Why does the leaf beetle galerucella lineola f. attack wetland willows?

The willow‐feeding leaf beetle (Galerucella lineola F.) attacks its host plant, tea‐leafed willow (Salix phylicifolia L.), heavily in damp habitats, but less so in well‐drained soils. To confirm this general observation and to explore the factors affecting habitat and host‐plant use by this beetle, we collected data from 12 study sites in Finland during two consecutive years. All sites included three habitats: (1) water, where the willows were growing in permanently waterlogged soils, (2) flood zone, where the willows underwent periods of flooding, and (3) dry zone, where the willows grew in well‐drained soils. G. lineola was, on average, 16 times more abundant on willows growing directly from water as compared to willows in well‐drained soils nearby. To explain this, we proposed three general hypotheses. (1) The food‐quality hypothesis states that the apparently stressed willows growing in waterlogged soils provide superior food for the beetles. (2) The predation hypothesis states that open water surface around the willows in wetlands impedes dispersal of predators providing enemy‐free space for the beetles. (3) The climate hypothesis states that wetlands provide better abiotic conditions for the adults, eggs, or larvae of this leaf beetle. These hypotheses were tested by using observational data and by performing both laboratory and field experiments. Contrary to the food‐quality hypothesis, adult beetles preferred S. phylicifolia grown in dry habitats both for food and oviposition medium in the laboratory, and produced fewer eggs when fed on willow leaves from wetlands. Consistently, larvae performed poorly on leaves from wetland willows in the laboratory, most probably due to low water content of the leaves, also rich in phenolic secondary compounds and poor in nitrogen. The densities of crawling predators did not differ between the habitats, and the predation experiments revealed no difference in predation pressure between the habitats. However, during a rainless period, the first‐instar larvae performed relatively poorly in drier habitats despite the high quality of food. When ramets of S. phylicifolia were exchanged between the habitats the first‐instar larvae performed best in sunny wetland habitats on willows originally from dry sites. Spraying with water improved the performance of neonate larvae more on willows growing in dry habitat than on wetland willows. The results show that G. lineola does not attack S. phylicifolia growing in wetlands because of the superior food quality or lower predation pressure, but because of the strong affinity by adult beetles to the moist habitat where also the first‐instar larvae, vulnerable to desiccation, usually perform best. An apparently ancestral physiological constraint hinders the beetles from using the best food resources and restricts their occurrence primarily to moist habitat patches with poor quality food and high risk of intraspecific competition.