Responses of the Weed Parthenium hysterophorus (Asteraceae) to the Stem Gall-inducing Weevil Conotrachelus albocinereus (Coleoptera: Curculionidae)

A stem-galling weevil Conotrachelus albocinereus Fiedler 1940 from Argentina is being field released to regulate populations of the weed Parthenium hysterophorus in Queensland, Australia since 1995. In this paper we report the tissue and metabolic responses in P hysterophorus in the context of the biology and feeding behaviour of the weevil. C albocinereus induces elliptical galls often on the main shoot axes and rarely on the terminal and axillary meristems of the host plant. From oviposition through gall maturation, the structure and metabolism of the host-plant tissues, especially at the gall region, change continuously to accommodate pressure inflicted by the feeding larva. The host plant shows re-direction of its vital metabolites to the gall, and to the metaplasied cells of nutrition in particular. In mature galls, the pith parenchyma elements turn lignified. Larval feeding fractures the vertical continuity of vascular tissues, which affects the host plant's overall metabolism. As the larva tunnels the shoot column, it places the frass at the fissured vascular sites. That activity initiates necrosis and eventual death of the living cells of the vascular tissue complex. Such a development induces water-logging stress in the gall and the evapotranspirational system displays contrasting responses. Permanently closed stomatal apertures and abnormally inflated substomatal chambers indicate that P hysterophorus suffers moisture-stress with cecidogenesis. The larval performance triggers moisture inundation in the galled shoot and this appears to be an advantage in using this weevil in the control of P hysterophorus.