DISPERSION OF GIPSY-MOTH LARV^ BY THE WHSTD. 



21 



Table VIII is here presented to show the annual advance of the 

 moth before and after control was attempted. This indicates that 

 the winds are chiefly responsible for the present conditions. 



Table VIII. — Spread of the gipsy moth in various directions by series of years from Jive 

 years after its introduction at Medford, Mass., to 1914. 



Periods of years. 



NE. 



N. 



NW. 



W. 



SW. 



S. 



SE. 



Average spread 



per year by 



periods. 



1875-1S90 



Miles. 

 15 

 5 

 27 

 41 

 114 



Miles. 

 10 

 4 

 11 

 60 

 16 



Miles. 

 5 

 5 

 12 

 25 

 28 



Miles. 

 10 

 4 

 10 

 19 

 15 



Miles. 

 6 

 4 



12 

 15 

 24 



Miles. 

 4 

 5 

 14 

 16 

 26 



Miles. 

 3 

 4 

 62 

 11 

 



Miles per year. 

 0.5 



1S91-1899 



.5 





3.5 



1906-1909 



6.7 



1910-1914 



6.4 







Total 



202 



101 



75 



58 



61 



65 



80 







2.3 





















Average spread per year in 



5 



2.5 



1.9 



1.5 



1.5 



1.6 



2 









It will be noted that the spread from the original center was very 

 slow, averaging only 0.5 of a mile per year up to the beginning of the 

 campaign in 1890. It was then held in control for a period of about 

 nine years, during which time there was very little advance into new 

 territory. The conditions in the old infested area were also much 

 improved. This shows the necessity for keeping the moth under 

 complete control if windspread is to be prevented. During the 

 period from 1900 to 1905, when no work was being done, the moths 

 spread at the rate of 3.5 miles per year. This had increased to 6.7 

 miles per year from 1906 to 1909, but this ratio has decreased to 

 some extent in the last period up to 1914, inclusive. 



The greatest distance gained in various directions has been to the 

 northeast, an average of 5 miles per year for 40 years, which is the 

 result of the favorable southwest winds. The combination of south- 

 east and southwest winds during the dispersion period has carried 

 the larvae northward at the rate of 2.5 miles per year. The spread 

 to the west, southwest, and south was accomplished at a slower rate, 

 owing to the less favorable winds blowing in those directions. 



SUMMARY. 



In 1913, as a resull of the several experiments conducted by using 

 fcanglefooted screens and cloth for (raps, there were caught on 977 

 Bquare feet 289 first-stage larvae which had been borne by the wind 

 one-eighth to 1 mile or more. In 1914 there were removed from 

 1,614 square feet of sticky surface 346 larva) which had been blown 

 from one-eighth to !•<' miles or more, as verified by the wind records 

 taken at or near those points. Three larva? were also taken from 

 two large creens on the bills in New Hampshire during 1914. 



