PARASITES IN CEREAL AND FORAGE CROP PRODUCTION. 97 



onic development we do not know, but where we rear such an abnor- 

 mal number of parasites from a sing:le host, and those from a single 

 host are all of the same sex, the case will at least bear investigation. 



These parasites have, however, one weak point. A sudden dis- 

 aster to their host reduces them almost to the verge of extermination. 

 For instance, over a considerable portion of the central Pacific 

 States during the past autumn there was so much rain just i)rior to 

 seeding time that the ground could not be prepared for wheat sowing 

 until late. Hence there are but few of the fly even in the earliest sown 

 fields, thus reducing the food supply of these parasites to the mini- 

 mum and preventing their breeding in any considerable numbers. 



There is another grou}) of parasites of the Hessian fly whose 

 habits are normal, but which have a greater number of host insects. 

 Among these may be placed EKpelmus allyiiU French. This species 

 ranges from New England to Montana, south to South Carolina, 

 Georgia, and Texas. The insect has two advantages in that it is 

 double brooded and is also a parasite of the several species of joint- 

 worms (Isosoma) that work in wheat, rye, and cheat, thus being 

 present among these grains whether infested by the Hessian fly or 

 not. AVe also rear it from Isosoma -infested timothy, orchard grass, 

 quack grass, Middenbergia, Elymus, and in the AVest from Bouteloua, 

 Stipa. Sporobolus, and Hordevm jiilxitinn. So, Avhile this parasite 

 does not increase as rapidly as Polygnotus and Platygaster, it has 

 the advantage of not being so dependent upon a single host for 

 sustenance. This double-brooded feature is also found in Seniiotel- 

 lus {St'ictonotus) isosornatis Riley, Sem'totellus chalcid'tphagus AValsh 

 and Riley, and Webste?'eUus tritici Ashmead, parasites of single- 

 brooded jointworms, but not, so far as is known, of the Hessian fly. 



Turning to the parasites of Isosoma, we find here also illustrations 

 of the value of parasitism. AVhile there are a few species that we 

 expect to breed from the larva^ of any or all of them, there are others 

 that seem to be restricted to a single species. As an illustration, we 

 rear an undescribed species of Cryptopristis from Ixo.sonKi tritici 

 Riley infesting wheat, but in over 400 rearings of Isosoma Ave have 

 failed to get it from any other host. Its area of distribution is not 

 that of /. tritici, however, as it seems to occur only from Xew York to 

 Virginia, Kentucky, and eastern Illinois. This is also two brooded, 

 while its host is single brooded, and from the enormous numbers that 

 emerge from wheat stubble infested by the Isosoma larvae, sometimes 

 in the ratio of 35 or 50 Cryptopristis to one of the Isosoma, it will be 

 seen that they are no small factor in holding the pest in check. But 

 here, again, the entomologist does not know enough about it to be 

 certain whether or not it is described, and the farmer is ignorant of 

 its existence. 



7487 — No. 07 — 07 7 



