Hello Susan,

There is very little research and remediation for ants on rhubarb, because they are not primary predators. They feed on sugary or rotting damage to the rhubarb plant. Common situations are insects (like aphids) that feed on rhubarb leaves and secrete sweet liquids. Another common ant attractor on rhubarb is crown root rot. Ants feed on the decaying material of the plant crown. Here is an article about aphids and ants. https://ask2.extension.org/kb/faq.php?id=756760

Here is an article on rhubarb care, including a description of crown rot. https://alaskamastergardener.community.uaf.edu/2022/01/13/what-is-wrong-with-my-rhubarb/

While there has been some feeding on the rhubarb leaves, the immediate issue seems to be physical abrasion to the stalk causing sugary liquids to ooze from the plant. The ants are feeding on that fluid secretion. Now let’s talk about why. This looks like some sort of physical damage. Was that string tying the stalks to that stake? Was there lawn mower or weed whip damage to the stalk? Could there be wildlife damage of the stalks? (This would require chicken wire around the plant to protect it.) Was there physical damage to the plant during transplanting?

The planting situation does not seem to be optimal for the rhubarb:

It looks like soilless growing medium that the plant was in and it was just placed in a well cut out of the grass. This can cause the bathtub effect in which the highly permeable soil around the roots collects water that cannot permeate the surrounding soil, resulting in wet roots and potential root rot. This can be exacerbated by how much run off comes out of the downspout near the plant base.

Plants like rhubarb should not be placed in a cutout of sod. If you continue to manicure the grass around the plant. You are likely to damage the stalks (possibly what happened).

If cheerful, rhubarb can grow to 3 feet in diameter or more. Where the plant is placed does not give it the area to grow to that size. I would allow at least 4 feet by 4 feet for a rhubarb plant that is just bare soil or mulch.

It would probably be best to move your rhubarb to a happier place before it dies completely. Rhubarb likes soil with good drainage and considerable organic matter. Here are some tips for successfully growing rhubarb. https://ohioline.osu.edu/factsheet/hyg-1631

I hope this helps.