If the prized Maple trees in your yard are dropping leaves like it’s fall but the calendar says it’s June…..don’t panic and take a “leaf” of absence. The culprits may be Maple Petiole Borers. If numerous green leaves are dropping and you’re raking like it’s October, look at the petiole or stem attached to the leaf.
Maple Petiole Borers are a type of sawfly or non-stinging wasp that overwinters as a pupae in the soil. They emerge as adults in spring to lay eggs in the petioles of Maple leaves. When the larvae hatch they tunnel through the petiole which disrupts the connective tissue. The leaves fall to earth, the larvae leave the petiole and burrow into the soil to hang out until the process starts over again the following year. Don’t lose your “composture!” At the most 10% to 20% of the leaves might fall. Trees can easily handle some defoliation. Management is difficult and unnecessary. The more important issue is keeping the tree healthy with proper feeding, watering during drought periods and maintenance pruning in winter. If the leaves are not falling off green, but rather shriveling and turning brown on the tree it could be shade tree anthracnose or frost damage. Anthracnose is a disease that occurs when spring rains coincide with foliage emergence. 
Combine this fungi with wet weather during emergence and throw in a frost or two and you can witness some pretty unsightly foliage. The upside is that large well established trees can withstand the damage without serious long term affects. The tree will push a new flush of growth from otherwise dormant buds. Clean up fallen leaves thoroughly and make sure fall clean up is done well. For older established trees make sure they are fed and watered properly. Consider winter pruning to improve light and air penetration through the canopy. For younger less established trees a Lime Sulphur spray could be considered each spring before bud break, usually spraying sometime in March. Finally it is not a “figleaf or your imagination” that we see a lot of leaf problems in June and July from damaging insects. One of those insects that does maddening damage because you don’t see them when they do damage (at night) is the Black Vine Weevil. 
They crawl out of the soil, mulch or debris at the base of a Rhododendron or Euonymus as an example and take “chunks” or “notches” out of the foliage in May and June. You can set out Tanglefoot sticky traps at the base of the plant or coat the lower stem with Tanglefoot to disrupt their journey up the plant. Removing lower foliage as part of their “staircase” will help also. In severe cases the plants could be sprayed with Orthene or Sevin for a measure of control.
There you have it, advice to good to “leaf” alone!