266 THtJ PRACTICAL APPLICATION OF ECOLOGY 



Observations conducted by Frew (1924) at Rothamsted likewise 

 show that time of sowing has a direct bearing upon the infestation 

 of barley by the gout-fly (Chlorops tceniopus). The comparative 

 immunity of early sown spring barley from the attacks of this 

 insect has been noted by previous observers, but apparently 

 without any experimental work having been conducted. In 

 Frew's experiments an area as uniform as possible was divided 

 into a series of plots which were sown with barley at successively 

 later dates. Although Frew's experiments were hampered by 

 heavy soil conditions, they nevertheless demonstrate the 

 advantages to be derived from sowing of spring barley before the 

 end of March (Fig. 73). Reference to the graph shows that 

 exceptionally high infestation figures were obtained for three of 

 the plots, which appear to have been due to retarded plant growth, 

 owing to the caking of the soil on the particular areas concerned. 



The ordinary culinary varieties of the broad bean ( Vicia faba) 

 are well known to suffer from heavy attacks of the aphid Aphis 

 rumicis over most parts of Europe. It has been previously 

 pointed out on p. 241 that this insect finds ideal conditions for 

 prolific agamic production on the growing, sappy, terminal shoots 

 of that plant. It has long been the practice of growers to sow the 

 seed as early as possible in the season, or even in the preceding 

 autumn. The advantage gained is in the advanced condition of 

 growth attained by the crop by the time the aphid becomes 

 prevalent. From among experiments conducted by Davidson at 

 Rothamsted, the reproduction of aphides on broad beans sown on 

 March 15th and April 27th was compared. In the former case 

 the mean number of young produced at the end of fourteen days 

 by single apterous females was 548, while on the later sown plants 

 the corresponding reproductive figure was 1,341. 



In 1918 Hopkins, in the United States, advanced the hypothesis 

 now generally referred to as the Bioclimatic Law. His contention 

 has a phenological basis, and it formulates the rule that such 

 events as the opening of the buds of a particular plant, dates of 

 appearance of species of insects, first dates of flowering, and other 

 phenomena, occur in North America according to a more or less 

 definite law. Phenological events of this kind occur later in 



