I have a couple of Elm saplings. About the only thing I've seen living on the Elm leaves is the Caterpillar of the Question Mark Butterfly. Here is a picture of an Elm sapling with a Question Mark Butterfly on it. It was taken from quite a distance, as the Question Marks are very easily frightened.
Second is the Butterfly, but from afar. Picture 3 shows the caterpillar in August 2015. What a beauty! Who would have matched up this Caterpillar with that delicate Butterfly?
The Caterpillar finally turned into this Pupa, but it didn't attach itself to the top of the little bottle I was raising it in. For some reason, it didn't develop to the adult stage. In June 2018 I found a couple of Caterpillars on an Elm sapling in the southeast part of the back yard. Unfortunately, neither of them survived even to the Pupa stage, not to mention to Adulthood. I never touched them, but a week or two after this picture was taken the Caterpillars both stopped eating and died. I haven't seen a Caterpillar at all in the intervening years! Sorry!
If you're lucky enough, you may find one of these two galls on an Elm leaf. They are caused by a so-called Elm Cockscomb Aphid. I've never been able to find the Aphid itself but we know it was there. Picture 3 shows another gall on Elms called the Elm Finger Gall Mite, and caused by a Mite of the same name. (A Mite is related to a Spider, has 8 legs and is usually quite tiny. Click on the third Gall to see a Whirligig Mite, which is the winner for jumping around.)
Or you may find a trail in an Elm leaf. A Fly (Agromyza aristata ) lays her eggs in the leaf and the babies burrow totally inside the leaf and leave this trail. Picture 3 shows a Weevil, the European Elm Flea Weevil, Orchestes steppensis. By the way, even if your Greek is a bit rusty, if the Generic name of a critter ends in -myza, as in Agromyza, it is a Fly.
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copyright Martha O'Kennon 2021