Chemoreception and feeding behaviour in a caterpillar: towards a model of brain functioning in insects

The quantitative relationship between chemoreceptor activity during food intake and total amount of food ingested per unit of time has been studied in last instar larvae of Pieris brassicae. Data on sensory responses and feeding intensity, derived from a study by Blom (1978), show a strong correlation between receptor activity during 1 sec periods of stimulation and food intake during 24 h. It is concluded that nerve impulses from chemoreceptors, which signal the presence of different phagostimulants, are summated algebraically in the central nervous system. Impulses from feeding deterrent receptors counteract the effects of phagostimulants. One impulse from a deterrent receptor neutralizes 2.5 impulses from phagostimulant receptors. Based on the relationships observed a simple model of a feeding centre in the central nervous system has been constructed (Fig. 8).     

Taste receptors in Colorado beetle larvae

Electrophysiological studies show that at least six sensilla on the various mouthparts of larval Colorado potato beetles (Leptinotarsa decemlineata) contain chemosensory cells. These sensilla are found on the galea, maxillary palps, and labial palps. Sodium chloride, sucrose, some amino acids, and chlorogenic acid are among the effective stimuli. The sensitivity spectrum of these cells correlates with known larval phagostimulants.     

Maxillary appendages used by western corn rootworms, Diabrotica virgifera virgifera, to discriminate between a phagostimulant and -deterrent

Ablation of maxillary appendages on western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), were used to determine which mouthparts were involved in discrimination between the phagostimulant, cucurbitacin B, and the phagodeterrent, strychnine. Strychnine deterred feeding in unablated beetles when its concentration on disks was equal to or higher than cucurbitacin B. Unilateral or bilateral lacinea ablation did not affect discrimination between cucurbitacin B and strychnine. Galeal ablations abolished strychnine deterrence at equimolar concentrations of both compounds but not at higher strychnine concentrations, whereas maxillary palp ablation had no effect. However, bilateral ablation of both galea and maxillary palps resulted in a loss of deterrence at all strychnine concentrations. These results indicated strychnine sensitive chemoreceptors were primarily located on the galeae and maxillary palps. However, beetles without galeae and palps still feed on disks suggesting that cucurbitacin B sensitive chemoreceptors were located on other mouthparts and contributed to cucurbitacin B phagostimulation.     

Chemosensory basis of behavioural plasticity in response to deterrent plant chemicals in the larva of the Small Cabbage White butterfly Pieris rapae

Behavioural and electrophysiological responsiveness to three chemically different secondary plant substances was studied in larvae of Pieris rapae L. (Lepidoptera: Pieridae). Three groups of caterpillars were studied that during their larval development were exposed to different rearing diets: an artificial diet or one of two host-plants, cabbage, Brassica oleracea, or nasturtium, Tropaeolum majus. In dual-choice leaf disc assays, caterpillars reared on cabbage were strongly deterred by the phenolic chlorogenic acid, the flavonol glycoside naringin and the alkaloid strychnine. However, behavioural plasticity was found in caterpillars reared on nasturtium or artificial diet in that these did not discriminate against chlorogenic acid. Caterpillars reared on the artificial diet were also significantly less sensitive to naringin and strychnine in the behavioural assay. Electrophysiological studies of the maxillary sensilla styloconica revealed that the deterrent neuron in the medial sensillum, but not in the lateral sensillum, of cabbage-reared caterpillars was more sensitive than the same neuron type of caterpillars reared on nasturtium or artificial diet. We conclude that (1) the diet-induced behavioural habituation to deterrents can at least partly be explained by chemosensory desensitisation of a generalist type of maxillary deterrent neuron; (2) behavioural cross-habituation to the three structurally diverse deterrent compounds can be traced back to cross-sensitivity for these compounds in the same gustatory neuron.     

Resistance of Pteridium aquilinum to attack by non-adapted phytophagous insects

Fresh bracken Pteridium aquilinum L. Kühn or crude bracken extracts deterred 9 non-adapted phytophagous insect species from feeding or settling. Feeding deterrent activity to 5 out of 7 insect species was present in crude bracken extracts at all times over the growing season and was significantly highest in May. Sequential isolation and purification of deterrents indicated that the sesquiterpene pterosin F (6-chlorethyl-2,5,7-trimethyl-indan-1-one) occurred in sufficiently high concentrations in late May/early June bracken frond (24–28 mg/kg fresh wt) to be partially responsible for the deterrent effects of crude bracken extracts to two insect species. A number of other fractions had feeding deterrent activity, but these were not characterized or quantified.     

