Taste cell responses in the polyphagous arctiid, Grammia geneura: towards a general pattern for caterpillars

The caterpillars of Grammia geneura are polyphagous as individuals. Electrophysiological responses of its medial and lateral galeal styloconic sensilla to 21 amino acids, 6 carbohydrates, 10 chemically diverse plant secondary compounds and two inorganic salts were examined. In the medial sensillum, a single cell responded to 8 amino acids, 3 carbohydrates, and the iridoid, catalpol, which is present in a favored hostplant. In the lateral sensillum, one cell responded to amino acids and another to fructose. Two cells in each sensillum responded to secondary compounds and it is suggested that the same cells are stimulated by inorganic salts. There was no evidence of a separate salt-sensitive cell. Phenylalanine stimulated a deterrent cell in the medial sensillum and was behaviorally deterrent. Some essential amino acids did not stimulate any cells and it is suggested that a small number of amino acids (sometimes non-essential) may serve as indicators of nutrient quality. Sugars probably serve as the primary phagostimulants because they are in relatively high concentrations in plants. It is proposed that taste receptor cells should be categorized primarily by their behavioral effects as phagostimulatory or deterrent, rather than their specific ranges of responsiveness. This would emphasize the basic similarities across taxa.     

Electrophysiological responses of taste cells to nutrient mixtures in the polyphagous caterpillar of Grammia geneura

In a normally feeding insect, the taste receptors are exposed to complex mixtures of chemicals, not single compounds. We investigate the responses of neurons in the galeal sensilla of the caterpillar of Grammia geneura to mixtures of nutrient compounds at concentrations occurring in plants. Compounds that stimulated the same neuron were generally additive in their effects in binary mixtures. Amino acids that did not stimulate usually had no effect in mixtures with a stimulating compound, but glutamic acid reduced the response to serine in the medial sensillum. Nutrient compounds that stimulated different cells in a sensillum acted independently of each other. Complex mixtures of amino acids resembling samples of free amino acids from three host plants were less stimulating than expected from their molar concentrations. In host plant selection, the response from the medial sensillum is probably dominated by sucrose; unless sucrose levels are low, amino acids will contribute little to sensory input because they stimulate the same cell as sucrose. In the lateral sensillum, amino acids act independently of sugars. The limited contact chemosensory array of caterpillars seems inadequate to allow them to make fine distinctions between plants on the basis of their free amino acids.     

Chemoreception of amino acids by female fourth- and sixth-instar larvae of the spruce budworm

An electrophysiological approach was used to record the responses of maxillary styloconic sensilla of fourth- and sixth-instar larvae of the spruce budworm Choristoneura fumiferana to 14 amino acids. One cell in the lateral styloconic sensillum was identified as an amino acid-sensitive neuron. All of the amino acids tested, except l-proline and l-arginine, were detected by this cell. Arginine did not evoke a response from either the medial or lateral styloconic sensilla. Proline evoked responses from a cell in the medial styloconic sensillum. It is known from previous behavioural work that l-proline is a phagostimulant and l-valine inhibits feeding in Choristoneura; we thus further characterized the responses to these two amino acids. For both instars, l-proline was detected as low as 0.001 mmol/l and the maximal response was at 50 mmol/l. Stimulation of fourth- and sixth-instar larvae with l-valine showed that the maximum firing frequency was obtained at 1 mmol/l. Above and below this concentration, firing frequency decreases. Sensory responses to the amino acids stimuli did not correlate with known behavioral responses to similar stimuli.     

Sex-specific differences in nutrient regulation in a capital breeding caterpillar, Spodoptera litura (Fabricius)

Nutrient requirements by male and female insects are likely to differ, but relatively little is known regarding how sexes differ in their regulation of macronutrient acquisition. The present study reports the results from a laboratory experiment in which behavioural and physiological components of nutrient regulation were compared between male and female caterpillars of Spodoptera litura (Fabricius). When provided with choices between two nutritionally complementary foods (one is a protein-biased diet and the other a carbohydrate-biased diet), both males and females adjusted their food selection to defend an intake target. However, the composition of diet preferred by the two differed, with females selecting significantly more protein than males with no difference in carbohydrate intake between the two. When confined to single diets with varying mixtures of protein and carbohydrate [P:C ratios, expressed as the percentage of diet by dry mass: protein 42%:carbohydrate 0% (p42:c0), p35:c7, p28:c14, p21:c21, p14:c28, p7:c35], females consumed more macronutrients than did males across on all P:C diets except the extremely carbohydrate-biased diet (p7:c35). Under both choice and no-choice feeding condition, such sex differences in nutrient intake were not expressed until late in the feeding stage of the final stadium. Sexes also differed in post-ingestive utilization of ingested nutrients. Females utilized ingested protein for body growth with greater efficiency compared to males, presumably reflecting provisioning their adult needs for protein to develop eggs, whereas males were more efficient at depositing lipids from carbohydrate intake than females.     

