Ants & Aphids in Plum Trees

I have ants farming aphids in my plum trees, with the result that a couple of the trees have been nearly defoliated. This also happened five or so years ago; at that time, I sprayed the trees with water with a certain amount of ferocity, but early in the morning. This time, I've been spraying in late morning or during the afternoon, because the ants aren't active in the early morning. However, I got to thinking that the damage is being done by the aphids, and I have no idea when they are most active or if it even matters.

So, any thoughts on the best time of day to spray the trees?

Also, any thoughts on how to get rid of the ants reasonably organically? Does diatomaceous earth work? This particular anthill is on a steep slope, which means it's difficult to get at.

Thank you!

When you get rid of the aphids you will get rid of the ants. I'm not sure what time of day is best but I think blasting the trees in the early morning is probably best. Doing it during the heat of the day can burn the leaves.

Try dusting the ant mound with DE. If it rains or gets irrigated you will have to reapply it.

You can try some insecticidal soap against the aphids. A bit more aggressive than just blasting them off.

I have an ornamental plum, not for fruit. I notice a lot more aphids in wet winters. The soil gets saturated below the tree and provides moisture well into the growing season. This means faster, more succulent growth that the aphids love. The past couple of years with greatly reduced water there have been few aphids.
On my fruit trees, there are some aphids- I need to keep them better irrigated for good fruit.

Thank you!

Good way to time the water blast method:
If you know what aphid predators and what types are in your area time the blast to help the predators:

An active predator that will search out aphids, even if they fell on the ground:
Spray right before their most active time.

A sedentary predator, or one that is confined to the plant:
I would be leery of blasting the aphids off, because this type of predator will also get blasted off. If they do not have a way of returning to their preferred location, their services may be lost.

Parasitic helpers pretty much need the aphids to stay on the plant, but for the ants to be removed. Then the parasitic wasps can lay their eggs on the aphids, and be assured of the next generation. I would not blast the aphids under these conditions, but would work on control of the ants.
-----------------------------------------------------------------------
Ant control: If the tree does not touch anything else (fence, another plant...) you can use a product called Tanglefoot around the trunk. This will prevent the ants from getting into the tree. If the plum tree touches something else the ants will use this as another route into the tree, bypassing the Tanglefoot.

Is Tanglefoot still available? I've tried a local nursery as well as online and it's been out of stock.

I am going to put DE down.

Thanks for replying. :-)

I have been picking it up at a local recycling place, not looking for it in stores. If it is not available under the Tanglefoot name, I'll bet a similar product is available. See if googling Sticky Barrier or something will bring up a useful product.