Food mixing by generalist grasshoppers: Plant secondary compounds structure the pattern of feeding

Plant secondary compounds can prevent feeding by phytophagous insects or, if ingested, can be poisonous to them. Less attention has been paid to the additional effects they have on feeding behavior when they are only weakly deterrent or not deterrent at all. Experiments were carried out on the generalist grass-hopperSchistocerca americana. Individuals were presented either with two cakes of high-quality artificial food with a single deterrent compound added or with two cakes, each with a different added deterrent compound. The deterrents consisted of single plant secondary compounds that were either marginally or strongly deterrent. There were profound differences in feeding behavior between those individuals given identical and those given different cake types, including longer feeding bouts on single cakes when a choice of different cake types was available. The behavioral effects demonstrate that the presence of secondary compounds in one food can influence the patterns of feeding on other available foods and suggest that such chemicals could impact foraging activities in a complex manner.     

Feeding Behavior and Survival of Glucose-Averse Blattella germanica (Orthoptera: Blattoidea: Blattellidae) Provided Glucose as a Sole Food Source

Glucose-averse (Glu/Glu) male German cockroaches, Blattella germanica, rejected brief exposures to glucose solutions despite food deprivation so extreme as to cause high mortality. In the 24-h period following 5 days of food deprivation, both Glu/Glu and wild-type (+/+) were given a continuous source of 1 M glucose solution. Although Glu/Glu visited the solution as frequently as +/+, feeding bouts were significantly shorter. These truncated feeding bouts were sufficient to extend Glu/Glu longevity to ca. 80 days, regardless of glucose concentration. When provided continuously, the glucose was completely rejected by Glu/Glu for the first 3 days. However, by day 9, glucose intake levels were similar to those of wild-type cockroaches. The initial feeding lag probably accounted for the lower long-term survival of the Glu/Glu vs the +/+ strain. Under the stress of food deprivation, glucose-averse B. germanica modify their feeding behavior toward glucose like many phytophagous insects provided non-host plants in no-choice situations.     

A neurophysiological study of sensitivity to a feeding deterrent in two sister species of Heliothis with different diet breadths

Heliothis subflexa and H. virescens are sister species that differ markedly in their hostplant specificity: the former is a specialist on one plant genus, the latter feeds on plants from many families. The behavioral threshold for rejection of deterrent chemicals is lower in larvae of H. subflexa than in those of H. virescens. In this paper, we examine the responses of the galeal styloconic sensilla of these larvae to stimulation by three chemicals, sucrose and inositol, which are phagostimulants, and sinigrin, a deterrent, in an attempt to determine the neural basis for the differences in feeding behavior between the species. The species difference could not be attributed to differences in firing rate of the deterrent-sensitive cells, differences in the ratio of responses to phagostimulants and deterrents, differences in the rates of adaptation of the sensory neurons, or differences in the extent of interactions between chemicals at the peripheral sensilla. We conclude that the differences between the species probably result from differences in processing sensory information within the central nervous system.     

Analysis of mouthpart movements during feeding of Frankliniella occidentalis (pergande) and F. schultzei trybom (Thysanoptera : Thripidae)

Scanning electron microscopy was used to elucidate the morphology and sequential movement of thrips feeding structures in the 2 species, Frankliniella occidentalis and F. schultzei, (Thysanoptera : Thripidae). The mouthcone consists of paired paraglossae, and fringed labral pad, a single, apically fused mandible and a pair of interlocking maxillae. The maxillae are open apically and form a feeding tube. Ten pairs of sensory pegs of 3 distinct morphological types (sensilla basiconica with a cuticular collar, sensilla basiconica without a cuticular collar, and sensilla trichoidea) were found on the paraglossae. The possible function of these sensory structures in host finding and choice are discussed. No structure for rasping the leaf's surface was found on the mouthcone or the ventral surface of the insect. Live specimens were observed feeding through Parafilm on artificial media (sucrose solution) and lettuce leaf tissue. These observations support earlier findings that thrips feed by piercing leaf cells with the mandible and ingesting cell contents through the feeding tube formed by the maxillary stylets. Based on these findings, we suggest that thrips be classified as piercing-sucking rather than rasping-sucking insects.     

