Been using Vine weevil killer all year, but now found lots of grubs
eating into roots of my potted plants.

Checked on label of bottle and it says ...

This Vine Weevil Killer is a systemic insecticide to protect plants
grown in pots & containers from damage caused by vine weevils & reduce
greenfly, blackfly & whitefly infestation.
One application gives protection for up to 4 months against Vine
Weevils & helps reduce Greenfly, Blackfly & Whitefly infestation for
up to 6 weeks.
Mix 30mls with 1 litre of water & use the resulting solution to drench
the soil or compost.
1 litre of solution will treat 10 litres of compost.

Does this mean that the treatment has to be via the roots only, and
not via the leaves as per other systemic insecticides ???

Is the Nematodes more effective. Which is more cost effective, as I
have lots of pots.

Can I keep Nematodes somehow till next year.