parts of the plant has not been studied 

 although thistles, generally, are not 

 affected much by grazing. 



I. PRELIMINARY STUDIES OF WEEVIL INFESTATION 



METHODS: To get a preliminary estimate of the number 

 of heads per plant of Cirsium longistylum infested by 

 Rhinocyllus conicus, five plants each were randomly 

 collected from five locations. The total number of 

 heads on each plant were counted and dissected, and 

 then scored as to whether or not they contained weevil 

 larvae. Collection site locations are provided on a 

 map, Figure 4, p. 19. 



RESULTS: The raw and summarized data from this study 

 are provided in Table 2, p. 20. The twenty-five C. 

 longistylum plants sampled contained a total of 366 

 heads. Two-hundred twenty-five of these heads 

 contained one or more weevils. Thus approximately 60 

 percent of the heads on plants were infested. The 

 degree of infestation and the impact on the seed 

 production were not measured. This would take an 

 involved study. However, different sites showed 

 different levels of infestation of individual plants, 

 with the lowest rate of infestation occurring at Kings 

 Hill and O'Brien Park. After adult weevils lay eggs on 

 plants in the spring, the eggs hatch and the larvae 

 burrow into the flower heads (Rees 1982) . Higher 

 elevation locations may limit weevil survival due to 

 variable temperatures. It is not known how weevil 

 infestation affects populations size fluctuations. 

 Monitoring data currently indicate that populations 

 contained a good mix of both rosettes and flowering 

 plants. It is possible that infestation rates are not 

 high enough to limit this species, or that the Cirsium 

 longistylum populations have very few predatory fauna; 

 a hypothesis put forth by Dr. C. Turner (U.S.D.A., 

 Albany, CA, pers. comm.). 



