20, 



ceptible Brassica species and plowing up tne 

 field before cysts had ripened. This appeared 

 to be either useless or too dangerous because 

 cysts ripened before the crop was destroyed. 



A refinement of the catch crop technique was 

 investigated by DEN OUDEN (1956). Instead of a 

 host plant he used a plant which produces a hatch- 

 ing agent and into which the larvae penetrate but 

 do not complete tneir life cycle. A promising 

 plant was Hesperis matronalis . However, the in- 

 crease of hatching over that with normal non- 

 susceptible crops appeared to be not more than 10% 

 and this is not sufficient to justify the cost 

 and trouble of growing this relatively uneconomic 

 plant. 



C. Other Heterodera species 



Both H. rostocniensis and H. schachtii are 

 characteri'zed by the fact that their eggs can be 

 stimulated to hatch by a substance which diffuses 

 out of the roots of host plants. This is not the 

 case with all other Heterodera species, and does 

 not affect their population dynamics. These other 

 species do not show fundamental differences with 

 the two most important Heterodera species. 



In almost all cases Heterodera spp. host 

 range comprises two or more crop plants and several 

 weeds. However, the effectiveness of these differ- 

 ent plant species as hosts varies widely. JOWES 

 and MORIARTY (195b) investigated the effectiveness 

 of several host plants of H. scnachtii , H. cruci - 

 ferae and H, goettingiana . 



Influence of soil type . In the preceding 

 talks the effect of soil type on tne population 

 dynamics of Heterodera species has not been men- 

 tioned. The impression gathered from most investi- 

 gations is that it generally is not of major im- 

 portance. Both increase and decrease of the popu- 

 lations seem to be largely independent of soil type. 

 However, upon closer examination this might appear 

 not to be quite true, JONES found tnat on a certain 

 Englisn soil the beet eelworm population was kept 

 low even after growing beets on it repeatedly, 

 Heterodera species differ from the species discussed 

 below in this respect. Results of experiments in 



