14 CIRCULAR 4 64, U. S. DEPARTMENT OF AGRICULTURE 



extent upon the length of silk attached to its body. Larvae have 

 been captured on screens more than 20 miles from the nearest known 

 infestation, and one larva has been taken on a screen attached to an 

 airplane at a height of about 1,800 or 1,900 feet above the ground 

 (2,000 feet above sea level). The height to which they may be carried 

 varies from day to day and may greatly exceed 2,000 feet. Once lar- 

 vae have reached such heights, they may encounter cross currents of 

 air and be blown in the direction opposite to which surface winds are 

 blowing. 



Transportation of larvae by the wind is, however, less serious than 

 it would appear. The farther the larvae are carried the more widely 

 they are separated and the less chance they have of falling close 

 enough together to establish infestations. There must be a large 

 number of such larvae present to offset natural mortality. Millions 

 of larvae, however, may be blown out of an infestation, fall nearby, 

 and succeed in spreading the infestation short distances. That 

 undoubtedly is why the infestation in New England has progressed 

 only a few miles per year. Were it not for the tremendous natural 

 mortality of larvae, the insect would have become established in new 

 localities far more rapidly than it has. For this reason special efforts 

 are made to destroy egg clusters before hatching, particularly those 

 that are in exposed or wind-swept locations. 



Wind spread takes place primarily on hot days when convection 

 currents from the heated surface of the ground are prevalent. The 

 appearance of cumulus clouds signifies the existence of convection 

 currents and the days when danger of wind spread is greatest. 



EFFECT OF CLIMATE 



The effect of climate, in all its phases, upon the gypsy moth has 

 never been fully investigated, and is therefore little understood. The 

 effect of extremely low temperatures upon overwintering eggs has 

 received more study than any other single factor. It has been deter- 

 mined that all egg clusters are killed when exposed to a temperature 

 of —25° F. and that some eggs are killed at —15°. 



In the field, however, temperatures fluctuate greatly within very 

 short distances. For instance, a temperature of —25° F. at a weather 

 station in a town or city does not necessarily mean that such a tem- 

 perature is common to the entire town. For this reason it has been 

 impossible to estimate accurately just what was happening to gypsy 

 moth eggs in towns from" which temperature records were available. 



Winter temperature is an important factor in gypsy moth abund- 

 ance in most of New England. In some winters the temperature 

 does not get low enough to be fatal, but in other years mortality 

 from this cause is heavy. In northern Maine, northern NewiHamp- 

 shire, and most of Vermont except the Champlain and Connecticut 

 River Valleys, minimum winter temperatures are low enough to kill 

 egg clusters that are not protected by snow, ice, or otherwise, and in 

 most of these areas infestations have become established slowly and 

 the increase of the species has been retarded. 



Late frosts in the spring sometimes cause severe damage to foliage, 

 and this usually results in heavy mortality of the small larvae. This 

 is most likely to occur locally, but was noticeable in many sections 

 of the infested areas during the spring of 1936. Frequently there is 



