What is the best way to save young trees from night beetle infestation?
The insect you refer to sounds like the brown or bronze beetle which is an adult grass grub. They wreak havoc with lawns as larvae and once they have pupated into beetles, they feed on the foliage of ornamental shrubs and trees for around 2-3 weeks in early Spring/early Summer on warm, still evenings.
There are a number of insecticides that specifically deal with these types of beetles which can either be sprayed on the foliage of trees from October onwards, or granules distributed on the ground around plants.
Another suggestion would be to also treat your lawn for grass grubs from late January/early February until the end of Autumn to attack the problem early on.
Talk to your local garden centre for product advice as they are typically aware of problems in your local area and can give you advice on which product has worked best for others.
Another tip - check your various plants to see which of them are less prone to major beetle damage, this will provide a guide for future plantings.