﻿STUDIES OF THE PINK BOLLWOEM IN MEXICO 43 



Table 35. — Average number of worms per green boll in cultivated and check plats 



Date of examination 



Plats 2 

 and 3 

 (culti- 

 vated) 



Plats 1 

 and 4 

 (check) 



Aug. 6 



16 



26—:... 



Sept. 5 



15 



25 



Average 



0.19 

 .60 

 3.58 

 5.86 

 6.18 

 6.47 



3.53 



3.81 



Although fewer Hving larvae were found in the cultivated than in 

 the check plats, there were also fewer dead ones. The proportion 

 of living to dead was about the same in the two cases. 



HEAVY WINTER KILLING IN WET SOILS 



Earlier in this bulletin attention was directed to the fact that the 

 survival of larvae in the soil decreases as the amount of moisture 

 increases. In unirrigated plats 12.8 per cent of the larvae were 

 ahve or had emerged as moths during May and June, while in several 

 irrigated plats no larvae whatever survived the winter. This heavy 

 mortality in wet or soaked soils during the winter indicates 

 perhaps one of the most important possibilities of control of this 

 pest wherever, through irrigation or the occurrence of winter rains, 

 the soil becomes and remains for considerable periods thoroughly 

 moistened. Under such conditions it seems probable that all of the 

 pink boUworm larvae entering the soil for hibernation will be killed. 

 On the other hand, it is known that the larvae in cotton bolls, either 

 on standing plants or on the surface of the ground, survive the winter 

 in large percentages. Such opportunity of carriage of the pest 

 over winter can be very largely eliminated by thorough cleaning 

 in the fields of all cotton plants, scattered bolls, or other rubbish, 

 and the burning of such material. It would appear, therefore, that 

 under the moisture conditions indicated and the thorough cleaning, 

 a method of effective control will be available for irrigated districts 

 and others where the winter rains are adequate to hold the soil 

 fairly moistened for a considerable period. Undoubtedly, the effec- 

 tiveness of the clean-up measures which have been carried out in the 

 United States in the effort to eradicate the pink boUworm has been 

 due quite as much to the mortality of the larvae in wet soil as to the 

 thorough collection and destruction by burning of all old plants and 

 scattered bolls. This is particularly true in southeastern Texas 

 and in Louisiana, where the winter rains are heavy. 



KFFICACY OF CLEAN-UP METHODS 



In th(5 wint(;r of 1922-2.3 an experiment was con<hictcd to deter- 

 mine approximately what prof)()rtion of the resting larvae in the 

 fiehl are destroyed by r^utling iirid })urning tlxs stalks. As pointed 

 out in the discussion of hiljcrnation habits (Ta])les 7 and 8), many 

 larvae leave the bolls and enter the soil when the stalks are cut. 



