The leaves of bay and laurels all eaten away around the edges, polyanthus suddenly turn yellow and die. Chances are you have the dreaded vine weevil. It has reached plague proportions in London gardens over recent years.
The adults, which are like tiny black beetles, can only be seen if you go out with a torch at night. Shaking the plants over a sheet will give you a clue to their numbers and give some measure of control. Unfortunately, while the adults disfigure plants biting out notches around the edges of the leaves, it their maggot-like larvae that do the most serious damage, gnawing away at the roots.
Vine weevils are hard to kill because you need to get the adults, the larvae and the eggs. There are two chemicals that will do the job but they are only for use in containers. Provado Vine Weevil Killer 2 (thiacloprid) and Scotts Bug Clear Ultra Vine Weevil Killer (acetamiprid) are both mixed with water. You need to really soak the compost for them to be effective. A single treatment can give up to two months control but my experience suggests that at least one repeat application Let your guard down once and they’ll be back in force.
Never uses these chemicals for any plants you intend to eat. The organic solution is a biological control in the form of a microscopic worm that parasitizes the vine weevil grubs. It works in the garden as well as containers but the soil needs to warm and moist. Putting the worms into action is easy. Just mix some powder into to your watering can and you are away. They work in the garden as well as containers but the soil needs to warm and moist. Google biological pest controls to find suppliers.