Comparative plant-herbivore interactions involving willows and three gall-inducing sawfly species in the genus Pontania (Hymenoptera: Tenthredinidae)

Abstract

The ovipositional preferences of female Pontania species, which induce leaf galls on willows, were examined in relation to the performance of progeny in terms of larval establishment and survival in galls. Three Pontania species separated widely in geographic location were studied: Pontania sp. 1 on Salix scouleriana near Flagstaff, Arizona, U.S.A., Pontania amurensis on S. miyabeana near Sapporo, Hokkaido, Japan, and Pontania pustulator on S. phylicifolia in eastern Finland, where two populations were examined. The species are uncommon to rare in the locations studied and depend on disturbance resulting in populations of young willows or young ramets. All species showed a strong and significant ovipositional preference for longer shoots and younger trees or ramets; older plants were not attacked. Survivorship differed considerably between species: Pontania sp. 1 had 80% of galls aborted, low parasitism and 16% survival, Pontania amurensis had few galls aborted, moderate parasitism and 70% survival, and Pontania pustulator was exposed to heavy parasitism and had a mean of 27% survival for the two populations. The pattern of ovipositional preference was not strongly related to probability of larval establishment in a gall for any of the populations studied. Only Pontania sp. 1 showed a weak but significant relationship between the pattern of preference and larval survival. The apparent weak linkage between preference and performance may be accounted for by four alternative hypotheses. i) Females are so selective that all galls initiated are situated on high-quality resources: females are specific to one species of willow, they are highly selective for very young plants, and on these plants they prefer the longer shoot-length classes. ii) Low predictability of larval survival negates any benefits of a preference by females. iii) Preference evolves independently of performance because young plants and long shoots provide better resources in which females oviposit. iv) Young leaves, on which galls are initiated and larvae develop, are a relatively uniform, nutritious and predictable resource, such that there is a broad range of shoot lengths similarly suitable for larval survival. The results provide insights into the factors contributing to the rarity of these sawflies, their highly patchy distribution over the landscape, and their population dynamics.