The grasshopper Cornops aquaticum (Bruner) lives permanently on aquatic macrophytes in floodplains of tropical South America. Its host plants and feeding preference were determined by field observations, crop analysis and feeding tests in the laboratory. Macrophytes were sampled and their petioles were examined for potential egg deposition in order to reveal host plant specificity. Sub-samples were dissected in the laboratory and eggs were counted directly whereas the rest of the sample was kept in net-covered water tanks aiming to verify hatching of nymphs. C. aquaticum fed and oviposited on Eichhornia azurea and Pontederia cordata (Pontederiaceae) in the field. Out of 140 fore-guts analyzed from free-living grasshoppers, 75% contained Pontederiaceae tissues. In the laboratory, however, it accepted plant species from other families. Tests of acceptance revealed that from 19 plant species offered, one by one, 16 were accepted. The seven plant species, which were accepted with highest frequency, were selected for tests of feeding rates. These rates were equal for all seven species. When four or five food plant species were offered simultaneously, the host species were not always preferred. The fact that in the laboratory more host-plant species were accepted than in the field indicates that the host range in the field is determined by other ecological factors than the plant chemistry. Host plant selection by C. aquaticum can be related to the relative abundance of the macrophyte species in the field and to the protection, which they offer against predation. Considering the emerging parts of the macrophytes, E. azurea represents the most abundant resource in the floodplains. Moreover, the cryptic coloration of the grasshopper on E. azurea lessens predation risk. That is why, apart from being potentially polyphagous, C. aquaticum presents a considerable oligophagy in the field, feeding on few species of Pontederiaceae.
Insecta; herbivory; diet
