Responses of the African Armyworm and three species of borers to carbohydrates and phenolic substances: an electro- and behavioural physiological study

Abstract

Electrophysiological recordings were made from the maxillary sensilla styloconica of fifth instars of the African Armyworm, Spodoptera exempta (Wlk.) (Lep.: Noctuidae), and the borers (Lep.: Pyralidae) Eldana saccharina Wlk. and Maruca testulalis (Geyer), and fifth and sixth instars of the borer Chilo partellus (Swinhoe). Stimuli were different concentrations of 13 carbohydrates and 11 phenolic substances. It appeared that a sucrose sensitive cell is present in the lateral styloconic sensilla of Spodoptera, Maruca and Chilo, and in both the lateral and medial sensilla of Eldana. The sucrose cell of fifth-instar Chilo is more sensitive than that of sixth instars. The additional responses of the medial cells to the phagostimulant sucrose are supposed to have contributed to the widespread infestation of sugarcane by Eldana. The medial sensilla of the Armyworm contain a cell type responding to both meso-inositol and D-ribose. Chilo appeared to be the only one of the three borer species able to perceive phenolic substances. It has a cell responsive to chlorogenic acid in its medial sensilla, which is a bit more sensitive in fifth- than in sixth-instar larvae. The presence of this cell may account for the positive correlation between total phenols content and resistance of sorghum cultivars to this borer. Spodoptera can perceive four phenolic acids. Its lateral sensilla contain a cell responsive to chlorogenic acid and its medial sensilla a cell responsive to 4-hydroxybenzoic acid, gallic acid as well as vanillic acid. Feeding experiments showed that the latter substances inhibit feeding in the Armyworm. So far, in the borers adequate stimuli could be determined for only two or three of the four taste cells present in each sensillum styloconicum. This indicates that we are still unaware of chemicals that may be important to these caterpillars. On the other hand, for the sensilla styloconica of Spodoptera, electrophysiology suggests the presence of five or six taste cell types, whereas only four taste cells are known from morphological studies.