Abstract:

Alternanthera philoxeroides, which originated in South America, is a noxious weed species in the warm temperate and subtropical regions of China, infesting rivers, lakes, ponds, and irrigation canals, as well as many terrestrial habitats. A host-specific flea beetle, Agasicles hygrophila, introduced from the USA in 1986 as a biological control agent, has become widely established in South China and has effectively controlled the weed in many water bodies. It has been observed, however, that the beetle’s ability in controlling the weed in terrestrial habitats has been limited, apparently due to its poor and/or greatly varying pupation rates. This study analyses the mechanism underpinning the relationship between the beetle’s pupation rate and the morphological and physiological characters of the three ecotypes of its host plant, viz. the weed growing in the water bodies (hydrophytes), that growing in wet soil with 15%-30% moisture content and the roots sometimes saturated with water (hygrophytes), and that growing in terrestrial soil with 5%-15% moisture content and the roots never saturated with water. Anatomical studies showed that hydrophytes do not have secondary structures in the stems, whereas stems of the hygrophytes and xerophytes have tight cortex cells and abundant crystals. The 7 morphological and anatomical characters of the alligatorweed analyzed in this study showed significant differences among the ecotypes of the weed as well as between the internodes of the same ecotype. Results of the principal component and clustering analysis placed these characters into three groups, which coincided with the distinctive levels of the beetle’s pupation rate and the effect of controlling the weed observed in both potted plants and the field experiments. A stepwise regression model was established between the pupation rate of A. hygrophiia and the 7 morphological and anatomical characters of the alligatorweed stems and internodes: y=1.533 2-0.248 3x3-0.051　2x4-1.634　9x5-0.957　7x6 (R2=0.999　9). The pupation rate of the beetle was found to be in simple positive correlation with the lengths of the internodes and the external diameter of the stems and the diameter of the pith cavity of the alligatorweed. The pupation rate was found to be in simple negative correlation with the density and thickness of the cortex parenchyma, and the thickness of the vascular cylinder and the pith cavity parenchyma. It is concluded that the pupation rate and the biological control effect of the flea beetle on A. philoxeroides are affected primarily by the thickness of vascular cylinder, cortex parenchyma, pith cavity parenchyma and the density of cortex parenchyma of its host plant. This is a rather different explanation for the poor performance of the beetle on terrestrial alligatorweed from the observations of other researchers in the USA, Australia and China.