528 



Du Porte (E. M.). Insect Carriers of Plant Diseases. — lltli Ann. 

 Rept. Quebec Soc. Prot. Plants Jrom Insects and Fungmis Dise/ises, 

 1918-1919; Quebec, 1919, pp. 59-65. 



There are many ways in which disease may be spread among plants 

 through the agency of insects, and there is a general similarity between 

 these and those by which disease is carried by insects among men and 

 animals. This paper summarises much of the present knowledge 

 on this subject. The question is raised whether the same organism 

 can normally attack both an animal and a vegetable host ; investiga- 

 tions up to the present have given mostly negative results. An 

 exception, however, is the causal organism of the bud-rot disease of 

 the coconut, a bacillus indistinguishable from Bacillus coli, which 

 lives in the intestines of man and other animals. OpJiionectria coccicola, 

 a fungus infesting scale-insects, has been known to pass from scales 

 to orange twigs, causing gummosis. A case in which the evidence 

 indicates that an insect may serve as host of an organism normally 

 parasitic on plants is that of the beet leaf-hopper, Euttelix teneJhi, 

 which spreads curly-leaf disease of beets in the south-western states. 

 Investigations have shown the necessity of an incubation period in 

 the body of the insects before they can transmit the disease [see this 

 Review, Ser. A, v, p. 492], and they are therefore not merely mechanical 

 inoculators. 



Among the insects that have been proved responsible for the spread 

 of fire-blight {BaciUus amylovonis) are various Rhynchota, such as 

 Aphis pomi, deG., Lygus pratensis, L., Adelphocoris rapidus, Say, Ca)ii- 

 pylommaverbasci, H.S., Orthotylusflavosparsus, Sahib., and Poeciloscytus 

 basalis, Rent., while any sucking insect feeding on infected trees is a 

 potential carrier. The European apple-tree canker {Nectria ditissima) 

 is often spread by means of Aphids. Boring insects that transmit 

 disease include the fruit bark-beetle, Scolytus {Eccopiogaster) rugidosii.i. 

 which is a carrier of fire-blight, and the Longicorn, Leptoslylus macula, 

 which carries the chestnut blight fungus, EndotJda parasitica. Leaf- 

 eating insects may convey disease organisms on their bodies and intro- 

 duce them into the tissues on which they feed. In this way Diabroiicn 

 vittata (striped cucumber beetle) causes the spread of bacterial wilt 

 of cucurbits {Bacillus tracheiphilus), D. duodecimpunctata being a 

 carrier of secondary importance. The larvae of Phytometm (Plusia) 

 sp. disseminates black rot of crucifers, and Leptinoiarsa decemlineala 

 (potato beetle) the brown rot of Solanaceous plants. 



Indirect inoculation may take place, owing to the presence of 

 wounds on the trees, depending largely on the ability of the virus 

 to enter through stomata or water-pores or to pierce the epidermis ; 

 such a case is instanced in the spread of fireblight by bees and flies. 

 The numerous wounds made by insects in plants greatly increase the 

 chances of infection, e.g., egg-punctures of Oecanthus nigricornis 

 (striped tree-cricket) in raspberry canes encourage the growth of the 

 bUght, Leptosphaeria coniothyrium, while in apple the punctures of 

 0. niveus (snowy tree-cricket) encourage the same disease. Curcu- 

 Uonids make j^unctures through which brown rot fungus can enter, 

 and various cane borers encourage the Trichosphaeria disease of 

 sugar-cane. 



The possibility of hibernating insects remaining infective throughout 

 the winter has received considerable attention, and it has been found 



