INSECTS AND PLANTS 35 



potatoes may be infected with the disease by means of the 

 Colorado beetle. Similarly, Bacillus tracheijMlus, the cause 

 of wilt disease in plants of the cucumber family, can be 

 and has been transferred from plant to plant by the beetle 

 Diabrotica vittata, and the bug, Anasa tristis. A common 

 disease of plants of the genus Prunus results in the leaves 

 curling to such an extent that they are unable to carry on 

 their proper functions, and the fungoid cause of the disease 

 thrives on the sugary secretion of aphides. The scale 

 insects, in the Florida citrus groves, are often attacked by 

 a fungus, Ophionectria coccicola. As these insects pierce the 

 orange twigs with their sucking beaks, the plants become 

 inoculated with the fungus, which sets up a disease called 

 gummosis, and gum, in considerable quantity, oozes from 

 various parts of the tree, much to its detriment. A boring 

 beetle, Xylcbonts perforans, and a weevil, SpfienopJiorus 

 sericeus, are given to making holes in sugar canes, thereby 

 enabling the spores of a troublesome disease to enter the 

 plants. The wood of a certain species of fir tree, Pinus 

 ponderosa, is often rendered almost valueless owing to the 

 fact of its being turned blue by a fungus, Ceratostomella 

 pilifera, and it has been discovered that the fungus enters 

 the tree through the borings of a beetle, Dendroctonus 

 ponderoscv. Some very common and destructive plant dis- 

 eases are known as " smuts," because of the coal-black- 

 coloured spores of the causative fungus. In one such 

 disease, Ustilago violacea, the spores take the place of 

 pollen in certain flowers, and are carried from flower to 

 flower by insects. Other similar examples, in plenty, could 

 be given, but enough has been said to show, in a general 

 manner, how closely related plant diseases are to insect 

 damage. 



Aphides, too, apart from direct damage, are injurious to 

 plants indirectly, for their sugary secretions, called honey- 

 dew, form an excellent medium for the growth of fungi, 

 which, while not parasitic on plants, injure them consider- 



