Hello Richard i don't think your problem will be depth of field coz small sensor point & shoot cameras have far more depth of field than a larger sensor dslr.What i mean by this is that say your shooting with your ixus at F4 then to get the same depth of field on a dslr you'd have to be shooting at F8.
I reckon your problem is your probably using autofocus & it's focusing on something else rather than your intended insect.
I use a point & shoot.But luckily i can change the focus points from large to only a small one in the centre of the frame.So when i focus on a butterfly the focus point will be covered by the butterfly.
Have a look in your manual & see if you can change your focus points.
If you need more help then you can try here
photography-on-the.net this forum is full of users with the same or similar camera to you.
Hope this helps.
If you have any problems with anything i've posted here(i dunno if i'm been really clear or not,lol)then just ask & i'll try again.
John