DETERRENCE CODING BY A LARVAL MANDUCA CHEMOSENSORY NEURON MEDIATING REJECTION OF A NONHOST PLANT, CANNA-GENERALIS L

The physiological basis of phagodeterrence was studied electrophysiolo gically and behaviourally in the phytophagous caterpillars Manduca sex ta and Manduca quinquemaculata. The model unacceptable non-host plant was the canna lily, Canna generalis. A strongly deterrent extract was obtained from fresh leaves of canna by extraction with hot ethanol or ethyl acetate in a blender. Behavioural rejection of these extracts wa s similar to that of fresh leaves, although less intense. In contrast, blender extracts using other solvents, as well as leaf surface rinses , were phagostimulant or neutral. Chromatographic fractionation of the deterrent ethanolic, extract showed the active principles to be moder ately polar and separable into two fractions. Previous ablation experi ments had shown that the medial maxillary styloconica and epipharyngea l sensilla are the two most important chemosensory organs in mediating behavioural rejection of canna leaves; if only one of these organs is spared, the animal completely rejects. canna. We investigated the neu ral responses of the medial styloconica and their contribution to the sensory coding responsible for this phagodeterrence. The active fracti ons of the deterrent ethanolic extract elicited a vigorous response fr om one chemosensory neurone in the medial styloconica. This neurone is distinguishable from others in the medial styloconica by its unique t emporal response parameters and the characteristic shape changes of it s action potentials. The response frequency of this neurone correlates with the degree of phagodeterrence in a dose-dependent manner. Thresh old deterrence occurs at a concentration of extract (1%) that elicits firing in this neurone at a rate of c. 50 spikes/s peak instantaneous frequency and 30 total spikes in the first 1 s.We conclude that this i s a 'deterrent neurone' in the sense that vigorous response from this neurone is a sufficient sensory code for behavioural rejection of cann a. Thus input from a single sensory neurone is capable of blocking fee ding, since only one (unilateral) medial styloconicum is needed to med iate this rejection.