GALLS 



is shiny yellowish-green, with dark green mottlings ; and the cornicles are 

 pale green. The winged viviparous females are bright green, with pale 

 olive head. This species often causes the leaves at the apex of the shoots 

 to curl and twist up. It attacks Gooseberries as well as Currants. Early 

 spraying with paraffin emulsion is the best treatment, but little can be done 

 when once the lice have got a hold. 



The Elm Aphis {Sckizoneura ulmi), frequently rolls, blisters, and distorts 

 the foliage of Elms. The galled leaves may be found from June to October. 

 Usually only one half of the leaf is attacked, curling downwards and under, 

 forming a roll within which the life stages are passed, as in the case of the 

 Currant Aphis. The unaffected part of the leaf does not appear to suffer 

 much from the attack, but the galled portion is gradually denuded of all 

 sap, and turns to a yellowish-green, and finally to a pale ashy grey. 



Pemphigus bursarius, or Aphis bursaria, forms an attractive gall on the 

 petioles of Black Poplar, and sometimes the Lombardy Poplar. The gall 

 consists of a pear-shaped or oval purse-like swelling, green in colour, suffused 

 with pink. It is usually % in. long and about 1\ in. in girth. It is very 

 plentiful from July to September, and on badly infected trees some branches 

 may have 25 per cent, of their leaves attacked. The queen aphis 

 punctures the petiole of the leaf, and the sap exuding, gradually surrounds, 

 and finally encloses her, and the work of reproduction goes on. The winged 

 aphides make their exit through a small opening at the apex of the gall. 



The Pine-apple Gall, so common on young trees of Norway Spruce Fir, 

 is the work of an Aphis {Adelges (Chermes) abietis). The queen emerges in 

 June, hybernates through the winter, and in the following spring punctures 

 the shoot at the axils of the young leaves, causing their bases to thicken 

 by an accumulation of sap. The thickened bases form a kind of false cone 

 in which innumerable eggs are laid, the resulting colony numbering perhaps 

 1800-2000 insects. The dry galls may remain on the trees for several years 

 before final disintegration, the infected shoots being often distorted and some- 

 times killed. 



The order of Diptera includes over a hundred gall-making insects, usually 



lvii 



