If scale insects spread, you can treat your lemon tree with the kerosene oil preparation Promanal (Neudorff) or with the rapeseed oil product Schädlingsfrei Naturen (Celaflor), the animals suffocate under the oil film. However, one must be very careful that all scale insects are covered with the oil film. Rubbing the leaves is probably not enough here, because if some animals are not hit, a new infestation will start from them. It is better to use a plant protection sprayer (with spray function) here. This way you reach all the animals.
For lighter infestations, you can also get at the animals with cotton swabs soaked in methylated spirits. This will wipe off the brown lids and the eggs of the lice and dispose of them. It is also important that the procedure is carried out very carefully, that is, you need to regularly inspect the plants for several weeks.
Of course, the fruits themselves are still edible with these means. However, they should be cleaned very well, preferably with a brush and plenty of water before eating.