16 



BULLETIN" 273, U. S. DEPARTMENT OF AGRICULTURE. 



Near the tank were a few low trees consisting of white pine and 

 gray birch; these were cleaned of egg clusters and the trunks tangle- 

 footed in order to prevent the larvae resulting from overlooked egg 

 clusters getting into the tops of the trees. The country about this 

 hill on all sides is generally infested (PL VI), so that almost all winds 

 tempered to 50° F. or above bore caterpillars. The weather data 

 used for comparison with the daily catch here was taken at Plum 

 Island, about 10 miles east. 



Table VI.— Dates and numbers of caterpillars removed from screen at Merrimac, Mass., 

 in 1914; also general direction of the wind during the period. 



Date and time removed. 



Number 

 borne by 

 west winds. 



Number 



borne by east 



and south 



winds. 



Direction of wind at time of examinations and a 

 few hours previously. 





19 

 22 



( 2 ) 

 



8 



32 

 2 



n 



3 12 







2 



( 1 



( 2 ) 

 1 

 1 





 



NW. and W., 8 a. m. to 5 p. m. 





NW. and W., May 19, 8 a. m., to May 20, 2 p. m. 



Mav21 





44 









 



NE., E., and SE., May 20, 3 p. m., to May 22, 5 



p. m. 

 W., NW., and SW., May 23, 8 a. m., to May 27, 



May 23, p. m 





10 a. m. 

 Do. 





Do. 



May 26 











36 







Do. 



May 27, p. m 



Changed to S., May 27, 11 a. m., to 5 p. m. 

 W. and NW., May 28, 8 a. m. to 5 p. m. 





May 29 









SW. and W., May 30, 8 a. m., to June 1, 11 a. m. 



May31 







 







W. and NW., June 5, 8 a. m., to June 6, 5. p. m. 







Total 



91 



50 









1 Some of these larvae were caught before this date, as 7 were dead on removal. 



2 No examination. 



3 Number accredited to different winds estimated owing to changes in direction. 



One hundred and forty-one larvae were trapped on this screen of 

 144 square feet between May 19 and June 2, inclusive. A few of these 

 undoubtedly were blown on before May 19, as seven of the nine were 

 dead on removal, indicating that the aviation of the small larvae 

 began probably two or more days earlier in Merrimac than along the 

 seacoast. 



With apparently an equal infestation on all sides of this screen 

 one-half mile or more distant, almost twice as many larvae were 

 trapped as a result of prevailing winds from the west as from the east. 



The data secured in this experiment indicate that all winds during 

 the day in a section generally infested bear caterpillars excepting the 

 north, which was not recorded during this period. Considering that 

 practically one larva was caught per square foot 300 feet above -sea 

 level, it is only by conjecture that we can estimate the maximum 

 height reached by them and the length of time they remain suspended 

 under average conditions. It is a known fact that large numbers of 