Cross habituation to feeding deterrents and acceptance of a marginal host plant byPieris rapae larvae

Sensitivity of the cabbage butterfly,Pieris rapae L. to feeding deterrents was compared for larvae reared on different food sources under laboratory conditions. Since cabbage-reared larvae normally reject nasturtium,Tropaeolum majus L., the effects of previous exposure to allelochemicals on larval acceptance or rejection of this plant were also examined. When compared with cabbage-reared larvae, nasturtium-reared larvae were less sensitive to feeding deterrents including cymarin, erysimoside and 2-O-β-d-glucosyl cucurbitacin E. Nasturtium-reared larvae were insensitive to chlorogenic acid, which was deterrent to cabbage-reared larvae. Feeding by larvae reared on a wheat germ diet was not deterred by these compounds. The results indicate that dietary experience can extensively affect larval sensitivity to feeding deterrents and that cross habituation of larvae to deterrents occurs in response to certain chemical constituents of nasturtium and wheat germ diet. Digitoxin, however, proved to be an exception. Larvae reared on either nasturtium or wheat germ diet were as sensitive to digitoxin as those reared on cabbage. Previous results have shown that rejectionof nasturtium by cabbage-reared larvae is due to the presence of strong feeding deterrents in this plant. However, more than 50% of 2nd instar larvae reared from neonate on cabbage leaves treated with strophanthidin, cymarin, erysimoside, digitoxigenin and digitoxin accepted nasturtium as a food source. 2-O-β-d-glucosyl cucurbitacin E, 2-O-β-d-glucosyl cucurbitacin I and rutin were also active in causing larvae to feed on nasturtium. Thus dietary exposure to unrelated plant chemicals can profoundly affect insect acceptance of a plant that contains feeding deterrents.     

The role of an epipharyngeal sensillum in the perception of feeding deterrents by Leptinotarsa decemlineata larvae

An epipharyngeal taste sensillum in Leptinotarsa decemlineata larvae was studied. Electron microscopy showed that the sensillum is innervated by five neurons. Electrophysiological experiments showed that one of these cells responds to water, a second to sucrose and a third to two feeding deterrents that were also effective in a behavioural test. Receptor cells sensitive to feeding deterrents were not previously reported for L.␣decemlineata larvae or adults. The response of the sucrose-sensitive cell was strongly inhibited by one of the two feeding deterrents and only slightly by the other feeding deterrent. The relationship between the behavioural and electrophysiological results is discussed in order to elucidate the neural code of feeding deterrents in L. decemlineata larvae. We conclude that probably both the response of the deterrent cell and peripheral interactions exerted by feeding deterrents on the sucrose-sensitive cell determine the potency of feeding deterrents. The present results provide a physiological basis for the hypothesis that the presence or absence of feeding deterrents in potential food plants is a decisive cue in food plant selection by L. decemlineata larvae. Accepted: 25 March 1998     

Endophyte isolate and host grass effects on Chaetocnema pulicaria (Coleoptera: Chrysomelidae) feeding

Endophytic fungi belonging to the genus Neotyphodium, confer resistance to infected host grasses against insect pests. The effect of host species, and endophtye species and strain, on feeding and survival of the corn flea beetle, Chaetocnema pulicaria Melsheimer (Coleoptera: Chrysomelidae) was investigated. The grass-endophyte associations included natural and artificially derived associations producing varying arrays of common endophyte-related alkaloids or alkaloid groups, peramine, lolitrem B, ergovaline, and the lolines. Preference and nonpreference tests showed that C. pulicaria feeding and survival were reduced by infection of tall fescue with the wild-type strain of N. coenophialum, the likely mechanism being antixenosis rather than antibiosis. In the preference tests, endophyte and host species effects were observed. Of the 10 different Neotyphodium strains tested in artificially derived tall fescue associations, eight strongly deterred feeding by C. pulicaria, whereas the remaining two strains had little or no effect on feeding. Infection of tall fescue with another fungal symbiont, p-endophyte, had no effect. Perennial ryegrass, Lolium perenne L., infected with six strains of endophyte, was moderately resistant to C. pulicaria compared with endophyte-free grass, but four additional strains were relatively inactive. Six Neotyphodium-meadow fescue, Festuca pratensis Huds., associations, including the wild-type N. uncinatum-meadow fescue combination, were resistant, whereas three associations were not effective. Loline alkaloids seemed to play a role in antixenosis to C. pulicaria. Effects not attributable to the lolines or any other of the alkaloids examined also were observed. This phenomenon also has been reported in tests with other insects, and indicates the presence of additional insect-active factors.     

