Diet preference of the different life stages of the aquatic grasshopper Cornops aquaticum (Orthoptera: Acrididae) and its implications for the biológica! control of water hyacinth

Abstract

Host specificity determination of weed biological control agents has historically relied on evidence generated through quarantine trials in the region of introduction. These trials are conservative and where possible, more research should be conducted in the region of origin. The oligophagous, semiaquatic grasshopper, Cornops aquaticum (Orthoptera: Acrididae), was released in South Africa for the biological control of Pontederia crassipes (Pontederiaceae). We conducted post-release laboratory no-choice trials, using early instar nymphs (instar I and II), later instar nymphs (instar III to VI), adult females and males, to determine fundamental host range; and gut analysis from field collected C. aquaticum to determine the ecological host range of this insect. In no-choice trials, survival rates of C. aquaticum were similar on Pistia stratiotes (Araceae), Oxycaryum cubense (Cyperaceae) and P crassipes for later stage nymphs and adults. However, under field conditions, P crassipes and the congeneric P azurea, were the only plant contents in the guts of early instar nymphs and the most abundant ones in later instar nymphs and adults. The results support the evidence that C. aquaticum is an oligophagous insect in the genus Pontederia, and that life stages should be considered when conducting host-specificity trials in externally feeding herbivore species. Diet composition of field-collected insects thus provides a better indicator of the field host range.