Many aphides, &c., puncture the leaves, suck out the sap, and induce va:ious local deformations, arrest of growth, pustular swellings, &c., and if numerous all the evils of defoliation may follow.
White or grey spots may be due to Peronospora, Erysiphe, Cystopus, Entyloma and other Fungi, the mycelium of which will be detected in the discoloured area; or they may be scale insects, or the results of punctures by Red-spider, &c. Yellow spots, and especially bright orange spots, commonly indicate Rust Fungi or other Uredineae; but Phyllosticta, Exoascus, Clasterosporium, Synchytrium, &c., also induce similar symptoms. Certain Aphides, Red-spider, Phylloxera and other insects also betray their presence by such spots.
In many cases the punctures of Aphides and Coccideae are shown to be responsible for such exudations, and at least one instance is known where a FungusClaviceps-causes it.
- Many ants feed largely and some almost entirely on the saccharine secretions of other insects, the best known of which are the Aphides (plant-lice or " green-fly ").
Species of aphides may be removed by tobacco infusion, soapsuds or other solutions.