Although there is no insect in the submitted photos, that type of leaf damage is associated with caterpillars or sawfly larvae. Slug sawflies usually skeletonize leaves rather than remove all leaf area. They damage roses at this time of year and could potentially feed on adjacent maples too. 

It's possible there is more than 1 possible culprit for the damage to the leaves. Gypsy moth can damage pines, while tent caterpillars and webworms are more likely to affect maples. Fly larvae do not cause the type of damage shown in your pictures. 

Maples can experience decline from a number of causes, including disease, insect, and soils with high pH. Sometimes, the damage is a result of freeze-thaw or other stressors from the previous season. If the tree is losing lots of leaves, this could be an indication of a soil-borne pathogen such as verticilium wilt. If there's no clear cause for the damage, a visit from an arborist may help diagnose the cause.

Pine and other conifers often lose their lower branches over time due to several types of fungus that restrict needle growth. If the middle and upper parts of the canopy are healthy, the trees are likely to do ok. 

Verticilium can only be confirmed by cutting open a living branch and searching for streaking and by sending in a sample to the Plant and Pest Diagnostic Lab in East Lansing.

Borer damage is usually accompanied by leaves disappearing at the top of the canopy rather than the bottom. There will also be circular or D shaped exit holes from larvae. 

Each of those tree types usually experience common issues from a variety of issues, and I don't have any further suggestions. If you are able to submit a photo, that might provide additional clues.