Feeding Behaviour of Maruca Testulalis (Geyer) (Lepidoptera : Pyralidae) in Relation to its Host and Non-Host Plants

Abstract

The host plant pods and flowers evoked higher feeding responses than the leaves among the fourth instar larvae in choice as well as no choice tests. \aV"Vo.e. First and fourth instar~did not feed on leaves of common bean because they were trapped by the dense trichome mat on both surfaces of the leaf and eventually died. This physical defence mechanism may be potentially useful in the management of H. testulalis. Fourth instar larvae unlike first instar did not feed at all on cotton leaves and flowers . There we~ no obvious reasons for this but physical as well as chemical factors may be playing an important role in bringing about the observed responses. Pure compounds tested using agar cellulose gel and cellulose acetate paper discs included, sugars (sucrose, fructose and glucose), amino acids (methionine, glutamine and glycine), and alkaloids (nicotine and tomatine). All the sugars were effective phagostimulants. Dose response curves relating to feeding bioassays indicated that 0.1- 0.2 M. are the optimum concentration range above which responses to the sugars declined. Methionine and glutamine also stimulated feeding but the response to glycine was very low. Nicotine and tomatine were deterrents. Aqueous extracts from pods and flowers of cowpea and common bean elicited much better feeding responses than the leaves extracts in the fifth instar larvae. Aqueous extracts from cotton leaves and flowers elicited much lower feeding responses than the host plant extracts. This reduced feeding on cotton extracts would reflect absence of chemical feeding stimulants and/or presence of non-polar deterrents. Electrophysiological bioassays showed that aqueous extracts from host and non-host plant parts elicited action potentials in the gustatory neurones of the lateral and medial sensilla styloconica on the maxillary galea as well as the sensilla on the maxillary palp. The palpal sensilla were sensitive to sucrose, NaCl, methionine, glutamine and aqueous extracts of host and non-host plants. Aqueous extracts from different host plant parts evoked higher impulse frequencies than the corresponding extracts from the non-host plant. Furthermore, the spike frequency evoked by a given extract appeared to relate well to results of the feeding bioassays. It appears that inputs into the CNS from the palpal tip sensilla complement those from the styloconica sensilla in order to bring about the observed feeding responses.