RELATIVE EFFECTS OF PLANT-RESISTANCE AND NATURAL ENEMIES BY PLANT DEVELOPMENTAL AGE ON SAWFLY (HYMENOPTERA, TENTHREDINIDAE) PREFERENCE AND PERFORMANCE

The relationship between oviposition preference and offspring performa nce is critical to the evolutionary ecology of the interactions betwee n herbivorous insects and plants, but is not well understood. To impro ve our understanding of this relationship we must critically evaluate the relative importance of the selective forces or causes of mortality that affect developing larvae. We documented a strong relationship be tween oviposition preference and larval performance for 2 species of g alling tenthredinid sawflies, a petiole galler, Euura sp., and a leaf galler, Pontania sp., near P. pacifica on the arroyo willow, Salix las iolepis. There was a correlation between attack and survival on differ ent ramet ages for both sawfly species. Mortality caused;by plant resi stance by ramet age primarily explained the pattern of attack of both sawflies. The effect of plant resistance was strongest on willow shoot s on very young ramets. This effect decreased rapidly as ramet age inc reased and then increased relatively slowly as ramets aged, becoming v ery strong on the oldest ramets. Mortality caused by plant resistance was 8-fold greater than mortality caused by natural enemies for Euura and 3.2-fold greater for Pontania. Natural enemies may play a lesser r ole in the population dynamics of the sawflies that may be subsidiary to the plant-herbivore interaction. We reason that selection has resul ted in ovipositing females flying primarily in the upper parts of will ow clones and avoiding the highly resistant younger ramets common in t he lower parts of clones. Within the upper parts of clones, the evolut ion of female behavior that discriminates against shorter, slower grow ing shoots on older ramets would be selected for by the decreasing sur vival of offspring on these shoots.