Hard maple tree dropping leaves in Spring - cause?

I live in SW Ohio (zone 6) and have a large mature maple tree in my back yard - I'd guess its 40 years old. For the past couple of years it has been dropping leaves throughout the summer. Its early May now, we've had above avg. temps the last 10 days, high 80s with no rain for 2 weeks. The tree has already started dropping a lot of green leaves. Any ideas why? Could it be heat stress? Or maybe over fertilization? I do use tree spikes twice a year, in spring and fall. Maybe its too much. The leaves are very green, and the tree is very full - it looks healthy.
Last fall I had it thinned out some, hoping all the leaf drop was due to its thickness and sun not getting to inner foliage. But didn't see much change and tree is just as full again. I'd like to stop raking a bag of leaves a day, and just enjoy my yard.

Thoughts on the possible cause? Thanks

My first thought was excessively wet soil but you mention little rain so that shouldn't be the issue. But it does cause leaf drop. The other most common cause, if no signs of pests and be sure to check carefully for that possibility, is over-fertilization.

The tree spikes are not generally recommended by arborists for several reasons. First because they have very localized effects - they feed only where used rather than the tree as a whole. Second because they feed only shallowly, only the superficial roots derive any benefits. Third, depending on the brand they can easily be too high in nitrogen and that N is delivered inconsistently.

You might want to ask this over on the Trees forum here for more details.
http://davesgarden.com/community/forums/f/tas/all/

Can you add some pictures of the leaves?
Are they showing any distortion? odd colors?

There is a tree disease called Anthracnose that can cause leaf drop in the spring. I do not know if it attacks maples. It is common on certain ash and sycamore (Fraxinus and Platanus) but not on the maples (Acer) around here. It is a fungus that is more prevalent in damp years, but the effects can linger and cause some leaf drop even when the weather is drier.

Thanks for the replies. I'll try to post some pics.

Take pictures of where the leaves used to be attached to the tree; pictures of the leaves as they lay on the ground; pictures of the leaf "stem" (that's the petiole) on the leaves that have dropped; and any other images you wish to share - including the whole tree(s).

I am certain you are noticing the activity of an itinerant insect in the Ohio River valley region known as the Maple petiole borer (Caulocampus acericaulis).

http://www2.ca.uky.edu/entomology/entfacts/ef405.asp

http://msue.anr.msu.edu/news/maple_petiole_borer_causing_leaf_drop_in_maple_trees

As you may read in the attached links, this is not a detriment to your otherwise healthy trees, which it seems like you provide more than enough care.

I disagree with the suggestion that the fertilizer spikes would cause this, but I agree with the idea that they are a waste of money. If fertilization is needed (pay for an inexpensive soil test to determine this), use the least expensive granular fertilizer of the proper composition and broadcast it over the entire root system - rather than poking it into the ground at just a few spots.

If the leaves are entirely green when on the ground, then it is unlikely to be one of the fungal problems such as Anthracnose, which would discolor or spot the leaves with infection before dropping.

Let's see those pics, and see if you find any larvae in your petioles...

VV - thanks for the info... that looks like a very good guess. Will grab some pics tonight.

Adding some pics from the tree. Maybe sawflies? If so, I'm assuming treating the large tree is not possible or easily done. But treating the ground/lawn they nest in until the next season is possible?

Over dry roots will cause leaf drop as the tree, even 40 years old can suffer from stress,
I dont know what type of soil you have, is it sandy, clay, rich loam or what, knowing that might give everyone other ideas to help.

I agree also about the feed your using, the best way to feed a tree or large shrub is to mulch it with plenty leaf mould either end of summer or early spring. while mixing the leaf-mould, add a few handfuls of granulated feed, however MOST established trees dont need feeing.

I would expect your soil beside the tree to be bone dry, and If you can, dig holes around the root area, try make the holes about a foot deep, get some empty juice containers and cut off the bottom, remove the cap, insert the containers narrow end downwards, leave about 2 inch of the containers above the ground and fill the containers with water from a hosepipe if you can or watering can, this will get water down to the roots of the tree, you will need to refill these containers several times at first. Back fill the soil around the containers so no animals can get into the roots or even diseases.
I suggest you refill the containers everyday several times to allow the roots to take up the water. after a few weeks cut this back to watering 3 times a week, then when the tree is looking happy, cat back again, BUT for this year, I would water as often as possible.
You can add feed this way too, HOWEVER, it is never a good idea to offer feeds to plants already stressed, the feed tells the plant / tree to take up the feed and put on more growth or flowers, this causes even more stress to a sick plant / tree as there is a lot of energy required by the plant / tree to make new growth or flowers.

I would suggest after a couple of months, you should see less leaf drop, at end of season, try give extra water, then add a good mulch as I mentioned above, the mulch will help prevent moisture loss and you lay off giving the water via the containers, but in spring, I would start the watering again soon as you find new bud / leaf, keep this up and it will tell you IF the main problem has been water shortage due to your climate as you said there has been no rain fall for a long time.
I live in a wet climate BUT, when I plant tree's or larger type shrubs, I always sink these drink containers into the planting holes as deeper down, come summer, my soil is so well draining that a foot down there is very little water getting down there.

Hope all this helps you out and gives you ideas to fix the problem,
Kindest Regards.
WeeNel.

Maple petiole borer is clearly the issue. This is not a cultural (watering, fertilizing, et al) problem.

If you reviewed the literature and others like it, you've seen that it is clearly the recommendation of professionals to do nothing. This is an ephemeral insect, and the tree goes on its merry healthy growing way after the larval cycle is completed.

I don't see the need for you to apply any pesticides/poisons to your tree or lawn or anything else. Providing moisture through hot dry periods is useful, but the dropping of full green leaves due to this passing pest will end very shortly. It is not an all-summer insect.