Parasitism of an insect Manduca sexta L. alters feeding behaviour and nutrient utilization to influence developmental success of a parasitoid

The effects of macronutrient balance on nutrient intake and utilization were examined in Manduca sexta larvae parasitized by Cotesia congregata. Insects fed an artificial diet having constant total macronutrient, but with varied ratios of protein and carbohydrate, with altered diet consumption in response to excesses and deficiencies of the individual macronutrients. Bivariate plots of protein and carbohydrate consumption for non-parasitized larvae demonstrated a curvilinear relationship between points of nutrient intake for the various diets, and the larvae grew best on carbohydrate-biased diets. The relationship was linear for parasitized larvae with the growth uniform across diets. On protein-biased diets, the larvae regulated the nitrogen content, containing similar amounts of nitrogen regardless of consumption. Efficiency of nitrogen conversion in non-parasitized larvae was greatest on carbohydrate-biased diets, while nitrogen conversion by parasitized larvae was greatest with intermediate nutrient ratios. Accounting for carbohydrate consumption, the lipid content decreased as dietary carbohydrate increased, but parasitized larvae contained significantly less lipid. The total biomass of parasites developing in individual host larvae was positively correlated with host protein consumption, but the individual parasites were similar in size. Parasitism influences host nutrient consumption in a manner that achieves uniform host growth under diverse nutritional regimes, thereby constraining blood nutrient concentrations within limits suitable for parasite growth and development.     

Feeding behaviour and nutrient selection in an insect <Emphasis Type="Italic">Manduca sexta</Emphasis> L. and alterations induced by parasitism

Manduca sexta L. larvae exhibit broad food acceptance with regard to nutrient content during the first 3 days of the last stadium. Larvae fed diets with a constant combined level of casein and sucrose, but variable ratios, display a linear relationship between protein and carbohydrate intake. Larvae grow best on a diet with equal nutrients, but will consume an excess of one nutrient in order to obtain an adequate amount of the other, as nutrient ratio shifts. Parasitized larvae feed similarly, but the nutrient ratio does not affect growth. Unparasitized larvae regulate intake of protein and carbohydrate when offered choices of protein-biased and carbohydrate-biased diets having combined nutrient levels of 120 g/l, but with variable ratios. Larvae normally consume equal amounts of nutrients, regardless of ratio, and grow similarly. As combined nutrient level is reduced in one diet, larvae abandon regulation and feed randomly. Parasitized larvae offered choice diets with 120 g/l combined nutrients do not regulate nutrient intake. Consumption of nutrients varies widely, but growth is unaffected. Larvae offered choices of diets having equal amounts of casein and sucrose but variable fat (corn oil), fail to regulate fat intake, although both unparasitized and parasitized larvae prefer a diet containing higher fat.     

Multiple mode of action of the feeding deterrent,toosendanin, on the sense of taste in Pieris brassicae larvae

Toosendanin, a tetranortriterpenoid isolated from the bark of Melia toosendan, is a feeding deterrent for larvae of Pieris brassicae. By using electrophysiological techniques, it was found that toosendanin stimulates a deterrent receptor cell located in the medial maxillary sensillum styloconicum. Toosendanin also inhibits responses of both the sugar and glucosinolate receptor cell, which are localized in the lateral sensillum styloconicum. The degree of inhibition of the sugar receptor increases with increasing sucrose concentration. The glucosinolate receptor cell shows a reversed reaction: inhibition by toosendanin decreases with increasing sinigrin concentration. Inhibitory effects occur at a toosendanin concentration as low as 10^–9 M and are dose dependent. The taste neurons that respond to amino acids or deterrents in the lateral sensillum, however, are not affected by toosendanin. It is concluded that the sensory code underlying feeding behaviour is modulated by toosendanin via several different peripheral sensory mechanisms.     

