Aphids attacking portulaca grandiflora (moss rose)

Last few days, I had seen aphids attacking the portulaca grandiflora. I had seen one aphid on the portulaca grandiflora (moss rose) in October. I removed it before I potted the plants. The plants were growing well in November. However, in December, I noticed that the plants are not growing well, leaves are turning yellow. I took a good look at the plants and noticed there were white aphids on the underneath of the leaves. I tried to remove as much as possible but after a couple of days, the aphids will return. I wonder where they come from. In December, it was raining most of the time and no direct sunlight reached the plants. I was thinking that the aphids grew rapidly as there was no heat from the sun to deter the population of the aphids.

Aphid

Today, I find two more aphids and already removed them from the plant. I hope the plants can survive this attack.