Potential for habituation to a neem-based feeding deterrent in Japanese beetles, Popillia japonica

We tested the potential for the Japanese beetle, Popillia japonica Newman, to habituate to a neem-based feeding deterrent applied to foliage of linden, Tilia cordata L., a preferred host for the adults. Female beetles' consumption of control foliage versus foliage treated with either a low or high rate of neem insecticide, corresponding to 9 or 39 pm azadirachtin, respectively, was tested in a series of 4-h choice or no-choice tests over four successive days. In another experiment, females were conditioned for 22 h with either control foliage, leaves treated with the low rate, or a mixture of both treated and untreated leaves. Deterrence of either the low or high rate of neem to these beetles was then evaluated in choice tests with control foliage, as before. In choice tests, mean consumption of control foliage was always greater than for treated foliage, regardless of rate. There was, however, proportionately more feeding on foliage treated with the high rate upon successive exposures. In no-choice tests, beetles initially deterred by the low rate were not significantly deterred by that rate by the third and fourth days of the experiment. Finally, beetles conditioned by exposure to leaves treated with the low rate were not deterred by that rate in a subsequent choice test, although they were deterred by the higher rate. Despite these trends, we suggest that Japanese beetles' polyphagy and mobility probably would reduce the likelihood for habituation to neem-based feeding deterrents in the field.     

Effects of quantitative and qualitative variation in phenolic compounds on feeding in three species of marine invertebrate herbivores

The deterrent effects of brown algal phenolic compounds and the terrestrial polyphenolic tannic acid on feeding by three species of invertebrate herbivores from central California, including the gastropods Tegula funebralis (Adams) and Tegula brunnea (Phillipi) and the echinoid Strongylocentrotus purpuratus (Stimpson) were examined. Algal phenolics used were the monomeric phenolic phloroglucinol and polyphloroglucinols from Fucus vesiculosus (Linnaeus), Halidrys siliquosa (Linnaeus) Lyngbye and Eisenia arborea Areschoug. All of the polyphenolics deterred feeding by all three herbivores at concentrations of 5 mg · ml⁻¹ in agar disks. Concentrations of 2 mg · ml⁻¹ also generally deterred feeding by the gastropods (these levels were not tested against S. purpuratus). Relative amounts of deterrence by different compounds were similar, especially for the gastropods. Phloroglucinol deterred feeding by the echinoids, but not by T. funebralis. Responses of the echinoids were otherwise similar to the gastropods, but more variable. I also demonstrated deterrence of S. purpuratus by tannic acid using the “tanned” kelp technique of Steinberg (1985). Reactivity of the different phenolic compounds in the Folin-Denis procedure, a common colorimetric assay used to estimate levels of phenolics in plant tissue, was similar. This suggests that measuring phenolic levels in brown algae by this technique will not be greatly confounded by the occurrence of different kinds of phenolic molecules in different brown algae. This result, in combination with the similarity of the deterrent effects of the compounds used in this study, increases the validity of previous studies in the northeastern Pacific Ocean which correlate algal phenolic levels and diets or feeding preferences of invertebrate herbivores. For plants and herbivores in this region, this assay is a reasonable measure of a biologically meaningful phenomenon — levels of phenolic deterrents in the algae.     

Sensing the Underground - Ultrastructure and Function of Sensory Organs in Root-Feeding Melolontha melolontha (Coleoptera: Scarabaeinae) Larvae

Introduction

Below ground orientation in insects relies mainly on olfaction and taste. The economic impact of plant root feeding scarab beetle larvae gave rise to numerous phylogenetic and ecological studies. Detailed knowledge of the sensory capacities of these larvae is nevertheless lacking. Here, we present an atlas of the sensory organs on larval head appendages of Melolontha melolontha. Our ultrastructural and electrophysiological investigations allow annotation of functions to various sensory structures.

Results

Three out of 17 ascertained sensillum types have olfactory, and 7 gustatory function. These sensillum types are unevenly distributed between antennae and palps. The most prominent chemosensory organs are antennal pore plates that in total are innervated by approximately one thousand olfactory sensory neurons grouped into functional units of three-to-four. In contrast, only two olfactory sensory neurons innervate one sensillum basiconicum on each of the palps. Gustatory sensilla chaetica dominate the apices of all head appendages, while only the palps bear thermo-/hygroreceptors. Electrophysiological responses to CO₂, an attractant for many root feeders, are exclusively observed in the antennae. Out of 54 relevant volatile compounds, various alcohols, acids, amines, esters, aldehydes, ketones and monoterpenes elicit responses in antennae and palps. All head appendages are characterized by distinct olfactory response profiles that are even enantiomer specific for some compounds.

Conclusions

Chemosensory capacities in M. melolontha larvae are as highly developed as in many adult insects. We interpret the functional sensory units underneath the antennal pore plates as cryptic sensilla placodea and suggest that these perceive a broad range of secondary plant metabolites together with CO₂. Responses to olfactory stimulation of the labial and maxillary palps indicate that typical contact chemo-sensilla have a dual gustatory and olfactory function.     