Sensitivity to chemically diverse phagostimulants in a single gustatory neuron of a polyphagous caterpillar

Caterpillars of the arctiid moth, Grammia geneura, are polyphagous, but species of Plantago are amongst their preferred food plants. A neuron in the medial styloconic sensillum on the galea has been shown to have a general phagostimulatory function. Experiments with binary mixtures and cross-adaptation have demonstrated that it responds to some sugars, to several amino acids, and also to catalpol. Catalpol is a plant secondary compound in Plantago and a phagostimulant for the caterpillars. The possible significance of combining sensitivity to nutrient compounds with sensitivity to a secondary compound is discussed. Accepted: 7 October 1999     

A comparison of dietary selection behaviour in larval Locusta migratoria and Spodoptera littoralis

ABSTRACT. . Final instar nymphs of the oligophagous acridid Locusta migratoria (L.) and larvae of the polyphagous noctuid Spodoptera littoralis (Boisduval) were fed for 4, 8 or 12 h, the conditioning period, on one of four artificial diets. Of these, diet PC contained 20% protein and 10% digestible carbohydrate; another, P, contained 20% protein but with the digestible carbohydrate component replaced by cellulose; a third, C, had the protein component substituted by cellulose, and the fourth, O, had both protein and digestible carbohydrate replaced. After this conditioning period, insects were given a choice of two diets, P and C, and hence an opportunity to select for the nutrients, if any, which were lacking in their previous food. Amounts eaten and selection behaviour were then recorded in detail for a total of 9 h. This paper deals with total amounts of diet eaten during the conditioning and choice periods. Spodoptera larvae were more sensitive than the locusts to being fed a nutritionally inadequate conditioning diet, and ate only small quantities of the P, C and O diets as compared with the PC diet, irrespective of the duration of conditioning. Locusts, on the other hand, when restricted to the P diet continued to eat relatively large amounts of it throughout a 12 h conditioning period. Those nymphs fed the C diet ingested large quantities (more than of the PC diet) up until 8 h, after which intake fell. When offered a choice, both species selected for the nutrients missing from the conditioning diet, even if the conditioning period had been as short as 4 h. During the first hour of choice locusts selected the P diet if they had been previously fed C and the C diet if previously fed P. Those deprived of both nutrients increased consumption of both P and C diets. Spodoptera larvae were more sensitive to prior deprivation of digestible carbohydrate than of protein. During the first hour of choice they selected the C diet if previously fed P or O but did not choose the P diet if previously fed C. In the subsequent 8 h of choice, however, a strong selection for the P diet after previous deprivation became apparent. In the locust, the selection for nutrients missing from the conditioning diet continued for the following 8 h of choice but became masked by a tendency, shown by all nymphs, to select C over P. The functional significance and possible physiological basis of all these responses is discussed.     

Nicotine moderates the effects of macronutrient balance on nutrient intake by parasitized <Emphasis Type="Italic">Manduca sexta</Emphasis> L

Effects of dietary nicotine and macronutrient ratio on M. sexta larvae were examined. Larvae were fed a carbohydrate-biased, protein-biased or diet having equal amounts of casein and sucrose, with and without nicotine. Without nicotine, larvae displayed compensatory feeding on the low protein diet, but despite consuming more, grew least on this diet. Nicotine at 0.5% had no effect on nutrient consumption. Nicotine at 1.0 and 2.0% reduced overall consumption and thereby also reduced nicotine consumption. Larvae parasitized by C congregata displayed reduced nutrient intake and growth on all diets. Parasitized larvae responded to 1% nicotine similarly to unparasitized larvae. At 0.5% nicotine, they displayed reduced consumption on all diets, possibly due to altered chemoreceptor sensitivity to nicotine. When offered a choice of two diets having different macronutrient ratios, one with and the other without 0.1% nicotine, all larvae preferred the diet lacking nicotine and failed to regulate nutrient intake such that the nutrient intake target, a ratio of nutrients supporting optimal growth, was achieved. Parasitized larvae consumed less nicotine on a fresh weight basis than unparasitized insects, suggesting that the feeding response of parasitized larvae to nicotine minimizes the exposure of nicotine to developing parasites.     

