There are aphids in all kinds of shapes and sizes. A distinction can be made between different types of aphids, mealybugs, soft scale and scale insects. What all aphids have in common is that they feed on plant juices, causing cosmetic damage to the plant. Aphids also inhibit growth and cause leaf deformation. Aphids can introduce toxic substances into the plant via their saliva, which cause deformation and/or discoloration. Leaf fall is also often a result of the infestation, and winged aphids can transmit pathogenic viruses.
All types of aphids, with the exception of scale insects, produce a lot of honeydew. Plant sap is rich in sugar, but poor in protein. As a result, the aphids have to absorb a lot of juice in order to get enough protein. The excess sugar is excreted by the aphids in the form of honeydew, making the crop very sticky. Black sooty moulds (Cladosporium spp.) can grow on the sugars, which infect (it causes cosmetic damage and not a real infection) the crop. An Anthurium pot plant, for example, becomes unsaleable as a result of this.
Aphids have a weak, oval body and are therefore more vulnerable and easier to control than scale insects and mealybugs. Scale insects and soft scales have a hard shield-like cover, under which the females lay their eggs. The female scale insects are constantly attached to the plant and do not move around as the mealybug can. However, the larvae of scale insects are able to move. This is the moment when they are also easy to control, because they have not yet formed a shield at this time.