THE WESTERN GRASS-STEM SAWFLY. 



13 



one stem. It is believed that the larva that finally reaches maturity- 

 has devoured its rivals. It is obvious that the remains of these 

 superfluous individuals would naturally be* counted when a census 

 of exuvise was undertaken and would complicate the result. But 

 from the best evidence obtainable it is almost certain that there are 

 five instars in the larval life of this species. 



The length of the larval period is probably about 60 days, varying 

 more or less with the warmth of the summer and the state of maturity 

 of the host stems. The acceleration or retardation of the oviposi- 

 tion period owing to an early or late spring has much to do with the 

 date of maturity of the larvse, and possibly with the length of the 

 larval period. August 29, 1911, at Kimballs, Utah, at an elevation 

 of 7,000 feet, the writer found mature larvas in stems of Elymus 

 condensatus. The next year, at the same place, 

 oviposition was beginning freely during the first 

 week of July. The determination of the larval 

 period is wholly inferential, based upon the find- 

 ings in a series of stems (figs. 7 and 8). 



The full-grown larvas vary greatly in size, their 

 growth being governed, as is usual in the case of 

 such borers, by the quality and quantity of food 

 consumed. Those living in wheat stems are 

 much smaller as a rule than those found in rank- 

 growing grasses such as Elymus. Measurements 

 of a series of individuals give variations of from 

 8 to 14 mm. in length and from 1 to 2 mm. in 

 diameter. 



When mature the larva always seeks the ex- 

 treme base of the stem, where it soon begins its 

 preparations for hibernation. Its first move is to fig. 8.- 

 cut a neat V-shaped groove entirely around and 

 inside the stem, usually at or a little above ground 

 level. This groove never severs the stem completely, but so weakens 

 it that the upper stalk, swayed by the wind, will break off com- 

 pletely when dry, leaving a stub that is very characteristic of the 

 work of this insect (fig. 5). In this simple manner the larva provides 

 for the easy escape of the adult in the following summer. The length 

 of the stub thus formed varies greatly. In Elymus condensatus the 

 stub sometimes will project above the ground as much as 3 or 4 

 inches, while in other grasses, and especiaUy in wheat, stubs easily 

 can be found less than an inch in length in all. 



Instances have been observed where two or more grooves had been • 

 cut inside the same stem, as if the larva had been uncertain as to the 

 best place for severing the grass. After cutting its characteristic 

 groove within the stem the larva forces a mass of the debris into the 



-Western grass-stem 

 sawfly: Mature larva. 

 Enlarged 5 diameters. 



