14 Farmers’ Bulletin 1262. 
part of the season), which may prevent the development of fall 
broods of the weevil. Birds are also agents in the destruction of the 
boll weevil, a fact which has been fully treated in the publications 
of the Biological Survey of this department. 
VARIATION IN NATURAL CONTROL. 
Winter killing during hibernation and summer killing due to heat 
are by far the most important types of natural control. If it were 
not for these agencies it would be impossible to raise cotton in the 
presence of the boll weevil. All control agencies, however, vary 
widely from field to field and season to season in their effect, and this 
explains the extreme variation in the amount of injury caused by the 
boll weevil. A mild winter followed by a rainy, cloudy summer 
favors rapid multiplication of weevils and the damage to the cotton 
crop is correspondingly increased, while the reverse weather condi- 
tions may practically prevent damage to the crop. 
DISSEMINATION. 
The boll weevil moves from place to place by flight. Although 
it is a weak flyer compared with many insects, it has been known to 
cover a distance of more than 40 miles in a very short time. Its 
flight can not be prolonged, but successive short flights, especially 
in connection with favorable winds, often carry the insect consider- 
able distances. This is the case, however, only during the so-called 
dispersion period, which extends from about the middle of August 
to the end of the season. During the rest of the year the weevil is 
little inclined to fly. There is always a movement from fields in all 
directions in search of hibernating quarters in the fall and a cor- 
responding movement from such quarters to the cotton fields in the 
spring. When the insects reach cotton fields in the spring there is 
little further movement until the general dispersion begins. Ordi- 
narily between the middle of August and the 1st of September the 
weevil seems to be seized with the instinct to migrate. It was thought 
at one time that this movement was forced by excessive reproduction 
and took place only when all squares and bolls, or the majority of 
them, became infested. Investigations have shown, however, that 
the dispersion takes place frequently when the fields are only slightly 
infested. In other words, the insect has a well-developed instinct 
for extending its range into new territory. It is this instinct that has 
caused the extension of the infested area in the United States year by 
year. The weevil is governed in flight by the wind. If there is no 
wind, or only a light one, a weevil is as likely to fly in one direction 
as in another. 
