When you access (not just plug) avast will scan the boot sector and, depending on your scanner sensibility, the files and folders when you access them (read).
If you want a particular scan, you'll have to edit the autoexec.ini file in the pendrive, for instance, using:
http://www.net-burner.com/WhatIs/Index.cfmand then using ashQuick.exe of avast folder.
The avast quick scanner (ashquick.exe) could be used in a link of your desktop to scan the pendrive.
But, as you've asked directly, there isn't an automated scan when you plug a pendrive...