Aphids attack a wide range of greenhouse crops including geraniums. Look on underside of leaves, buds and young growth for aphids, their whitish shed skins and honeydew. Foxglove aphids tend to cause more leaf distortion than some other aphids. Foxglove aphids have black lines on the legs and antennae and dark green spots at the rear of their body which distinguishes them from green peach aphids.
Managing Aphids in the Greenhouse on the UConn IPM Web site.