Clonal Development of Coyote Willow, Salix exigua (Salicaceae), and Attack by the Shoot-Galling Sawfly, Euura exiguae (Hymenoptera: Tenthredinidae)

Five clones of the willow, Salix exigua Nuttall, along the Weber River, Utah, produced shorter shoots with increasing ramet age. The number of ovipositions by the shootgalling sawfly, Euura exiguae Smith, correlated positively with shoot length, so that sawfly density was high on younger ramets of a clone and low on older ramets. The sawfly attacked the most vigorous parts of the clones. Survival of progeny in galls decreased with ramet age from 60% survival on 1-yr-old ramets to 26% survival on 6-yr-old ramets when plant-induced mortality was considered alone. The effects of parasitoids on the pattern of sawfly attack was not significant. Survival of progeny in young ramets (2 and 3 yr old) was higher in the egg to very early first-instar stage (50%) than in older ramets (37%) (5 and 6 yr old), but differences were slight after that stage. The major determinants of Euura population distribution on willow clones were the female selective oviposition behavior on longer shoots on younger ramets, which probably evolved in response to higher survival of progeny in the younger ramets. When the relative effects of attack and survival were evaluated in response to ramet age, age accounted for 89% of the variance in attack and 95% of the variance in survival. The contrast between this species and others that attack vigorous plants and plant parts and those that attack stressed plants is emphasized, and an explanation is proposed for the difference between epidemic pest insect herbivores and those that remain as endemic species.