onions got et

hi, I have been growing onions for thirty years now, and never saw this.
The healthy looking onions 'got et' by something, whatever it is attacked them from the bottom leaving a clean hole and what looked like a tunnel system which I couldn't trace and didn't seem to have any nests or the like. I maybe didn't look properly yesterday, but if what ever it is propagates itself, my onions will soon have disappeared.
Any ideas? I would like to avoid poison if possible..

From a search of the forum, it seems it is likely to be onion fly, or allium leaf miner

Have a look at these two links and see if either seems more probable than the other.  It's my first year with onions so no experience to speak from 

http://www.growingyourownveg.com/onionpestsanddiseases.php
http://apps.rhs.org.uk/advicesearch/profile.aspx?PID=643