- Ferrier, S. M. and P. W. Price
Oviposition preference was examined in relation to offspring performance in a bud gall-forming sawfly, Euura sp., which attacks the Scouler or fire willow, Salix scouleriana (Barratt). The only known locality for this sawfly in Arizona is in the inner basin of the San Francisco Peaks, at 3,100-3,450 m above sea level. A random sample of shoots was taken in October of 1995, 1996, 1997, and 1998 to record shoot length, number of buds per shoot, number of galls per shoot, percent mortality, and overall percent survival for each generation. Oviposition preference was quantified for the first 3 yr, and larval performance was measured over all 4 yr. We found a strong oviposition preference for longer shoots. The longest shoot-length classes contained the smallest proportion of buds, and yet they were attacked more frequently than the short shoot-length classes. Therefore, the rarest shoot-length classes were the most preferred. Larval establishment and survival in galls were used to evaluate offspring performance. No significant correlation existed between larval establishment and shoot length, thus negating the prediction of a preference-performance linkage. Overall survivorship was high in 1995 (77%), 1997 (80%), and 1998 (82%), but drastically dropped in 1996 (16%), possibly because of a severe drought during that year. We discuss two alternative hypotheses to possibly explain these results: (1) all modules (long and short shoots) could provide sufficient nutrients for larvae to survive; and (2) selective pressures acting on female oviposition behavior are independent of larval performance.