Nature of damage
Depending on the species, aphids can inflict various kinds of damage on a crop: nymphs and adults extract food materials from the plant and disturb the balance of growth hormones. As a result, the plant’s growth is retarded giving rise to deformed leaves or even, if the infestation occurs early enough in the season, the death of young plants. Plant sap has low protein content but is rich in sugars. Aphids therefore need to extract large quantities of sap in order to get sufficient protein. As a consequence the excess sugar is secreted in the form of honeydew, making the crop and its fruit sticky. Black fungal moulds grow on this honeydew, contaminating fruit and ornamental crops and rendering them unsuitable for market. At the same time, photosynthesis in the leaves is reduced, affecting production – the aphid’s saliva can induce strong “allergic” reaction such as malformations of the growing tips – pathogenic organisms and particularly viruses are introduced. Cucumber mosaic virus (CMV) is thus transmitted in cucumber.
An aphid colony has a clear effect on a plant. A growing plant will trans-locate more food materials to an affected part in order to maintain growth, which of course further advantages the aphid colony.