31. 



'^' ^thods for quantitative mvestiRations on Heterodera popu- 



Population studies on Heterodera species either in re- 

 lation to crop rotation or chemical control have for a lonp 

 l7Z^^f "^ T^"" i"^d«q^acy of methods for measuring the 



Sin f°"':^ ""^^ "°^ ^^^^ ^^^^^"^^ (19UU) devised a good 

 method for the extraction of cysts from soil that an accuracy 



conlTi"^ K !^P ^S.^^^ standard required for experiments 

 could be reached. The method based on the fact that dry cysts 

 ;.,^hl ^^«q"fte^y described by GOODEY (19^7). However, cyst 

 numbers only still do not mean very much. The only reliable 

 measure of degree of infectivity is the number of hatchahle 

 nS^J^H f ^^ ''^'*^* ''^'"''' ^e^^a^c" workers helped thanselves 



rv^t^ n.r^'''"° ^"^ ""^"^^ ^" ^'"P^y ^"d billed ("viable") 

 cysts. Others estimated the number of eggs in the cysts 

 either directly or after squashing (OOSTENBKINK 1950). The 

 error involved in all these methods was still tremendous. 

 Estimations could be as far off the truth as being only I/3 

 01 the actual number of eggs present. 



The number of viable larvae present in a certain number 

 01 cysts is now estimated in two ways: 



A. Counting the number of eggs with larvae present 

 in the cysts. 



B. Detennining the number of larvae whicn can be in- 

 duced to hatch from the cyst. 



Direct coun ting of eggs . The use of direct counts of 

 eggs as an estimate of infectivity is based on the assumption 

 that the actual number of infective larvae is not much lower 

 than the total number of full eggs. Hatching experiments show 

 plild to'the son!''' "''" "° '''™'''' treatment has been ap- 



r-oi ^''°l^^'' ^° set a reliable count of eggs, these must be 

 released from the cyst and brou,T;nt into suspension in water 

 so that their number in aliquots of this suspension can be 

 counted. The best way to do this is to squash and rub the 



onH th!° f ^Y; l"l^ ^^ S^^ ^"^"^ ^P^^^' a"d wash these eggs 

 and the cyst wall debris in a measuring cylinder with water. 



Tho ^^^.^^i^^shing and rubbing can be done in different ways. 

 The simplest is to rub the cysts between two r Lass slides 

 For certain species this is quite sufficient: In others 'the 

 eggs stick together too much. Better results are obtained 

 than by rubbing them with a glass bar on a channelled metal 

 plate (see GOODEY 19^7). 



