Estuarine gradients determining the quality of tidal marsh halophytes as host plants for endophagous insect larvae: Experimental evidence

Phytophagous insects of estuarine tidal marshes which live inside their host plants, are, in contrast to the plants, not directly exposed to the estuarine salinity gradient. Previous field studies, however, have shown that patterns of growth and development ofAgapanthia villosoviridescens larvae, stem-borers of the halophyteAster tripolium, gradually change on tidal marshes along the Westerschelde estuary (HEMMINGA and VAN SOELEN, 1988). In the present study we carried out a laboratory experiment in which we followed growth ofA. villosoviridescens larvae from two different Westerschelde tidal marshes; the larvae either were kept inA. tripolium stems from their own marsh, or they were kept in stems from the other marsh. It was found that larvae from both tidal marshes showed larger weight increases inA. tripolium stems from the least saline marsh. Apparently, differences in host plant quality between stems of the two marshes exist. The results lend support to the hypothesis that growth and development, and distribution and abundance of phytophagous insects in estuarine tidal marshes may be indirectly influenced by estuarine gradients,via the host plant quality which changes along the estuary.