STUDIES OF THE PINK BOLLWORM IN MEXICO 27 
this out also. The field in the first group that showed a damage of 
32 per cent and another that showed only 3 per cent were both 250 
meters from the 1920 fields. Loftin made some counts on this plan- 
tation in 1920, and some of his most heavily damaged fields were 
near fields that showed little damage in 1921. 
A relatively heavy infestation was frequently noticed at the edge 
of a field. This would indicate either migration from a near-by field 
or possibly greater concentration of moths at the edge of the field 
on the side toward which flight directed them. The more or less 
simultaneous beginning of the infestation in different parts of large 
fields, noted at Tlahualilo in 1921, must be attributed either to a 
rather general flight of the moths first emerging in the spring or in- 
effectual fumigation of planting seed. As long as such possibilities 
remain, the data on distribution of the infestation can not be con- 
sidered solely with reference to fields of the previous year or seed 
storehouses as sources of infestation. 
VOLUNTEER COTTON (ZOCA) 
Under favorable conditions in the Laguna district, volunteer cotton 
sprouts from stalks of the previous year to such an extent that a 
considerable crop has often been produced on such fields. This 
cotton is commonly called "zoca" and its destruction in the spring 
has in recent years been required by the Mexican Government in its 
program of pink bollworm control. On account of the extreme 
shortage of irrigation water in the fall and winter of 1921, the growth 
and cultivation of zoca was permitted in 1922 in order to offset to 
some extent the small acreage that could be planted. 
The influence which zoca has on the damage of the pink bollworm 
to planted cotton probably depends greatly on seasonal conditions. 
With early zoca and late-planted cotton, early food is furnished the 
worm, and greater numbers of the insects are present when the 
planted cotton becomes subject to attack than if the zoca is kept 
down, causing many of the early emerging moths to die without 
finding cotton on which to deposit their eggs. On the other hand, if 
the zoca and the planted cotton begin fruiting at about the same time, 
the former, being on the insects' hibernating grounds, is attacked 
first and may retard the infestation in the planted cotton. But in 
that case a rapid increase in the infestation of the planted cotton may 
be expected when moths in the zoca become so abundant that they 
begin to seek other cotton. 
In referring to the records of nonpickable cotton for 1922 (Table 
17), it is noted that the percentage nonpickable for zoca is about the 
same as that for the irrigated planted cotton and much higher than 
for the unirrigated planted cotton. Also many more green bolls were 
left on the zoca than on the planted cotton at the time of the exami- 
nations. Ordinarily the stand of zoca is very poor and it has a heavy 
late growth, which accounts for a high percentage of nonpickable 
cotton. Although infestation develops early in zoca, it does not 
necessarily follow that severe damage occurs much earlier than in 
planted cotton. This is indicated in Table 22, in which the infesta- 
tion of a field of zoca is compared with the average for three fields of 
planted cotton on a plantation near Torreon in 1921. 
