lemon tree in pot in greenhouse

Without seeing the leaf and the way in which the holes are eaten it is not 100% sure to advise you, but it sounds like the Chafer Beetle which eat the leaves in May and June, if it is, you need quickly to spray the leaves with a Systemic insecticide as the bugs from these feed on the roots of your shrubs and are serious They take a few years to hatch, in which time they will have devoured all the roots of your shrubs, plants or lawn which it is nursed by, the bug thrives and hatches your shrub dies and dispatches!!!! It depends on the shaps and formation of the holes as to what your pest is. Most of those leaf eating pests are treated in the same way, By spraying Systemic insecticides