23 



the data have not been completely analyzed, it 

 was his opinion that the weevil was having 

 little effect on this rare thistle that is 

 restricted to serpentine warmspring sites. He 

 also made the observation that new world 

 members of the Cirsium genus were very 

 depauperate in herbivorous fauna in comparison 

 to old world species. It should be noted that 

 the weevil has infested a number of native 

 species in the genus Cirsium (Turner et al. 

 1987) . 



While the rate of seed predation is not clear 

 in the case of C. lonqistylum , it is a matter 

 of concern because seed predation can have a 

 magnified effect on succeeding life history 

 stages, resulting in greatly reduced seedling 

 establishment and recruitment of new plants 

 (Louda et al. 1990) . Herbivory of other parts 

 of the plant has not been studied, although 

 thistles generally are not affected much by 

 grazing. 



PRELIMINARY STUDIES OP WEEVIL INFESTATION 



METHODS: To get a preliminary estimate of the 

 number of heads per plant of Cirsium lonqistylum 

 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 2, p. 24. 



RESULTS: The raw and summarized data from this 

 study are provided in Table 2, p. 25. The twenty- 

 five C. lonqistylum plants sampled contained a total 

 of 3 66 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 (Dr. Charles Turner, pers. comm,). It 

 is not known how weevil infestation affects 



