With scale insects the mantra is you have to hit them in the crawler stage, before they settle down and make a protective shell. The window for this can be narrow. Maybe try Pennsylvania Cooperative Extension for help with identification and control. There is also a trees forum on this site, the forum your question is currently on is set up to talk about orchard fruits rather than ornamental trees.

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!

Ken, The problem with this particular plum tree is twofold: (1) it belongs to the co-op, not to me, and I don't think they'd take kindly to me cutting it down and (2) it's right in the middle of my HOSTA GARDEN, providing the main shade for it. The hosta garden gets the late afternoon sun, it's an eastern exposure, and the oak trees behind the plum tree (oak trees are in the strip between the sidewalk and the street) gradually shade over everything. Various neighbors have told me that, when they inquired about having a dead or dying tree replaced, they were told that the tree would not be replaced. So, if this plum tree up and dies, I suppose they will saw it down and leave my hosta garden to be shaded only by the oak trees in the late afternoon. I wonder how that will affect my hosta garden...

I don't use any oils or pesticides for insect control as I grow organically. google Arbico. they specialize in beneficial insects. I ordered three praying manti egg cases. one for my yard and for my neighbors on both sides. one egg case hatches abou two hundred manti. They devour every insect you hate. and once they live out their life cycle they lay their own egg cases. so now I have an ongoing population of manti. I live an hour north of death Valley so its a desrt hot blazing summers freezing winters if my manti can stay alive year after year your area will support them as well. also consider Ladybugs these are literalyy the toughest bug in the pack they eat every single pest bug you can imagine problem is when they eat everything they move on to find more food. I even take my baby manti indoors to work over my bonsai and houseplants I leave a couple of windows open all the time for my cats to get in and out. so manti have an escape, or entrance if they choose. I grow vegetables, tobacco, fruit, housplants and bonsai. this was a permanent and inexpensive solution to my bug problem. Good Luck!!!!