Thanks for raising this question. I had noticed that my ladybirds are mating on plants that don't have any aphids, and when I relocated them to aphid colonies, they jumped ship! I have just found this on the web, see the link for more info.
"So, can ladybirds really make any difference to aphid population? Well yes, to some extent. The females lay their eggs very early in the aphid infestation, and crucially, don’t lay any more. This is because ladybird larvae are cannibalistic and will eat any (other ladybird) eggs that are laid later.

Female ladybirds have developed a clever way of knowing when to stop laying eggs. The UEA scientists discovered that ladybird larvae leave chemical “footprints” behind as they crawl over the plant. Females can detect these tracks, and know that any more eggs would probably be eaten, so stop laying."
http://www.nrp.org.uk/enews/edpladybird.htm