How to encrypt USB drive on a Mac?
I didn't find a way to do this from Disk Utility, but you can accomplish it from Terminal using diskutil.

Open Terminal and run diskutil list to show all devices currently connected. One of the entries should look similar to

/dev/disk7 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *1.0 GB     disk7
   1:                  Apple_HFS LOGBOOK                 1.0 GB     disk7s1


with the name of your USB stick instead of LOGBOOK. Take note of the device shown in the first line.

Then run

diskutil eraseDisk JHFS+ NEWNAME GPT /dev/diskWHATEVER-NUMBER-YOURS-IS


to erase all content and create the required partition scheme. Replace NEWNAME with whatever name you want to use (enclose in "" if it contains anything else than letters) and use the device number found above.

Once done you can encrypt in Finder by right-clicking on the stick name in the sidebar.