My first bonsai tree is struggling with pests. It starts off fine, but once a few leaves have developed, tiny white creatures appear on the newest leaves. They are very small, and look like specks of dust until I watch them closely and see them moving. Once they have appeared, this seems to stop further growth, the leaves and branches get no bigger, and eventually the leaves fall off. Once this has happened, new branches and leaves appear, grow nicely for a few weeks, and then the tiny pests come back again. Can anybody tell me what these are and how to get rid of them? My tree is a Chinese Elm. Thanks.