As a horticulturist one of the most common questions I'm asked is...
"My plant has little black ants running all over it. How do I get rid of them?" This is a classic case of asking the wrong question when looking for a solution.
The questions should actually be... "Why are there little black ants running all over my plants?" The answer is quite an interesting one.
There's one simple reason you may have black ants on your plants - food. They are feeding on something.
The 'something' is likley reasonably immobile whereas the ants are very busy so they tend to be seen while the root-cause is overlooked.
So just what is the 'something'? Many pests that attack our plants are sap-suckers. They feed off your plant's sap and most often this will be around the new growth tips, whether that be leaves or flowers. This is where sap flow is highest, closest to the surface and plant tissue is softest so therefore easiest to penetrate. These pests digest much of what they are sucking and then they excrete, yes, poop or wee out, sugars. Ants love sugar so they come looking for this waste material. The most common pests where this occurs are scales, aphids & mealy bugs.
Sometimes, if you don't have ants, you'll see black powdery material on leaf surfaces. This is called sooty mould and it too lives on the execrated sugars.
Here's the interesting thing. Ants get very clever and very protective when it comes to their sugar-sources. They in-fact look after them like a dairy farmer would their cows. They will move pests around the plant to find better feeding spots and grow the population. They will build little shelters to protect them. They'll move them to underneath leaves where they are protected & then bring them back out. The simple fact is that these pests can cause an enormous amount of damage causing flower buds or young fruit to drop and destroying new growth tips. The speed and severity of damage will be amplified by the ants helping them to thrive. So what you need to be doing is treating the pest, not the ants. Most of these bad critters can be easily knocked-off with one of the quality spray oils. I'm a fan of eco-oil as it also contains a compound that attracts beneficial insects that will feed on the pest, is based on natural, not petroleum, oils and has no withholding period on food crops.
With any of these products you do have to contact and soak the pest for them to work so you need to get complete coverage of the plant. If you're an organic or food gardener just select one that works for your situation. Bear in mind that the bugs with their own protection like scale and mealy-bugs will generally need an oil-based spray to knock them off. Repeat applications may be needed for them to work effectively. If the infestation has happened around flowering or peak growth time then I like to prune afterwards and I will carefully collect each clipping and put it in a bucket for disposal. So... hope this sheds some light on that classic little black ant problem!
Black ants tending to and 'milking' juvenile mealy-bugs in the base of a gardenia flower after the bloom has dropped.Zoomed in to show their busy work!