HGter: 11 



be quite possible. If one vrenb iior-th, say into Scandanavia or norUrrn 

 Europe, it may very well be that the uiaxiiiium munibor of generations 

 reached is 3. 



POPULA TION STIIUIKS; HOot ef fect 



\-Je shall next turn to the effect of the host on the soil nematode 

 population. This work, which I am going to describe to you in outline, 

 has been done in two ways. The early work v:as done in fields using 

 gridded plots to try to reduce effects of variability. Later, I turned 

 to microplots which were 2 feet long by 3 feet wide and 2 feet deep. The 

 soil before being put in the microplots was mixed to destroy uneven fer- 

 tility effects and to even up the cyst nematode population. This re- 

 duces to a considerable extent the within-field variability. Most of 

 this vjork has been done with sugar beet eelvjorm and one can get a fairly 

 good measure of the effect of the host on the soil population. In the 

 first experiments I grew the host olants as crops, that is, the plants 

 vrere spaced in the plots as they normally would be in the field. Thus 

 there would be more plants of one kind per microplot than there would 

 be of others. For example, there were only four sugar beet plants in a 

 plot, whereas there were many rape plants per plot. The nematode popu- 

 lation levels can be set up as low, medi^mi, or high, as desired. Nema- 

 tode population levels in the fallow plots, in a sense, represent the 

 check diiring the course of the expr^riment. The levels in these plots 

 taken at the end of the season represent the maintenance level for the 

 nematodes without plants being present as an influence. After the 

 plots are used for a year, the eelworm populations may be considered 

 to be the one factor which has varied the "-iost, whereas there is still 

 a fair degree of homogeneity in fertility and soil structure, although 

 these will be shifted a little. In the next year, having created dif- 

 ferent cyst nem.atode population levels, you can do fui^ther vrork with 

 them. 



Using these microplots it has been possible to study the effects 

 of numerous kinds of plants on the soil population, ■«■ 



The findings can be summarized into a graph in Figure 3* 

 Haintenance level 



Log FIi'^AL 100 

 POPULATION 



fficient host 



Inefficient host 



100 



Log Initial population 



Figure 3. Effects on final nematode populations on hosts of different 

 suitability or efficiency. 



-"-''■.ditor's note. At this Doint Mr. Jon'^s prenontod and discussed con- 

 siderable data in tabular fonn on lantoni slides, but those ai-c not i-e 

 produced here. 



