The Society for the study of flies (Diptera)

I have not heared of specific trapping Sylvicola before so I would be surprised if anyone had actually sorted that out. Depending on the present population size it might be worthwhile just going out into 'the field' on a sunny day and inspect tree trunks. At times when there are many Sylvicola about you can find them resting on tree trunks and branches in considerable numbers.
If you insist on using a trap (assuming you mean a water trap) I wonder if it is an option to add some fermenting stuff to it. Larvae develop in sap runs and the midges might be attracted to a light alcoholic smell. Perhaps use some stuff the lepidopterists use to attract some moths?

Just letting my thoughts flow so it may all be worthless... Geoff Hancock is the main UK Anisopodidae guy. You might also approach him.