STUDIES OF THE PINK BOLLWORM 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.-- . . . --. -. 
0.35 
.65 
2.94 
5.35 
5.86 
6.05 
0.19 
16 
.60 
26 
3.58 
Sept. 5 
5.86 
15.. . 
6.18 
25 
6.47 
Average . ... . . __ .. 
3.53 
3.81 
Although fewer living 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 
alive 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 bollworm 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 bollworm 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. 
1FFICACY OF CLEAN-UP METHODS 
In the winter of 1922-23 an experiment was conducted to deter- 
mine approximately what proportion of the resting larvae in the 
field are destroyed by cutting and burning the stalks. As pointed 
out in the discussion of hibernation habits (Tables 7 and 8), many 
larvae leave the bolls and enter the soil when the stalks are cut. 
