I found one white curl grub near roots of a Cousin It which had died - it was planted 5 months ago.

There are 29 other Cousin It which have grown well but now seem to be getting 'brown' and I am worried they may also be affected by curl grub. I cannot really dig them all up to check. Is there any pesticide or anything else I can apply to the soil that will kill possible grubs but not the plant? Many thanks