Thanks Alistair and David B for the various comments - 50g per litre would I suppose give me 20 trees-worth, so about 1.50€ per tree, which if it works would be ok (the best price for the green stickies is over 4€)...I think the loved-up male (according to Cheverel's article) is supposed to be attracted and then drown... Both methods of course are likely to attract other flies...tant pis...the only realistic organic alternative is spraying with a clay solution, which of course would have to be repeated every time there was a storm, and even so to get the right coverage seems like a very tiresome occupation...
I shall carry on regardless, and report what happens - i might try the yeast as well...thanks Marilyn in an earlier post..
Sandra