Popu2:]. 



PRINCIPLES OF POPULATION MEASUREMENT 

 F. G. W, Jones 



The remarks below apply primarily to cyst-forming nematodes, but the 

 principles raised have a much wider application to many types of non- 

 mobile or semi-mobile organisms found in soil. 



Cyst-forming species of nematodes (Genus Heterodera ) are important 

 pests of many field crops in Europe, North America, and elsewhere. 

 Quantitative estimation of soil nematodes dates from Cobb (I9I8); and 

 the first attempts to estimate the cyst-forming species in Great 

 Britain were those of Morgan (192^), during investigation of the 

 potato-root eelworm in Lincolnshire. Soil population estimates of 

 cyst-forming nematodes are now extensively used for the determination 

 of presence or absence, in connexion with the so-called certificates 

 of freedom frcm infestation (Peters 1951) that are required for the 

 export of produce, such as seed potatoes. 



Soil population estimates are also used as the basis of advice to 

 farmers, on the risks of planting susceptible crops on infested land, 

 and in research into population dynamics. Table 1 gives some practical 

 figures for judging the situation. The standard of accuracy demanded 

 in research is generally higher than for advisory work. There is no 

 fundamental reason why this should be so; but in advisory work, time is 

 often pressing, and accuracy may have to be sacrificed for speed. 



Soil Extraction 



The extraction of cysts from soil is the basis of all population 

 estimates. For this purpose, water flotation is universally employed. 

 Morgan (1920) used ordinary flasks, and these are still of value where 

 the soil contains much floatable debris (Jones 19U5) . Most frequently, 

 however, the Fenwick can is used (Fenwick I9U0) . In many advisory 

 centres throughout Europe, whole batteries of Fenwick cans can be seen 

 in action; and the yearly through-put of samples is very great. The 

 chief modification of Fenwick 's original model is the provision of a 

 sloping floor and sludge drain to facilitate clearance of waste soil 

 between samples. 



Other methods of cyst extraction worth mention include those of Hellinga 

 (I9U2) and Bvhr (195U). In the former, the soil containing cysts is 

 first washed through two sieves of appropriate mesh, and then the con- 

 tents of the lower sieve are transferred to a porcelain dish in which 

 flotation occurs. In the latter, the soil is placed in a glass cylinder, 

 and a strip of filter paper is fitted around the wall. liJhen water is 

 added and the contents stirred, cysts and debris rise to the surface 

 and adhere to the filter paper, which, when unrolled, gives a continuous 

 band from which cyst collection is relntive]y easy. 



