[jonewer]

They are eating my quince tree! 

How to stop them?

[Trillium]

Is this the caterpillar eating your tree?

http://en.wikipedia.org/wiki/Antheraea_polyphemus

You'll need to buy a nematode spray system to rid yourself of these as the tree will harbor many eggs as well as caterpillars. The system is organic and safe, and available almost anywhere garden products are sold.