Aphids are small, soft-bodied insects, which are usually 2 – 4 mm long. Aphids often cluster on young shoots and flower buds or underneath older leaves. There are many different species of aphids which vary in colour from green to yellow and black.
Aphids attack milkweed, fruit trees, roses, camellias, chrysanthemums other ornamentals and a wide range of vegetables. Aphids also transmit virus diseases such as broad bean wilt. Small colonies multiply rapidly in warm weather, and large infestations can develop in a number of days. Check your plants a couple of times a week at the beginning of warm weather so that aphids can be controlled when populations are relatively small. Look out for natural predators such as ladybirds and parasitic wasps.The tiny wasp lays eggs inside the aphid giving it a hard light brown shell like appearance. The ladybird larvae which looks like tiny bird droppings are actually voracious eaters of aphids. Both of these predators will reduce aphid numbers rapidly once they arrive.
Buds may fail to open and leaves are twisted and distorted. New growth may be stunted. Because aphids excrete a substance called honeydew, this provides an environment on which sooty mould fungus can grow. Removing the source of the honey dew will usually solve the sooty mould problem as well.
REF: Yates NZ Website
Because this website is mainly about Butterflies and especially Monarch Butterflies, I'll address the aphid problem on milkweed, the Monarch's food supply.
Aphids must be removed from milkweed plants as soon as you see them, otherwise they will destroy the plant.
Important !! First remove any caterpillars and watch for eggs.
You can trim out portions of milkweed leaves that have eggs on them and place these into a dish, on a dampened paper towel. See more about that here.
If anyone knows of another successful method for controlling Aphids on milkweed, that does not involve using chemicals or anything else that will harm Monarch Butterflies, their eggs or their caterpillars, please do let me know. Contact me here.