[Observium] Large number of defunct shell processes

Hello. We're running Pro build 6606 on an Ubuntu 14.04 machine. I'm consistently seeing large (>100) numbers of defunct processes lingering on the box. E.g.:

... www-data 31956 32711 0 14:09 ? 00:00:00 [sh] <defunct> www-data 32623 17403 0 11:37 ? 00:00:00 [sh] <defunct> www-data 32641 23519 0 11:37 ? 00:00:00 [sh] <defunct> www-data 32661 23519 0 11:37 ? 00:00:00 [sh] <defunct>

Can anybody shed some light on why these are hanging around and what to do? They're causing our system monitoring to make grumpy noises...

Thanks!

-Pete