EXPERIMENT STATION REPORTS. 



2t3 



Parasite of wheat joint-worm (Euroloma boltcri), enlarged (original). 



Mild there, it is only at long intervals that it seems to be relieved of all 

 restraint and to take possession of fields of wheat all over the country. 

 Such occasions are undoubtedly brought about by weather conditions un- 

 favorable to the parasites which ordinarily keep the pests in check. There 

 is a continuous rivalry between the pest and its parasites as between the 

 hunted and the hunter, and while the joint-worm occasionally gets 

 ahead and makes trouble, just so surely in the past, the parasites have 

 searched them out and destroyed all but an insignificant remnant — sub- 

 sequently^ almost disappearing themselves for lack of sufficient food. 



In the case of the common wheat joint-worm, Isosoma tritici, there 

 is but one annual generation. The tiny, winged wasp-like insects emerge 

 from their cells in the straw, usually in the stubble-fields, and lay their 

 eggs in the wheat just after it begins to lengthen out in the spring. Each 

 egg is pushed into a hole that is bored into the succulent straw at the 

 tenderest spot, which is just above a node or joint. Sometimes a dozen 

 or so eggs are scattered above each joint, although more often only part 

 of the straw is utilized in this way. One can form a more or less accurate 

 estimate as to the time when the eggs were laid in each field since the most 

 tender, new growth is selected; for instance — straws attacked near the 

 lower part of the plant show that the eggs were laid while the plant was 



Fig. 3.— Swellings made by common joint-worms (Isosoma tritici) in straw, enlarged, 



