Ladybirds eat aphids, so if you have plenty of Ladybirds on your plants, that will help.
Aphids, sometimes referred to as greenfly, are small, soft insects with pear-shaped bodies and long & thin with sharply bent legs. They can be coloured brown, black, yellow, pink or green.
Aphids survive during the cooler winter months and are usually hidden within leaf/flower buds. They multiply rapidly during warmer months in spring or summer. Aphids can be usually found in clusters on young shoots, flower buds or underneath leaves.
Aphids are sap suckers. Sap is the plant´s food, nutrient and water circulation fluid. When aphids or other sap sucking insects suck sap from a plant, the plant is weakened and there is a risk of the insect infecting the plant with disease.
Check out the "Rediscover" website for recipes.
If you are a Monarch butterfly enthusiast who is raising Monarch caterpillars on milkweed, then it's a whole different story. Do not spray!
Here's what I do to reduce aphids.
Basically, I wash off the aphids with a strong hosing of water. Only water.
Most of my swan plants are grown in pots. I find it's way easier to manage them this way and you can chop and change plants as the caterpillars eat them.
I remove any caterpillars from the plant, then gently lay the plant and pot down sideways on the concrete. Then I hose the leaves and stems as hard as is reasonable, to wash off the aphids.
I also use my fingers to rub them away, at the same time.
I go over and over each plant until I'm sure there are no aphids left.
Then I let the leaves dry a little. before putting the caterpillars back onto the plants.
I carry this out on concrete, away from the area where the plants will usually grow.
This is because, if you just wash off the aphids in the same area where the plants are growing, ants will most surely bring them all back onto your plants.
Ants 'farm' aphids for the dewy substance they excrete so, if you take the aphids away, the ants will just bring them back. If you hose off the aphids in a different area, the ants don't find the plants so easily.
Yes, the process is time consuming and labour intensive, but its the only 'safe' method I have found that doesn't hurt either the caterpillars or the plants.