July 7, 1923 
Platygaster vernalis, Hessian Fly Parasite 
39 
CERTAIN ECOLOGICAL CONSIDERATIONS 
Various factors in the environment of Platygaster vernalis have a marked 
effect in modifying its normal rate of multiplication and help to produce 
a correspondingly high or low annual rate of mortality. 
By making a series of examinations of Hessian fly forms taken from one 
certain field or farm throughout the season, it has been possible to obtain 
interesting data bearing on this point. The individual puparia in each 
examination were selected impartially as they were met with, while the 
wheat tillers were being inspected for them. They were dissected under 
the binocular microscope and the contents of each carefully classified. 
Table III shows the results obtained during the summers of 1918, 1919, 
1920, and 1921. The same results are graphically represented in figures 
4, 5, 6, and 7. The collections were made on a farm at Carlisle, Pa., in 
1918, on a farm at Mount Holly Springs, Pa., in 1919 and 1920, and on 
a farm at New Windsor, Md., in 1921. 
Table III .—Mortality of Platygaster vernalis , as observed in collections of Hessian fly 
forms made at intervals during the years 1918, 1919 , 1920 , and 1921 
1918: FIVE COLLECTIONS FROM FARM, CARLISLE, PA. 
Date of collection. 
Fly forms 
examined. 
Results of examination of Hessian fly forms. 
Contain¬ 
ing living 
P. vernalis. 
Contain¬ 
ing dead 
unrecog¬ 
nizable 
matter 
and dead 
P. vernalis. 
Otherwise 
parasitized. 
Living un¬ 
parasitized 
fly forms. 
Mortality 
of 
P. vernalis. 
Number. 
Per cent. 
Per cent. 
Per cent. 
Per cent. 
Per cent. 
May 7. 
58 
79-31 
O. OO 
O. OO 
20. 69 
O. OO 
June 17. 
184 
76. 09 
IO- 33 
4.89 
8. 69 
4. 06 
24 . 
52 
59.62 
15- 38 
7.69 
I 7 - 3 I 
24.83 
July 19. 
18 
44.44 
. OO 
50.00 
5 - 56 
43-96 
Aug. 27. 
8l 
14. 81 
28. 40 
S 3-°9 
3 - 70 
8i- 33 
1919: NINE COLLECTIONS FROM FARM, MOUNT HOLLY SPRINGS, PA. 
May 27. 
i 39 
57 - 55 
1.44 
0. 00 
41. 01 
0. 00 
June 5. 
100 
46. 00 
4. 00 
4. 00 
46. 00 
20. 07 
12. 
no 
28.18 
15. 46 
8. 18 
48. 18 
51-03 
July 11 . 
100 
23. 00 
21. 00 
36. 00 
20. 00 
60. 03 
21. 
100 
22. 00 
23. 00 
46. 00 
9. 00 
61.77 
Aug. 7. 
150 
12. 67 
27 - 33 
52.67 
7 - 33 
77.98 
14 . 
200 
11. 00 
25.00 
60. 00 
4. 00 
80. 89 
Sept. 25. 
200 
4. 00 
29. 50 
62. 50 
4. 00 
93-05 
Oct. 10. 
200 
3 - 5 o 
27. 50 
66. 50 
2. 50 
93-91 
1920: FIVE COLLECTIONS FROM FARM, MOUNT HOLLY SPRINGS, PA. 
June 10. 
14 
186 
51.00 
7-5i 
6. 00 
40. 86 
7 . 00 
50.55 
36. 00 
1. 08 
0. 00 
8S- 27 
Aug. 31. 
200 
9. 00 
5 1 * o° 
38.00 
2. 00 
82-35 
Sept. 8. 
100 
4. 00 
5 1 - 00 
45.00 
. 00 
92. l6 
Nov. 27. 
100 
2. 00 
55- 00 
42. 00 
. 00 
96. 08 
