1914] Natural Control of Toxoptera. 8 
OO 
die in about seven days, it is found that the fifty adults would 
have produced about 500 young during the first two or three 
days after the egg of the parasite had been laid in their bodies. 
These young would have a very small chance of being destroyed 
by the parasite and would reach maturity after the death of 
their mothers or about three or four days before the offspring of 
the Aphidius which had “‘stung”’ the 200 Aphids, had emerged. 
During these three or four days, they would be producing 
young, so that when the Aphidius emerged, there would be 
between 2000 and 2500 Toxoptera on the plant. 
When the offspring of this female parasite emerged, however, 
the chances are that all the Toxoptera would be destroyed. 
From the above, it will be seen that starting with one Aphidius 
and 200 Toxoptera on one plant, there would be at the end of 
about fourteen days, 2000 or 2500 Toxoptera, while the plant 
would not be entirely clean of Toxoptera until about twenty 
days had elapsed. On the other hand, if an adult ladybird had 
been introduced with the 200 Toxoptera, every individual 
would have been cleaned away from the plant in ten days at the 
most, while it probably would have been within five or six days. 
When the ladybirds are present in the field with Aphidius, 
another fact must be remembered, namely, that if Aphidius 
has “stung’’ a number of Toxoptera and a ladybird later 
ate these parasitized Toxoptera, the ladybird is also destroy- 
ing the parasites as well as the Toxoptera. This is even 
carried further by the ladybirds, inasmuch as they will eat the 
parasitized forms even when the Toxoptera is dead, and the 
parasite is in the pupal stage. It is no doubt due to this 
reason that one does not find wheat plants covered with para- 
sitized forms in the field in South Africa, as one does in the 
United States. When a number of infested wheat plants were 
enclosed with wire netting so that Aphidius could gain entrance, 
but a larger insect, as a ladybird could not, it was soon found 
that the wheat plants were crowded with parasitized forms 
similar to those described by Webster as occurring in the 
United States. There seems to be but little doubt. that in 
South Africa, the ladybirds are of more value in combating 
Toxoptera than Aphidius. This is of particular interest 
inasmuch as an attempt is now being made to introduce A phid- 
tus testaceipes into British East Africa where Toxoptera is 
found near Njora. 
