33. 



of this line witn tne line indicating the level of hatching 

 in water can be detemiined accurately. FEIWICK inaxcates the 

 concentration of the hatching agent by the so called L.A. 

 (log activity) value. This is the logarithm of the strength 

 of the original solution compared to the strength at which no 

 activity of the hatching agent is noticeable any more. So 

 if the level of the "water hatch" is reached at a concentration 



of 1 of the original solution the latter has an L.A. value 



1000 

 of 3> which is a very good concentration for experiments. 



The actual hatching test is done by immersing a sufficient 

 nvmber of cysts of the population to be tested in root dif- 

 fusate and reiicwing the latter once or twice a week. The de- 

 gree of variation among numbers of larvae hatching per cyst 

 is tremendous. Therefore at least 200 cysts should be investi- 

 gated at a time. 



The test is continued until no material hatching of lar- 

 vae is found anymore. 



Eighty to ninety percent of the larvae in cysts of Heter - 

 odera schachtii and H. rostochiensis kept \inder natural con- 

 ditions will hatch in root diffusate of sufficient concen- 

 tration. The remaining 10^-20% are not all dead. They may 

 hatch in due time. 



Hatching tests are particularly suitable for comparison 

 of viability of cysts which have undergone chemical treatment. 



However, such a treatment may not only have killed the 

 eggs, it can also delay the beginning of hatching for several 

 weeks (FEIf^WCK). 



Hatching agents have not been demonstrated for all Heter- 

 odera species. No effect of host root diffusates was found 

 with H. major , H. goettingiana and H. glycines . In such cases 

 only anount of root invasion is a reliable measure for nrmbers 

 of infective larvae in the soil. No stimulating substances 

 may be present in these cases or they may be broken aown in 

 tne soil so rapialy tnat extraction is impossible* 



Distinguishing living and dead larvae by staining . 

 H»lEr.^ CI953; and v.d. LAAN and BYLOO (19^5) investigated 

 the effectiveness of staining with acridin orange in distin- 

 guishing living from dead nnmatodes. It appeared impossible 

 to distinguish nematodes killed or moribund by the action of 

 a nematicide from untreated living nematodes with a sufficient 

 degree of accuracy. 



