DIRECTION AND VELOCITY OF WIND. 31 
not live upon this tree. This being the case, dispersion by the wind, 
which is concerned chiefly with small caterpillars, is limited in its 
results by the food plants which the young caterpillars are able to 
reach, and this depends somewhat upon the ability of the larva? to 
crawl. For instance, if caterpillars are blown by the wind and drop 
on cultivated ground or in pastures, they must perish on account of 
scarcity of food. Similarly, if caterpillars lodge in solid pine 
forests (PL XIII) they will perish for want of suitable food. 
In fact, the establishment of the pest in this way depends upon the 
larva? being able to find suitable food. Either the larva? must be 
dropped by the wind on a food plant that is favorable for their de- 
velopment or they must fall within crawling distance of it. In many 
sections of the country the foliage is unfavorable for gipsy-moth food ; 
hence colonies have not become established. 
CONDITION OF FOOD PLANTS. 
The condition of the trees in an infested area has an important 
relation to the liability of the spread of the insect. Unfavorable 
food, coupled with warm weather, naturally stimulates the activity 
of the larva?, and this increases the chances for spread. If there are 
dead trees in the infested area the chances of wind spread of the 
larva? from them are greater than on healthy trees, as the caterpillars 
are unprotected from the sun and wind and move about continually 
in search of food if the temperature is high enough. 
DIRECTION AND VELOCITY OF THE WIND. 
Of all of the factors thus far enumerated perhaps no one is as 
important with regard to dispersion as is the wind. The others 
may be regulated or controlled to some extent but it is impossible to 
do this with the wind. For instance, it is possible to reduce the in- 
festation in any area so that wind spread is nearly precluded, because 
it is dependent upon the factors Avhich have been mentioned, which, 
with the exception of temperature, can be controlled. 
The natural trend of dispersion of the gipsy moth has been toward 
the north and northeast. The spread of the brown-tail moth has 
been chiefly in a northeasterly direction, and the reason attributed 
to this has been that the prevailing wind during July, when the 
moths are flying, was from the opposite direction. Since the aviation 
of first-stage gipsy-moth larva? has been proved, it is evident that 
a part, at least, of the more newly infested area is due to the action of 
the wind. It is also true that the prevailing winds in April and May, 
when the larva? are small enough to be distributed in this way, is 
chiefly from the south and southwest. Table II gives the record of 
wind direction for 20 days in April and May, when the temperature 
