Severe consumer impacts in early succession: Influence of a non-native weevil on willow (Salix sitchensis) colonization on Mount St. Helens volcano

Background/Question/Methods Sitka willow (Salix sitchensis) is the first colonist to provide three dimensional physical structure on primary successional sites at Mount St. Helens, thereby altering plant communities and forming habitat for several mammalian and avian trophic guilds. Sitka willow stems are attacked and killed by the non-native willow stemboring weevil Cryptorhyncus lapathi (Curculionidae) and a native clear winged moth Paranthrene robiniae (Sesiidae). In upland habitat, stemborer attack radically alters plant architecture and reduces biomass accumulation and thicket formation.  The goal of this study was to quantitatively evaluate the impact of these stemborers on willow colonization and stand formation.

Results/Conclusions

We surveyed stem damage on ~600 willows at 154 permanently marked sites from 2007-2009.  Across all plants and years, stemborers damaged 62-73% of stem cross sectional area. Damaged stem area was upwards of 80% in drier upland areas, where it generally caused stem mortality, and was lower in riparian areas, which generally tolerated attack.  To better understand the impact of stemborers on willow growth, we excluded them from 60 naturally established plants in upland for one year, using a pyrethroid insecticide. Stemborer-free plants increased in stem cross sectional area by approximately 100%, while stem area of control plants decreased ~5%.   We conclude that stemborers severely diminish biomass accumulation and inhibit the formation of dense willow stands, although their impact on other aspects of colonization remain to be evaluated.  In light of other examples of severe herbivore impacts on plant colonization in this same system, we propose that this example represents an expected phase of early succession during which the effects of consumers on colonizing plants are greatly amplified relative to their effect in more mature systems.