THE BLACK FLY OF CITRUS. 
13 
to be as follows, the count of the leaves being made from the bottom 
to the top of the shoot. 
Table II. — Eggs of black fly on 19-inch orange shoot at end of dry season. 
LeafNo 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24. 
Egg spirals 0321241015012000000000 0. 
■,-%* This shows that the rate of the infestation of young growth on a 
tree is dependent (1) on the degree of infestation of the older leaves 
on the tree which, in a large measure, determines the number of 
adults present on the young growth; (2) on the season. This is 
discussed further under the heading, "Seasonal history," page 42. 
For an idea of this tree at the times these counts were made see 
the photographs (PI. I). 
That the degree of infestation or rate of spread on a given plant is 
determined by the species of plant is shown by Tables III and IV for 
the lime trees at Ancon. 
Table III. — New egg spirals of the black fly on old and new lime 
I 1919. 
, experiment No. 8, 
Leaf No. 
May 17. 
May 24. 
May 31. 
June 7. 
June 14. 
June 21. 
June 28. 
July 5. 
1 

1 




1 








1 



1 
2 
1 





1 
2 
2 

3 


3 

1 
1 
1 

3 
1 

1 

1 

2 
1 


1 
1 
1 
1 

1 
3 

1 
2 


1 




1 

(?)3 
2 
1 

2 




3 



4 







o 
2 
2 
3 
4 
o 
5 
o 
6 
o 
7 
o 
8 
1 
A*. ... 
B 

C 
1 
D : 
o 
E 
1 
Total 
2 
5 
14 
10 
11 
7 
7 
7 
i The leaves on the new growth were lettered A, B, etc., upward from the junction of the new growth 
E with the old and the leaves of the old growth were numbered 1, 2, etc., upward, starting with the junction 
Jfof a much older shoot or branch. 
The observations on this shoot were begun on April 10, 1919, at the 
end of a dry season, the rainy season beginning on April 13. 
Table IV. — New egg spirals of the black fly on old and new lime leaves, experiment 9, 1919. 
Leaf No.i 
Apr. 17. 
Apr. 26. 
May 3. 
May 10. 
May 17. 
May 24. 
May 31. 
June 7. 
June 14. 
June 21. 
June 27.* 
1 „ 
1 
2 

1 
1 

1 



















1 



1 

1 

1 
2 
2 
1 

1 
1 

3 







1 
2 



1 






5 
5 
2 
7 
1 

4 






2 
1 

1 
1 
1 

1 

1 




1 
x 
2 
1 
3 

A» 
o 
B 
o 
c 
o 
D 
1 
E 
o 
*F 
2 
#6... ::::.: 
1 
H 
1 
I 
1 
J.-..„ 

1 In this experiment the lettering of the young growth and the numbering of the old are the same as in 
the preceding. 
J This experiment was accidentally destroyed by the pruning of the trees on June 30. 
8 This shoot started to grow on May 3, but no eggs were observed on it until May 24. 
