I had a problem with aphids on my vegetables. I used my garden hose w/ nozzle to knock them off the plants and I will follow up with some Neem oil once the temperatures dip later this week. I noticed a bunch of ants on my tomato plant, they protect the aphids and eat the honeydew droppings, so I need to treat for ants too. I have some Borax powder I spread around my planters that the ants will tramp through and take back to their nests. The stuff is non-toxic in reasonable quantities. Check the label before using it around fruits or vegetables.

Do you have ants on your tree? They farm the aphids. They will go after ladybugs to protect the aphids. There is a product called tanglefoot that keeps ants from climbing the tree. That is what I use, then after a couple of weeks the ladybugs usually have them all eaten up.

What I would do, and have done with success, is to blast off all the bugs with lots of forceful garden hose water. After, I would wrap a couple rings of tightly stretched 1" wide masking tape over smooth areas of the lower trunk bark with the sticky side facing outward. The bugs trying to walk across the tape will either turn around or get snagged by the glue as long as the stickiness doesn't decline. They will be able to crawl beneath the tape if there are any bark grooves below the tape. It would be good to check the tape each day to see if any small lizards happen to need gentle rescuing. Once the tape loses it's stickiness, just put new tape on top of them, or replace the tape if it is clogged with bugs.

When you've had enough, spray with Malathion.

Thanks everyone! My wife did tell me about the ant issue, and she ringed the trunk with some white stuff that should keep their traffic down. I'm going to blast them with a hose, and maybe some more soapy water- that worked in the past.

Malathion is highly toxic to bees. Plums need bees.

Just use the hose.