Steve Harris wrote:

I had loads of hoverflies before I even bought a packet of marigold
seed. Once the marigolds were up, the hoverflies ignored them. Maybe
they did eat a few aphids but not all.

That's right. If they ate them all, then they would have nothing
left to eat.

Then it's also well known that ladybirds eat aphids. I had loads of both
but the ladybirds couldn't be arsed to wander over to the aphid-ridden
plants.

Ditto. The aim of attracting predators of pests is not to wipe out the
pests but to reduce them to manageable levels.

So I sprayed. Job done.

Joni Mitchell sang "give me spots on my apples, leave me the birds
and the bees".

Evidence-Based Gardening anyone?

Your mileage obviously varied but sounds like you were trying to hit a
different target.
--
Nick Wagg