THE CANADIAN ENTOMOLOGIST 



55 



Observations on the Number ok Aphids Destroyed by Svrphid Larv.'E. 



In order to obtain specific data on the number of Aphids destroyed by a 

 single lar\a during the larval stage, newly-emerged larva were isolated on 

 plants and a count kept of all aphids consumed. This was done by adding 

 fresh aphids eack day, in the numbers likely to be eaten. No count was kept 

 of the aphids which were born on the plants, but these would not be numerous. 

 However, it may be taken for granted that the figures given in the accompanying 

 table are slightly below^ the average, although they are the average of numerous 

 experiments conducted. 



t.\ble showing duration of immature stages and number aphids destroyed. 



Note. — The laiva were fed upon Aphis pomi and .A. malifolise in these experiments. 



As shown Ity this table there is considerable variation, between species, 

 in the total average number of aphids destroyed, and in the average daily con- 

 sumption. 



It may be noted that the number of aphids eaten or destroyed by a larva 

 will depend, to a great extent, upon the abundance of food and the size of the 

 aphids. In the field it was found that where food was very plentiful, the larvae 

 often killed two or three aphids in succession, but did not suck them dry. Also, 

 in such cases, the larval and pupal stages appeared to be of shorter duration. 

 As the development of the larva depends to a very great extent upon the 

 abundance of food, it is safe to state that such abundance stimulates the insect 

 to more rapid maturity. This fact was clearly demonstrated in the insectary, 

 as the duration of both the larval and pupal stages was markedly shorter in 

 cases where larvae were reared on aphid colonies, rather than upon a certain 

 number of aphids per day. 



Again, the time of the year has a great effect upon the development of the 

 larvae. In warm weather growth is much more rapid. Larvae feeding upon 

 colonies of stem mothers are of more importance than those feeding upon sum- 

 mer colonies, as the stem mothers are more productive, and their destruction 

 means that for each one destroyed, possibly millions of aphids will not be brought 

 into the world, as each stem mother is the progenitor of inestimable numbers. 



This brief outline of one phase of the work on the immature stages of 

 Syrphida? merely furnishes the basis for further extensive work. It might be 

 pointed out that besides the determination of the economic value of the various 

 species, considerable information on the systematic classification of the various 

 species and genera is obtained from such investigations. 



