157 



is short. Cnaphalodes is a twig- and leaf-feeder on larch. The 

 colonici do damage similar to that by colonici of Chertnes. In the 

 author's opinion the principal damage is done by the progredientes. 

 They are frequently so numerous as to whiten the larch needles. To 

 repair the damage, the dwarf shoots and dormant buds begin to grow. 

 These, together with the elongating terminal shoots, provide new and 

 succulent food for the succeeding generations of progredientes. Thus 

 the struggles of the host to free itself only result in its more complete 

 subjugation. 



The non-migrating species of Chermes and Cnaphalodes are more 

 serious enemies to spruce than are the migrating species. The Cher- 

 MESiDAE, however, are serious enemies of spruce only when allied 

 with unsuitable soil or atmospheric conditions. The collective damage 

 to larch by the colonici of Chennes and Cnaphalodes and the pro- 

 gredientes of Cnaphalodes is serious in Britain. 



The marked parthenogenetic developments of the species of Chennes 

 and Cnaphalodes on both spruce and larch make it useless to eliminate 

 either host from any particular plantation. As Chennes quickly 

 increase in the event of any decline in the health of their hosts, great 

 care should be taken that the planting area is clearly suitable to the 

 conifer which it is desired to plant. The author is convinced that 

 there is no practical method of controlhng these pests after a plan- 

 tation has been formed. His observations lead to the conclusions 

 that these Aphids are frequently widely present in forest nurseries, 

 and that they often seriously damage their hosts immediately after a 

 plantation has been formed. Thorough fumigation of the nursery 

 stock immediately before despatch to the planting area is therefore 

 strongly advocated. A table is given showdng the results of fumigation 

 \^'ith hydrocyanic acid gas, nicotine and carbon bisulphide, and their 

 effect is discussed. Hydrocyanic acid gas, generated from potassium 

 cyanide at the rate of 1 oz. to 100 cub. ft. of space, or from sodium 

 cyanide, 1 oz. to 130 cub. ft. of space, is apparently the most useful 

 fumigant, and the process proves to be a safe, efficient and inexpensive 

 method of killing Chermesids on nursery stock. Fumigation should 

 be done not later than 1st April, as the insects begin to oviposit after 

 that date. The temperature at which fumigation was successful 

 proved to be much lower than that considered the optimum ; this is 

 impoii:ant, as the average air temperature at the time when fumigation 

 must be carried out is low. The methods of fumigation described 

 in these experiments will probably require elaboration in practice, 

 but this control should prove of special value in this country at this 

 time, as very extensive planting of conifers wall have to be carried 

 out in the years following the close of the War. 



Various theories have been suggested as to the origin of migration. 

 Early authors were agreed that spruce is the original host of Chermesids ; 

 Cholodkovsky considers the cycle to be an annual one and on spruce 

 only [loc. cit.], sexuales being produced towards the end of summer 

 and the Tvinged forms transported by wind to trees of other genera, 

 where they adapted themselves to feeding and breeding. The migra- 

 tion back to spruce then took place in a similar way to the first migra- 

 tion. Borner in 1907 introduced the theory that the Picea host was 

 intermediate, pine being the primary host. Tliis theory was later 

 abandoned for that of Mordwilko, who considers that all Aphids were 



