How sure are you that they're ants? It sounds like aphids.

btw, the only thing I can think of that is "different" is that.... about 2 weeks ago, I topped-off the soil with some very-finished compost....

Tx.

Not sure exactly what aphids are, but upon closer inspection, this is what I see: really, really tiny black things clustered together that don't seem to really move. I am also definitely seeing a few small black ants as well, going back and forth on the branches.

Tx.

Ok, yeah, that sounds like aphids. Aphids are little insects that suck on plant juices. They have to suck a lot of it to get the nutrients they need, so they excrete the excess water, sugar, etc. as a sticky liquid called "honeydew." Ants really like to eat honeydew, so they hang around and protect aphids from their natural predators (such as ladybird beetles).

Aphids prefer to feed on fresh, succulent new growth, so they often become more of a problem after a plant has been fertilized.

Anyway, aphids on a potted plant are easily controlled with soap sprays. You can get Safer Insecticidal Soap (which is formulated to minimize risk of damaging the plant), or make your own. Mix 1 teaspoon of a non-detergent dish soap (such as Ivory dish soap) with a quart of water. Spray the bugs with a trigger sprayer. Soap kills them on contact, but it's not a preventative spray, so you should revisit the plant periodically to see if it needs to be resprayed.

Thanks so much for your very clear, informative explanation. It's very helpful. I do have a plant soap solution called Pyola that I'd bought last year, but hadn't used yet. It does specifically say it helps get rid of aphids, so I will try that.

I'm just curious... if I did nothing, would the aphids eventually take over and kill the tree? Also, you were exactly right about their liking fresh new growth, as I did notice they seemed to be concentrated on the newest leaves. Interesting. I love when I can find an exact correlation between a problem and its cause!!

...although... I just thought of something else. Every year my plant develops "fresh new growth", so why did the aphids appear just now, and right after I added the compost, but yet I've never seen them in past years when I had normal new growth? Could it have been the compost itself that somehow attracted them, versus the resulting new growth per se?

Good question. Depending on the source, maybe the aphids travelled in with the compost?

I think that the aphid's arrival was probably just luck (good for them, bad for you). But nutrients tend to extend the time that a plant puts out new growth, allowing more time for the aphids to establish before the plant tissues "harden off."

Anyway, in most cases aphids don't become a real concern for the long-term health of a tree. I would be a little more concerned if the potted plant was indoors, where the aphid predators couldn't find them. If left unchecked, aphids can cause deformity and damage to the new leaves and tender shoots, and unsightly "black sooty mold" can grow on the honeydew.

So just squish them by lightly rubbing the infested area lightly they will be gone soon. Even if they just fall off they cannot return as some of them lose their wings after they are on the branch for a while. Unless Ants are farming them, that is.

Thanks everybody. I doused the tree with Pyola, so hopefully that will do the trick!

One thing about the aphids is that when they feed they excrete a sugary substance called Honeydew which then attract spores which cause the area to turn black (called Sooty Mold if I remember correctly). It's what totally knackered my Malus (Crabapple tree) last year so this year I've watched out for it industriously. Then yesterday I was painting my pergola next to the tree and spotted the distinct curled up leaves and sure enough the damn tree was being attacked by the Blackfly (aphids) again.

With the soapy spray remember to check weekly to make sure the little buggers don't come back to chomp on the new growth. Also remember because of the leaf curling it protects the feeding aphids so you need to open up the leaf when you spray it or they don't get zapped. Another thing to do is try to get ladybirds into your garden as aphids are their version of Macdonalds heh.