noun: pest; plural noun: pests
1. A destructive insect or other animal that attacks crops, food, livestock, etc.
So is an insect that attacks garden plants a pest? Not for me, otherwise every single moth would be classed as one.
But we do have a few insects that seem to make a point of proliferating as such a rate that they are damaging the host plants. Over the last couple of years or so we have two garden beetles that are on a sharp increase and starting to become pests!
One is bright red, appears from early April-ish and might still be knocking around in one form or another into late summer. I first saw this in the garden in April 2011, and it is now a dead cert to arrive. They are always on my Snake's-head Fritilaries to start with, and now we have some lilies growing in pots they'll be smothered in larvae in a few weeks. The larvae try to hide themselves by hiding within a cloak of their own shite. Nice.
Scarlet Lily Beetle
The other is less obvious. We've had them in the garden for a while and I've never considered them as a potential pest despite their reputation for the larvae attacking roots. They can reproduce parthenogenetically, so potentially could increase in numbers quite easily. Up to now I've seen the odd one, sometimes in the moth trap. But we now have quite a large hydrangea growing in a large pot, and whilst looking for something else I noticed one on the outside of the pot ... and then at least eight on the soil-surface inside the pot. Looks like they've made the hydrangea their home. To be honest I'm not that bothered - it's not a garden plant that I chose or particularly like ........