Restricted diet breadth of the larvae of Antheraea assamensis and the role of the labrum‐epipharynx and galeal sensilla

The silkworm, Antheraea assamensis Helfer (Lepidoptera: Saturniidae), grows primarily on Persea bombycina and Litsea polyantha. To understand if the restricted diet breadth is due to the specific role of gustatory sensilla of the larvae of A. assamensis, the same fifth instar larvae retaining only labrum‐epipharynx or galeal sensilla were subjected to food choice tests. The foods used were leaves of two host‐plant and two non‐host‐plant species. Mean per cent consumption and per cent of choosing larvae were used as parameters for drawing conclusions. The finding indicated involvement of the labrum‐epipharynx for acceptance and galeal sensilla for rejection of a non‐host‐plant species. Scanning electron microscope studies revealed the presence of two sensilla on the galea, one lateral and one medial sensilla styloconicum and two gustatory sensilla in the epipharynx of A. assamensis. The study revealed the key role of galeal sensilla in the restrictive diet‐breadth of A. assamensis.     

Chemoreception of amino acids in larvae of two species of Pieris

ABSTRACT. Specificity and sensitivity of gustatory neurones in response to twenty-two amino acids were studied in larvae of Pieris brassicae L. and Pieris rapae L. (Lepidoptera: Pieridae) using electrophysiological methods. Twelve amino acids stimulated a specific amino acid receptor cell in the lateral styloconic sensillum on the maxillary galea of both species, and a further two evoked single unit responses in the same sensillum of P.brassicae only. Histidine, phenylalanine and tryptophane were the weakest stimulants for P.brassicae , but were among the four best stimulants for P.rapae . In both species, eight amino acids were ineffective. Significant differences in stimulatory effectiveness were found between amino acids. Nutritionally essential amino acids were more effective in both species, as in five other lepidopterous species. Similarities with postulated sites for amino acid recognition in the dipteran Boettcherisca peregrina were found.
Concentration-response (C/R) relations were studied for five amino acids. Significant differences were found in saturated response levels. Parameters characterizing C/R relations were estimated using a logistic model. Comparing C/R parameters with phytochemical data on concentrations of free amino acids in a common host plant, Brassica oleracea L., shows that amino acids are effective stimuli at their natural concentrations. The amino acid chemoreceptor seems able to transmit information about concentration differences of amino acids in the plant tissue.     

Do food protein and carbohydrate content influence the pattern of feeding and the tendency to explore of forest tent caterpillars?

This study examines whether the ratio of protein to carbohydrate affects the timing of meals and the propensity to explore of forest tent caterpillars (Malacosoma disstria). The behavior of fourth instar caterpillars was observed on three semi-defined artificial diets varying in protein (p)-carbohydrate (c) ratio. These diets were (a) p14:c28, (b) p28:c14, and (c) p35:c7. The probability of initiating feeding at first contact with the food and the duration of the first feeding event did not vary across diets, suggesting not much difference in phagostimulatory power. There was also no difference in the total time spent eating, at rest and in motion between diets. However, the timing and duration of meals varied significantly; more short meals were observed on the carbohydrate-biased diet. The duration of pauses between meals also increased with food protein content. Furthermore, caterpillars on the carbohydrate-biased diet were more likely to leave the trail leading to the known food source and to discover a second food source, suggesting that protein deprivation promotes exploration. These findings shed insight into the physiological responses to protein and carbohydrate ingestion and demonstrate how post-ingestive effects can favor consumption of foods containing protein without invoking an explicit mechanism of independent nutrient regulation, but simply by influencing the pattern of feeding and the propensity to explore.     

Morphological and electrophysiological characteristics of the epipharyngeal sensilla of the silkworm, Bombyx mori

One pair of gustatory sensilla was found on the epipharynx ofBombyx mori larvae, and some morphological and electrophysiological characteristics of the epipharyngeal sensilla were investigated. They are sensilla coeloconica composed of a small papilla with a pore at the tip and a swelling of cuticle encircling the papilla. Three bipolar neurons innervate each sensillum. One neuron is an inositol receptor which responds to inositol only. Another cell responds with action potentials of relatively large amplitude to some feeding deterrent substances, such as strychnine nitrate. The thresholds of these cells for inositol and strychnine nitrate are approximately 10⁻⁴ M and 10⁻⁷ M, respectively. At least two kinds of spikes can be observed when these sensilla are stimulated with some salts and acids. Dose-response relationships and time courses of responses to inositol and strychnine nitrate were also examined in this study.     

