash off in the rain? I thought it would go into the planc

All chemicals will be washed away from the rain unless you are using a systemic granular.

"Rain washes off the (Sevin) dust, rendering it less effective. Likewise, if the garden needs to be watered, do it before applying carbaryl. Allow a minimum of seven days between applications." http://homeguides.sfgate.com/apply-sevin-dust-garden-83474.html  ; This site will give you specifics about using Sevin safely in your yard. Good luck!

Yes it can wash off and needs to gently be reapplied, but not too much, you do not want to harm your plants.

It will wash away, it depends on if it was on the plant long enough to do its job.

Can I wash it off enough to make my bee's come back?