Leaf beetles come in many colors and sizes, they are usually specific to a few related plants. The ones which I have are Green Dock Leaf Beetles (Gastrophysa viridulametallic) which are rather pretty, but feed on foliage making it very unsightly. They are quite common in grassland areas and perforate the leaves of ornamental dock and rhubarb. They could be sprayed but it would be pointless as the surrounding countryside is full of wild docks, sorrel and knotweed, which they seem to prefer and harbour a continuous supply. They usually appear in late April and the females develop enlarged abdomens so the elytra (hard wing case) cannot close properly. This is due to the eggs which they lay in clumps on the underside of leaves.
The small black first instar larvae, about 2mm long, graze on the lower side of leaves; the later, larger black instars perforate the leaves like the adults.
The most immediate and effective option is to pick off the larvae and adults, and crush the groups of orange-coloured eggs to try and reduce the damage. A fatty-acid based spray forms a scum on the body of the adults and larvae causing them to expire.
Other damaging Leaf Beetles include the Red Lily Beetle Lilioceris lilii, which attacks bulbous plants, especially lilies and fritillaries.