Some species inject toxic salivary secretions into plants during feeding. As aphid populations develop, infestation sites become littered with cast skins. Many aphid species secrete a sticky substance called 'honeydew' which is similar to sugar water. This energy-rich anal secretion falls on leaves and other objects below the infestation. As a result, sunlight is unable to reach the leaf surface, restricting photosynthesis that produces the plant sugars.
Honeydew also attracts ants, flies and other insects.
Some species are heavily dependent on ants for survival and dispersal. The honeydew-loving ants 'tend' the aphids and prey upon natural enemies of aphids and otherwise unhealthy aphids. Ants also carry aphids to uninfested parts of plants. Aphids are important vectors of plant diseases, particularly viruses. However, any aphid may be considered a potential pest when conditions are favorable for reproduction.