Aphids on milkweed

Aphids come in a variety of colors - yellow, red, black, etc. The "honeydew" secreted by aphids is used by ants as a nectar or food source. The ants collect honeydew from aphids like we collect milk from cows. The ants will ferociously defend their "cows.' Even though people are enormous compared to ants, if you disturb a heavily colonized plant the ants will vigorously attack your hand!