Gelechiidae 341 



greater immunity to disease epidemics than any other species of grain 

 moth. 



Sitotroga feeds on a number of different grains, among which are 

 wheat, corn, and oats. The dry kernels are used in bulk for rearing pur- 

 poses. The hygroscopic moisture within the kernels should not be 

 below 8' , . During the feeding and pupal stages, the insect occupies 

 the interior of the individual kernels so that the interspaces between the 

 kernels are not clogged with excreta or webbing. The newly emerged 

 adult moths, therefore, always have free egress from the mass of grain. 

 Upon emergence from the kernels the moths worm their way upward 

 and outward until they reach a space of sufficient size to permit their 

 wings to expand and mating to take place. 



By day the moths tend to climb upward and come to rest in positions 

 of positive thigmotropism and negative phototropism. By night they 

 become active and oviposit in crevices about 0.23 mm. in width. The 

 newly hatched larvae are negatively phototropic and positively geotropic, 

 so that they readily permeate a mass of grain. If the moths are col- 

 lected daily, few, if any, deposit their full quota of eggs or die within 

 the production unit. 



The type of unit used depends to some extent on the rearing medium 

 and the size of the grain used. Small kernels yield smaller moths than 

 do large kernels, and the smaller moths deposit fewer eggs. Moths 

 reared in corn deposit over three times as many eggs per moth as those 

 reared in wheat. On the other hand, given equal weights of grain, the 

 rapidity with which the grain is utilized varies inversely with the size of 

 the kernels; so that a much greater number of moths is obtained in a 

 shorter period of time with wheat than with corn. This is due to the 

 fact that a kernel of grain, irrespective of its size, is usually inhabited 

 by only one larva at a time. 



Laboratories producing for seasonal field requirements find that soft 

 varieties of wheat form the most satisfactory rearing media. In experi- 

 mental work where a uniformly non-seasonal supply of moths is needed, 

 without the necessity of frequently replenishing the food, soft varieties 

 of corn are used. 



The "tilted bin" type of production unit adapted particularly for 

 holding corn, has been developed by the University of California. A 

 "vertical bin" type of unit for holding wheat was developed by the 

 U. S. Bureau of Entomology. 



Of these two types of production units, the "tilted bin" type (Fig. 66) 

 is the simplest in construction but is not as readily refilled as the "ver- 

 tical bin" type; therefore it is only well adapted for use with a slow 

 productive medium such as corn. Essentially it consists of a series of 

 similar shallow trays (36" x 26'" x 3") placed one on the other and 



