J. Davidson 135 



the end of June the numbers of aphids produced on these bushes were 

 very small. In fact the colony on the Euonymus bush placed in the 

 compartment C of the tent made little or no progress, so that results 

 were not obtained for this compartment. 



Now if we compare these results with what happened in the tent 

 when the winged migrants settled on the Broad Beans, we find that in 

 two or three weeks these latter plants became very heavily infested 

 by the rapid reproduction of the aphids feeding on them. 



Also in series A the Rumex plants. Broad Beans, and Poppies infected 

 from Euonymus B with winged viviparous females became heavily 

 infested in a few weeks. 



The plants in series A were all infected with winged viviparous 

 females, with the exception of Rumex No. 2, which was infected with 

 apterous viviparous females. Winged forms were used because it is 

 the winged migrants which infect new host plants. It will be seen, 

 however, that the apterous viviparous females from Euonymus flourished 

 and reproduced in great numbers when transferred to Rumex sanguineus, 

 winged migrants being produced in due course. 



A noticeable feature throughout the experiments was the migratory 

 tendency of the winged viviparous females. Soon after the winged 

 generation was produced on the various plants, the winged forms showed 

 a desire to migrate and collected in vast numbers at the top of the 

 muslin bags which covered the plants. They seemed active and restless, 

 and apparently not feeding on the host plant on which they were pro- 

 duced. However, many undoubtedly produced " lice " on the host 

 plant, and several could usually be found sitting beneath the leaves, 

 but the majority of the winged forms crowded round the top of the 

 muslin cover, even when the host plant was still in a healthy condition. 

 When the top of the cover was opened they immediately took flight. 



It would appear from these observations that the winged females 

 demand a change of host plant, and it is for this reason that certain 

 plants which may be heavily infested early in the summer suddenly 

 become free from aphides, owing to the production of winged migrants 

 which, when they are produced, tend to leave the original host plant. 

 In the case of Aphis euonynii I found hundreds of young nursery Euony- 

 mus bushes smothered with these aphids in May, whereas a few weeks 

 later the bushes were almost free from them. Two or three of these 

 bushes were kept under close observation. On May 23rd, they were 

 heavily infested with Aphis euonymi, many winged forms being present. 

 A week or so later the bushes were almost free from these aphids, and 



