Are they white or green caterpillars hanging by silk threads from the trees?  Most likely they are the larvae (caterpillars) of either oak leafrollers or oak leaftiers, two different species of moth. Leafrollers are from the genus Archips and leaftiers are from the genus Croesia.

Thank you for the info.  I'm not sure which one we have because they are still in the caterpillar stage.  What is strange is that there were thousands of them and most of them were climbing up a pine tree that was dieing this spring, some of them had gone over to a maple tree. I've occasionally seen these type of caterpillars but never so many. 
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As you know, most insects have population cycles.
This may be the year for your infestation.
Bacillus thuringiensis (Bt) is a group of soil microbes that are used to control certain insect pests and a common treatment for caterpillars.
https://agbiotech.oregonstate.edu/video/bacillus-thuringiensis-bt

Thank you!! 
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