Aphids feeding on Fennel, they are very common sap-sucking insects that can cause a lack of plant vigour, distorted growth and often excrete a sticky substance (honeydew) on foliage which allows the growth of sooty moulds. Some aphids transmit plant viruses. There are a number of different growth stages in the picture ranging from L1 larvae to adults, the white objects are the moulted skins remaining after ecdysis