1914] Natural Control of Toxoptera. 85 
Syrphids become overly abundant in the field, the parasites 
find them and they so reduce their number that they are of 
very little value. 
In conclusion, it might be stated that the cause of a bad 
outbreak of Toxoptera in South Africa is due to the same 
causes as a bad outbreak in the United States. If the early 
winter months are abnormally cold, while the middle months 
of the winter are warmer than the average, followed again by a 
cold spring, there is a long period of from five to six months 
during which time Toxoptera breeds more rapidly than the 
ladybirds or Aphidius, the result being a bad outbreak of 
Toxoptera. 
Another factor which sometimes tends to cause bad out- 
breaks is long periods of drought since, under such conditions, 
plants are not able to withstand the number of Toxoptera 
which normally they could carry without showing any ill effects, 
thus dying, not entirely from injury by Toxoptera, but, with 
drought as a secondary factor. If such land be irrigated, the 
plants are enabled to survive the attack. 
