My plum tree has recently started leaking a nice sticky substance that gets on everything; dogs, deck, barbecue, etc...I noticed a white substance sticking to the stickiest of leaves, and on closer inspection they looked like little white bugs. Are those aphids? If so, what do I do? I've read up a little on aphid control but I'm not sure if the things I have read would work on a large tree that seems (to me) pretty infested. Oh, and none of the bugs I saw seemed to be alive, just stuck in the sticky mess they created! Help!