Heter: 10 



ficient hatch, larval movement, and proper invasion of the roots, there 

 is created a kind of hiatus in the population development. There are also 

 regions in England certainly, and probably elsewhere, on muck soils 

 where there is a high water table and that hiatus does not occur and the 

 nematode generations go straight through. 



Suppose we are working with soils in favorable conditions and vre 

 start with the beet nematode. It 30 happens that if we take the size 

 of the nem.atode, starting with the larval of about 500yL/up to a fully 

 grown female which is about 1000^, certain stages in development occur 

 at fairly fixed points in size so we can set a number of relatively well 

 defined stages. You can pull up the roots and taking always the largest 

 cyst, which represent the products of earliest invasion, you can chart 

 the stage of development reached. Sow the host plants at different 

 times and you can produce in that way what I call developmental con- 

 tours. 



Let us consider as an example, work done with H. carotae on carrots. 

 Carrots were grown at intervals. One finds that as soon as decent 

 sized roots are produced they are invaded almost at once. Then you next 

 find the swollen females, or early cystic forms as I call them. The 

 cortex is ruptured, the eggs are forming and becoming fully embryonated. 

 There is another stage beyond that, which is the liberation of larvae, 

 which in a sense completes the life cycle. This latter is a very dif- 

 ficult thing to observe in the soil. Now in this particular case with 

 the carrot, there were 2, or perhaps, 2|- generations. If you plant as 

 early as February you get slow development first, because of the low 

 soil temperatures. At the later higher temperatures you get more rapid 

 development and then in the autumn, as the temperatures begin to cool 

 down, we obser' ^ rapid earlier development and later dying off. 



With the pea eelworm I have actually taken it right through the 

 winter on beans as a host. Development is merely arrested and the nema- 

 tode by no means killed under our conditions so development again com- 

 mences as soon as soil temperatures rise. It looks as though under ovir 

 conditions, provided the crop grows long enough, for most species except 

 H. niajor, which is rather different, you can get the maximum of about 2|- 

 generations per year. On sugar beet \xihich grows for us from about March 

 right through to November we could get 2| generations. The last generation 

 is often in the form of very many small cysts which you can see on the 

 roots in October and which don't develop any further. Potato root eel- 

 worm with us probably doesn't go through more than say l|- or, at the very 

 most, 2 generations, more likely 1 generation; H. major only 1 generation. 

 Beet eelworm development depends on the vegatative period of the crop and 

 whether there is an adverse drying condition in the soil during the sum- 

 mer. That seems to be the situation with us. V/hether it would be the 

 same under your conditions I don't know. In some of the soils, for 

 examnle in Kr, Thome's area in Utah where you have irrigation and high 

 soil temperatures, I wouldn't be a bit surprised but that the siigar beet 

 nematode goes through more than 3 generations. It would seem to me to 



