142 BIOLOGICAL PROBLEMS 



half-grown scale, the spraying compound must con- 

 tain an ingredient capable of dissolving the wax 

 which cements the scale to the leaf. Once the defen- 

 sive armour is penetrated, the spray poisons or as- 

 phyxiates, as the case may be. 



Caterpillars, when migrating, are stopped by a 

 trench, which may be made impassable. Otherwise, 

 soil insects, being underground, cannot be reached by 

 " shell-fire ", but must be gassed by injecting or 

 mixing with the soil some substance such as carbon 

 bisulphide or naphthaline. Beetle-borers, in their 

 labyrinthine " dug-outs", may be attacked by spray- 

 ing with a compound which gives rise to an as- 

 phyxiating gas, and gas masks have to be worn by 

 those who apply the spray as in spraying the roof of 

 Westminster Hall. Certain insects, such as conifer- 

 feeding sawflies, cannot, as a rule, be touched by 

 sprays; these may perhaps be controlled by winged 

 parasites aeroplanes ! 



Consider first of all the remedies to be employed 

 our munitions, as it were, in this mimic warfare. 



At a time like the present it is best to choose certain 

 general preventive remedies to check the more trouble- 

 some pests: lime-sulphur winter washes for fruit 

 trees to check fungi or insects awakening from winter 

 sleep; mixtures containing lead chromate or lead 

 arsenate in spring to protect fruit trees from green- 

 fly and caterpillars; and an efficient fungicide. Cura- 

 tive treatment is useless in the war against potato 

 blight. Unlike the American gooseberry mildew, 

 which simply adheres to the skin of the plant, potato 

 blight is caused by a parasitic fungus, Phytophthora 

 infestans, known in 1845 under the name of Botrytis 

 infestans, which actually effects an entry into the 

 plant, absorbs food, and in the act of doing so 

 poisons and destroys the tissues. The conidia or the 



