rockm

I doubt this is from an aphid attack. For that much damage, the aphids would have to be carpeting the plant. Aphids also generally attack NEW growth and shoots, not old leaves or tougher woody growth. Typically, older leaves, as these are, remain unaffected, unless the aphids are so thick they look like woolen mittens on the ends of shoots.

More likely this has to do with placement of the tree and watering. Since you've had it only a few days, the tree is most likely acclimating itself to its new surroundings. It may have been kept in a brighter location before you bought it and now is in a place where it doesn't get as much light. In a less bright location, it cannot sustain as much foliage and is "self pruning." That's pretty typical behavior for many species. This could also be helped along by overwatering --which is a common cause for yellowing foliage on chinese elms.

You have to take a step back and let things be for a while. Make sure the soil approaches dryness (dry half an inch down or so into the soil) and then water thoroughly.

Also you have to get past the notion that a tree is going to look presentable ALL the time. They will not. Expecting them to be in constant "show" shape leads to overcare and alot of problems. Every bonsai goes through ungainly, unnattractive phases, as well as phases where they can be spectacular.