Electrophysiological studies of gustation in lepidopterous larvae

Summary The electrophysiological responses of the maxillary gustatory sensilla of ten species of lepidopterous larvae to sugars, polyhydric alcohols, amino acids, organic acids, glycosides, and sterols were tested. Of the caterpillars examined three were monophagous (Calpodes ethlius, Ceratomia catalpae, Danaus plexippus), three were oligophagous (Pieris rapae, Papilio polyxenes, Malacosoma americana), and four were polyphagous (Helioihis zea, Estigmene acrea, Isia isabella, Porthetria dispar). The sensilla of all species are structurally similar to the extent of having four chemosensory bipolar neurons in each peg.It is not possible to account for the wide range of responsiveness to diverse compounds in terms of narrowly specific receptors characterized as water, sugar, salt, inositol, or amino acid receptors. Bach receptive cell was found to have only a limited degree of specificity. Not all amino acids stimulated the same cell. Some amino acids stimulated only one; some stimulated two; some inhibited all cells; some inhibited one while stimulating another. Differential sensitivity to amino acids varied from species to species and even between the two sensilla in an individual.The diversity of relationships found with amino acids existed also with respect to acids and glycosides. Ascorbic and hydrochloric acids at equivalent pH values each stimulated a different cell. Glycosides stimulated one or two cells depending on the species and sensillum.Synergism occurred nearly as often as inhibition. Among the instances were synergistic reactions between sucrose and inositol, proline and inositol, sucrose and sinigrin, glycine and sucrose. The occurrence of the phenomenon was specific to certain species and sensilla.Results point to the conclusion that the basis of difference in sensitivity to different compounds from species to species and from receptor to receptor reflects variations in the molecular structure of the receptor substrate. Whatever the mechanisms involved it is clear that caterpillars are able to detect not only compounds that may be of nutritive value but also non-nutritive compounds some of which are secondary plant substances. When the total taste sensitivities of the various species of caterpillars are compared, there is no obvious difference between polyphagous, oligophagous, and monophagous species.This work was supported by National Science Foundation Grant GB 1472.     

Gustatory responsiveness to pyrrolizidine alkaloids in the Senecio specialist, Tyria jacobaeae (Lepidoptera, Arctiidae)

Abstract.  Electrophysiological recordings from taste sensilla of the caterpillar Tyria jacobaeae with the pyrrolizidine alkaloids, characteristic compounds from their host plants, demonstrated sensitivity of a pyrrolizidine alkaloid-sensitive cell in the lateral galeal sensilla at concentrations as low as 1 × 10⁻¹¹ M. Another pyrrolizidine alkaloid-sensitive cell in the medial galeal sensilla responded at higher concentrations. Both pyrrolizidine alkaloid-cells were maximally sensitive to seneciphylline N -oxide and senecionine N -oxide. Seven other pyrrolizidine alkaloids were less stimulating. Monocrotaline N -oxide was the least stimulating. Observation experiments demonstrated that differences in sensitivity to different pyrrolizidine alkaloids at the electrophysiological level were correlated with differences in feeding behaviour; the first feeding bout was of longer duration on diet containing seneciphylline N -oxide than on diet containing monocrotaline N -oxide, and a plain diet was generally not accepted.     

A taste receptor neurone dedicated to the perception of pyrrolizidine alkaloids in the medial galeal sensillum of two polyphagous arctiid caterpillars

It is shown that pyrrolizidine alkaloids are phagostimulants for the caterpillars of two polyphagous arctiid caterpillars, Estigmene acrea and Grammia geneura. The caterpillars will also eat dry glass‐fibre discs containing only pyrrolizidine alkaloid ? an example of pharmacophagy. The tip‐recording technique is used to demonstrate that each species has a neurone in the medial galeal styloconic taste sensillum responding to pyrrolizidine alkaloids, although the species differ in their sensitivities. This neurone responds to at least four different pyrrolizidine alkaloids and their N‐oxides, and experiments indicate that it is dedicated to perception of these compounds. The sensory response is phasic–tonic and during the tonic phase remains unchanged for at least 500 ms, resembling the type of response often seen in neurones that are sensitive to plant secondary compounds producing deterrent effects.     

