150 MICROSCOPIC ANIMAL FORMS AS ANTAGONISTS 



common fat solvents. It becomes inactivated when allowed to act upon 

 bacteria and appears to be adsorbed by killed bacteria, even by species 

 that are not destroyed by it. This substance does not have the properties 

 of either bacteriophage or lysozyme. 



The presence in certain insects of a variety of other substances, such 

 as allantoin, which affect bacterial activities has also been established. 

 These observations give rise to the hope that man may in time succeed 

 in developing and utilizing microorganisms for the biological control of 

 injurious insects (849). 



RELATION OF NEMATODES TO SOIL 

 MICROORGANISMS 



Nematode worms are represented in the soil by a number of sapro- 

 phytes as well as by many plant and animal parasites. The latter vary 

 greatly in their relation to the host. The larvae of the cereal parasite 

 Tylenchus tritki penetrate the wheat seedlings between the leaf 

 sheaths, near the growing or apical points. When the head is formed, 

 the larvae enter the flowering parts and form galls. They become sexu- 

 ally mature, mate, and lay eggs which hatch in the galls, and then be- 

 come dormant. When the galls fall to the ground and decompose, the 

 larvae are liberated and proceed to find and attack new wheat plants 

 and cereal plants. 



Some nematodes attack plants by feeding upon the roots. The meth- 

 ods of control require, therefore, a knowledge of their life history. Some 

 species produce resistant forms or cysts that may survive in the soil for 

 many years, even in the absence of the host plant. Soil sterilization by 

 steam or by chemicals is frequently employed as a measure of nematode 

 extermination. 



Antagonistic relationships may be utilized for the control of nema- 

 todes. Linford et al. (528) found that the root-knot nematode of pine- 

 apple {Heterodera marioni) may be controlled by heavy applications of 

 organic material. The decomposition of this material results in a greatly 

 increased population of saprophytic nematodes in the soil. The decom- 

 posed organic residues also support large numbers of such other soil 

 microorganisms destructive to the parasitic nematodes, as the nema- 



