PLANT DISEASES. 85 



forms of life are distributed in the top three inches of soil, and 

 thus within easy reach of the plough. The layers between 3 and 

 5 inches contain small numbers, and below 5 inches the numbers 

 rapidly decrease until at a depth of 9 inches the numbers are 

 neghgible. A census of the soil population has been taken, and 

 shows that the numbers per acre present in the soil down to a 

 depth of 9 inches is dependent on whether or not the land has 

 been dressed with farmyard manure, the manured plot containing 

 three times as many organisms of this type as the unmanured 

 plot. The vast majority of the animals are non-injurious. In 

 the arable land studied the chief injurious insects are wireworms, 

 leather- jackets and the larvae of the swift moth. These are 

 present in almost equal numbers in the manured and unmanured 

 plots, showing that the continued use of farmyard manure does 

 not materially affect the numbers of these injurious animals. 

 Another definitely injurious group is the millepedes, which are 

 frequent pests of potatoes, roots, &c., and in this case the 

 manured plot contained more than double the numbers present 

 in the unmanured. The numbers of earthworms present in the 

 soil was found to be many times more numerous than had 

 previously been supposed. 



The work is now being extended to another of the Rothamsted 

 experimental fields, which is divided into a large number of 

 plots manured in different ways. It will thus be found whether 

 different manurial dressings or methods of cultivation have any 

 marked effect on the higher forms of animal life present in the 

 soil, particularly in regard to the pests harmful to crops. 



The soil at Rothamsted is a loamy clay, and it does not follow 

 that the distribution of insect Hfe will be the same on other types 

 of soil. The same methods of investigation are now being 

 carried out by other Institutions, representatives of which have 

 visited Rothamsted to discuss the subject, and it is hoped that 

 comparative information will thereby be obtained for soils of 

 a different character. 



Plant Lice or Green Fly (Aphididse). — Aphids, as is well- 

 known, are amongst the worst enemies of the farmer and fruit- 

 grower. With few exceptions all plants are susceptible to attack 

 by one or another of these pests. Hitherto the methods of 

 repression have been limited to the application of sprays, but 

 this is expensive, for the spraying needs to be carried out at 

 frequent intervals owing to the extraordinarily rapid multiplica- 

 tion of the aphids, complicated by migrations from other plants 

 on which they can live. There is urgent need for a thorough 

 study of the various members of the aphid family, to find out 



