988 Distribution of the Large Larch Saw-fly. [march, 



infection appears to have been near Dinas Mawddwy in 

 Merionethshire, and from thence it has spread westwards to 

 the sea, and north, east, and south in the form of a rough 

 ellipse, with its major axis running N.E. and S.W., and with 

 a major semi-diameter of about 32 miles. 



The West and South- West winds have probably played a 

 leading part in the distribution of the insect. 



In a period of about six years some 1,800 square miles of 

 country have been infected, as follows: — 



Intensity I : ... 1-2 square miles. 



,, II. ... v ... ... 6-8 ,, s „ 



,, III. 4 00 » 



,, IV 1,400 



Compared with the area over which it has as yet spread 

 comparatively little damage has been done. As pointed out, 

 the pest begins to damage the trees seriously .when the 

 intensity of attack rises above III., and this is the case thus 

 far on only 1 per cent, to 2 per cent, of the whole area. The 

 danger lies in the future, for one may suppose that the insect 

 will continue to spread over Britain in search of larch, and 

 that finally, finding its advance checked, it will begin to 

 become a serious pest in those districts where it is already 

 established. 



Scotland. — The distribution of Nematus Erichsoni differs 

 materially in Scotland from the distribution in the Lake 

 District and in Wales in that the intensity of attack varies 

 but little over wide areas. In the last two districts, as has 

 been pointed out, it is possible to point with some degree 

 of probability to a focus from which the insect has spread in 

 all directions. This is not the case in Scotland. The 

 presence of the saw-fly has been noted in all localities where 

 a search has been made. The intensity of attack rarely rises 

 above IV., that is to say, the saw-fly is present, but in too 

 small numbers to do damage. In the accompanying sketch- 

 map the localities have been marked with a numeral showing 

 the intensity of attack. It will be seen that the chief localities 

 visited are Glenluce and Stranraer; Girvan and Ayr; 

 Callander and Loch Vennacher; Perth to Dunkeld and Forfar. 

 Signs of attack were also noted in the vicinity of Galashiels. 

 It seems very probable, then, that a complete inspection 



