This weekend I found bushes with tons of spirea aphids on them. In some cases 50% of
the growing tips had clusters of the light green aphids. The plants most affected seemed to be in full sun. Shaded ones had fewer. Heavy infestations on landscape or nursery plants can cause twisted or deformed foliage. Especially in the landscape predators may take care of them pretty quickly. In the nursery though you may not be able to wait, or risk damage, if you are moving plants out for sale. In many cases horticultural oil, soap, or even a stream of water is enough to reduce populations of these exposed aphids. Other insecticides can be found on the aphid note.