Effects of artificial and natural diets on success in tip recording and on galeal chemosensillum morphology of European Corn Borer larvae

Abstract. Larvae of Ostrinia nubilalis (Hübner) (Lepidoptera: Pyralidae) were reared on artificial diet, fresh cut corn, or switched from diet to corn for 5 days prior to tip recording. Electrophysiological responses to a standard mixed chemical stimulus were obtained from the galeal uniporous chemosensilla of the early fifth-stadium larvae. Mouthparts of some larvae were briefly treated with protease, then recorded again. Larvae were then processed for scanning electron microscopy (SEM) of chemosensilla terminal pores. Spike:noise ratios were significantly higher for corn-reared, switched-to-corn, and protease- treated larvae, relative to diet-reared larvae. Morphology of the chemo- sensilla terminal pores, as assessed by SEM, reveals that apparent occlusion of the pore tip is significantly greater for diet-reared larvae relative to the other groups. The data suggest an association between quality of the electrophysiological recordings and physical blockage of the terminal sensillum pore. These data should serve as a caveat for those engaged in chemosensory studies of insect herbivores.     

An electrophysiological analysis of the effect of phagostimulant mixtures on the responses of a deterrent-sensitive cell of gypsy moth larvae, Lymantria dispar (L.)

Gypsy moth larvae, Lymantria dispar (L.), are polyphagous feeders. The medial styloconic sensillum of this species bears a taste receptor cell that responds to alkaloids and another that responds to the sugar alcohol, inositol. The lateral styloconic sensillum bears a taste receptor cell that is sensitive to the sugar, sucrose. We tested the effect of two phagostimulants, namely sucrose and inositol, on the response of the deterrent-sensitive cell and found that both phagostimulants suppressed its response, equally, while their combination was significantly more effective. We also tested the effect of two alkaloids (i.e., strychnine and caffeine), which deter feeding in this species, on the response of the inositol- and sucrose-sensitive cells. Although both of these deterrents had no effect in suppressing the response of the sucrose-sensitive cell, they both had an effect in suppressing the inositol-sensitive cell. We also found that sucrose suppressed the response of the inositol-sensitive cell, whereas inositol had no significant effect on the response of the sucrose-sensitive cell. In this paper, we examined the effect of mixtures of these compounds to determine the nature of their interaction. In the context of host–plant interactions and, for example, host recognition, whereby host plant acceptability depends on the total sensory impression acquired from responses to multiple plant components rather than the presence or absence of single stimulant or deterrent compounds, this study could have a direct bearing in the development of natural compounds (i.e., alkaloids) for pest control and crop protection. It will also contribute to our understanding of the neural basis of the feeding behavior of this insect.     

A highly sensitive taste receptor cell for pyrrolizidine alkaloids in the lateral galeal sensillum of a polyphagous caterpillar, Estigmene acraea

Adult males of Estigmene acraea use pyrrolizidine alkaloids to produce pheromones and all stages probably use pyrrolizidine alkaloids for defense. The alkaloids are obtained from plants by the caterpillars. We demonstrate that a contact chemoreceptor neuron in the lateral galeal sensillum exhibits a dose-dependent response to seneciphylline N-oxide, a widely occurring pyrrolizidine alkaloid, down to concentrations of 10(-9) x mol l(-1), and even at 10(-12) x mol l(-1) the response is greater than to salt alone. At concentrations of 10(-6) mol x l(-1) and above the instantaneous firing rate is very high, and at 10(-4) mol x l(-1) initially exceeds 500 spikes s(-1). The firing rate declines in the 200 ms following stimulus onset but then is sustained with an instantaneous firing rate in excess of 100 spikes s(-1) for at least the next 800 ms. At lower concentrations a delay occurs before firing is initiated, and then the pattern of firing is irregular. The cell is equally sensitive to some but not all of several other pyrrolizidine alkaloids tested as free bases and their N-oxides. It also responds to ouabain, which may also serve as a defensive compound, and to asparagine and fructose but with much higher thresholds than to the pyrrolizidine alkaloids.