J. Davidson 141 



Throughout my experiments it was observed that, when the plant 

 had finished its young active growth, or became heavily infested with 

 aphids, the changes resulting cither in the quality or cjuantity of the 

 cell sap (or both) seemed to induce the production of winged forms, 

 which wanted to migrate to other plants. 



It may be that in the case of the Compartment A in the tent, the 

 Shirley Poppies and Broad Beans afforded sufficient food, and thus the 

 Mangolds and Beet did not become heavily infested. 



Gaumont (1913)^ records a heavy infestation of Beet by Aphis 

 euonymi in the South of France during 1911. 



Theobald (1912, p. 471, oj). cit.) records migration flights of Aphis 

 rumicis in the South of England during 1911. Poppies became very 

 heavily infested, and then when these became " seedy," masses of 

 winged migrants were distributed over many different plants, but only 

 on Dahlias, Beet, and Mangolds did they flourish to any great extent. 



It would seem that under natural conditions winged migrants 

 of Aphis rumicis are produced on the host plant, and they take flight, 

 being carried partly by their own powers of flight, partly by the wind, 

 to many different species of plants. On some plants such as with 

 Onions in these experiments, they soon die oft', on others they live 

 happily, and form small colonies, while on others they reproduce in 

 enormous numbers and heavily infest the plants. In the latter case, 

 it would appear that the cell sap is best suited for the particular species 

 of Aphis and affords the necessary stimulus for the rapid reproduction 

 of young. 



It remains yet to be proved whether the stimulus derived from the 

 cell sap of the previous host plant has any influence on the degree of 

 infestation of the succeeding host plant. 



Some seasons are much more favourable than others for the distribu- 

 tion of aphids. The present season of 1913 has been a particularly 

 favourable one for Aphis " blight^." Winged aphids are very fragile, 

 and, if the weather conditions are wet and severe, the winged migrants 

 are unable to withstand the journeys from plant to plant. 



In favourable seasons the distribution of aphids over a district from 

 plant to plant may be very extensive. 



^ Gaumont, L. (1913), " Le Puceron de la betterave," Revue de Phytopathologie, i. 

 No. 1, April 20th, 1913, pp. 12-13. 



- Gardeners' Chronicle, London, 7th June, 1913, p. 377. 



10—2 



