Estuarine gradients and the growth and development of Agapanthia villosoviridescens, (Coleoptera), a stem-borer of the salt marsh halophyte Aster tripolium

Phytophagous insects of estuarine salt marshes which live inside their host plants are not directly exposed to estuarine gradients. Host plant quality, however, may change along the estuary as a result of the direct effects of these gradients; as a consequence growth and development of endophagous insects may be influenced. The results of a study of the life cycle of Agapanthia villosoviridescens (Coleoptera, Cerambycidae), a stem-borer of the halophyte Aster tripolium, on three salt marshes along the Westerschelde estuary (the Netherlands) are in line with this hypothesis. It was shown that in upstream direction (1) mean larval weights were consistently higher during the entire period of larval development; (2) the percentage of late instars on a given sampling date generally was higher; (3) the percentage of larvae which underwent successful metamorphosis increased. Furthermore, (4) dry weight of the imagos was highest on the least saline marsh. The effects of estuarine gradients on the Aster host plants was indicated by differences in growth and chloride content between the populations of the three marshes. The non-overlapping geographic distribution of Agapanthia villosoviridescens and its host plant Aster tripolium on the Westerschelde salt marshes may be related to the effects of estuarine gradients on the suitability of the host plant.