APHIDES. 



239 



destructive to some of them. The Cotton Aphis is dealt with above 

 (page 110). Other species attack tur and leguminous plants, 1 mustard, 

 rape and cruciferous crops, 2 sorghum and maize, wheat, indigo, etc. All 

 are destructive when circumstances are favourable, and a long spell of 

 cloudy weather will bring them out upon the crops. 



An important feature in these attacks is the vigour of the plant ; a 

 strong healthy plant resists attack and is less liable to it ; an exotic 

 plant, not fully acclimatised, suffers more than a hardy indigenous 

 variety. Crops growing under bad conditions, with too much moisture or 

 on exhausted land, suffer far more than the same plants growing under 

 good conditions. As a rule, an attack of plant lice is produced by these 

 causes, and only a removal of them effects a cure. 



The artificial remedy for plant lice is spraying, a perfectly efficient 

 and simple business which should be carried out on experimental farms. 

 It is not at present possible to introduce it into general use and will never 

 be, until the cultivator is able to watch for his pests and check them at 

 the start. A field once thoroughly infested with aphis can be cured only 

 by a considerable expenditure in labour and insecticides, an expenditure 

 exceeding two rupees an acre under the best possible conditions. The 

 actual destruction of the first colonies of the plant lice by spraying is a 

 matter principally of watchfulness, the necessary materials costing but 

 very little ; but this requires an intelli- 

 gent foresight and much care in finding 

 and thoroughly destroying the first colo- 

 nies. As a rule nothing can be done to 

 check plant lice because the matter is 

 not taken in time ; the adverse climatic 

 conditions cannot be altered; the vigour 

 of the plant must be maintained in 

 every possible way and so far as possible 

 the first colonies of plant lice destroyed. 



Further investigation of plant lice is 

 required to discover their alternative 

 food-plants and generations, as they live 

 only for part of the year on crops and 

 probably have wild food-plants at other 

 seasons ; it may be possible to check them 

 by the destruction of these food-plants, 

 this being done at a time when the attacked crop is not growing. Plant 



1 83. Aphis cardui. Linn. (Aphidse.) 



2 162, Aphis brassier. Linn. (Aphldee.) 



tV ;i ! (>///' 



FIG. 2S7. 



The scale enclosing the young 

 Mealy Wing. (Much magnified.} 



