Contact chemoreception of host plant specific chemicals in larvae of variousYponomeuta species (Lepidoptera)

Abstract

Gustatory sensitivity of larvae belonging to nine differentYponomeuta species was studied. A conspicuous behavioural difference between these closely related species is represented in their host plant preferences. Electrophysiological and morphological examination of the anteriorly-located sensilla revealed that a restricted number of contact chemoreceptors are present. A tip recording technique was used to record gustatory responses of neurones in the lateral and medial styloconic sensilla. The following chemicals were applied as stimuli: sucrose, sedoheptulose, sorbitol, dulcitol, coumarin, phloridzin, salicin, prunasin, rutin, (+)-catechin, the trisodium salt of isocitric acid, ZnCl2, NaCl, and KCl. Most of these stimuli were selected on the basis of chemical composition of the array of host plants of the larvae studied (Table 1). Different spike amplitudes displayed in the recordings indicated that most constituents tested are perceived by single cells. Ionized substances sometimes excited more than one cell. In several recordings, a delayed response appeared. This phenomenon seems related to variation in diameter of the distal pore by which inward diffusion of the stimulus takes place. Analysis of the time course of the recordings indicated that the effect of diffusion of stimulus molecules on the initital neural response may be considerable for the non-ionized substances tested. The functional diversity displayed in the interspecific sensitivity patterns can be partly explained by composition of the host plants on which the larvae feed (Table 2). Compounds of one type act as phagostimulants, and some of these compounds are sufficiently host-specific to act as host-recognition factors. A second type of chemical signal inhibits “wrong” food intake; these substances are classified as deterrents. A few gustatory responses appeared non-adaptive, this type of sensitivity is discussed from an evolutionary point of view.