14 



Control of the Greenhouse White Fly 



noted. The limits of the scale stage in the greenhouse were found to 

 vary from 45 days in February to 17 days in July and bore a close 

 correspondence to the temperature (see Tables I and III). There was 

 however some variation with the different species of plants, since in one 

 experiment when a variety of plants were infested on April 23-25 and 

 the eggs on all hatched from May 5-8 the duration of the scale stage was 

 on fuchsia 26-31 days, on thick-leaved zonal geranium 24-30 days and 

 on tomato, potato, calceolaria, and thin-leaved variegated geranium 

 21-29 days. 



Eggs 



Age of Scales in Days 



Diagram I. Showinj? the time spfent in the four larval stages by A. vaporariornm on French 

 beans at optimum temperature (mean 74° F.). TJie maximum number moult where 

 the curves cross at 50 % . 



In July young bean plants, known to be free of the insect, were taken 

 one each day for 19 days and exposed for 24 hours to a large number of 

 adults. These were then cleared off and the plants were kept in a fly- 

 free muslin cage. When emergence of adults commenced on the plant 

 first infested the series was stopped, each plant holding developmental 

 stages of the same age from egg to adult. A portion of each plant holding 

 200-450 scales was then examined and each scale was assigned to its 

 particular instar, the condition of 4900 scales being thus noted. The 

 percentages in the various stages on each plant are shown in Diagram I. 

 The mean temperature during the experiment was 74° F. with a range 

 of 54-193°. Hatching began on the 8th day after oviposition and was 

 practically complete by the 10th day. The figures showed that on the 