Specialist deterrent chemoreceptors enable Pieris caterpillars to discriminate between chemically different deterrents

Pieris butterflies (Lepidoptera: Pieridae) are specialist herbivores of cruciferous plants. They exploit glucosinolates, secondary plant metabolites chemotaxonomically characteristic for this plant family, as token stimuli. In addition to particular glucosinolates, some genera of the Cruciferae contain cardenolides, steroidal allelochemicals that act as potent feeding and oviposition deterrents to several Pieris species. We investigated the sensory mechanisms by which these compounds are perceived in larvae. Pieris caterpillars and many other lepidopterous species are endowed with so-called generalist deterrent receptors, that respond to a broad spectrum of secondary plant substances. In Pieris caterpillars we found a second type of deterrent chemoreceptor in maxillary styloconic taste sensilla. This neuron is very sensitive to cardenolides (threshold 0.1–0.3 M). The generalist deterrent receptor also responds to these substances but its threshold lies at 50–100× higher concentrations. In behavioural preference experiments Pieris brassicae L. caterpillars preferred cardenolide-treated cabbage leaf discs when confronted with a choice between them and a deterrent substance that does not occur in the Brassicaceae. The cardenolides acted as potent deterrents when offered against untreated cabbage leaf discs. This demonstrates that the balance of activity elicited in the two types of deterrent chemoreceptors determines the behavioural decision.     

Behavioral and physiological resistance of the German cockroach to gel baits (Blattodea: Blattellidae)

A gel bait-resistant German cockroach, Blattella germanica (L.), strain Cincy was collected in Cincinnati, OH. This strain exhibited a high level of behavioral resistance to Avert (0.05% abamectin) and Maxforce FC (0.01% fipronil) gel baits. Topical application assays indicated moderate levels of physiological resistance of the Cincy strain to abamectin and fipronil. Resistance ratios (based on LD50 values from topical applications) to abamectin and fipronil were 2.5 and 8.7, respectively. The Cincy strain of had a significantly lower LD50 value to abamectin than a nonaverse field strain (Dorie) and similar LD50 values to fipronil as the Dorie strain. The aversion behavior (avoidance of gel baits) was therefore caused by food ingredients in the gel baits. The Cincy strain showed avoidance of agar containing fructose, glucose, maltose, and sucrose, which are phagostimulants to the laboratory strain. Modifications of the inert ingredients in the Maxforce FC gel bait significantly improved the efficacy against the Cincy strain. The Cincy strain produced significantly smaller oothecae and lower numbers of eggs in each egg capsule than the nonaverse Jwax and Dorie strains of cockroaches, suggesting fitness costs are associated with resistance.     

亚洲小车蝗触角转录组及化学感受蛋白基因表达谱分析(英文)

【目的】化学感受蛋白(chemosensory proteins, CSPs)是一类小分子的可溶性蛋白,在昆虫中发挥多种作用。本研究旨在组装中国北方主要草原害虫之一——亚洲小车蝗Oedaleus asiaticus的触角转录组,鉴定出化学感受蛋白基因以及分析其在成虫不同组织中的表达水平。【方法】利用RNA-s eq 亚洲小车蝗成虫触角进行转录组测序和组装;通过筛选转录组数据库、克隆及测序,鉴定出化学感受蛋白基因;应用qPCR分析CSP基因在成虫不同组织(触角、去除触角和口器的头、上唇、去掉下唇须的下唇、下唇须、下颚须、胸部、跗节、翅和腹部)中的表达模式。【结果】成功构建了亚洲小车蝗成虫触角转录组,共获得61 629个unigenes,平均长度为733 nt,总长度和N50分别为45 175 449和1 130 nt。其中26 064个unigenes(42.29％)注释到6个数据库(NR, NT, Swiss-Prot, KEGG, COG和GO)。通过Blast验证、克隆和测序鉴定出17个CSP基因;BlastP和系统发育分析表明亚洲小车蝗CSPs(OasiCSPs)与东亚飞蝗Locusta migratoria CSPs(LmigCSPs)和沙漠蝗Schistocerca gregaria CSPs(SgreCSPs)关系最密切。qPCR分析表明,8个CSP基因在成虫不同组织中的表达水平存在显着差异,特别是,OasiCSP8在下唇须和下颚须中高表达,而OasiCSP11和OasiCSP13在触角中表达量最高。OasiCSP15在化学感受器官(触角、上唇、去掉下唇须的下唇、下唇须、下颚须和跗节)中的表达量远高于非化学感受器官(去掉触角和口器的头部、胸部、翅和腹部)中的表达量,而OasiCSP12在几乎所有测定的组织中具有相似的表达分布。【结论】OasiCSPs在亚洲小车蝗化学感受和发育过程中可能起着多种作用,这些结果为进一步研究这些化学感受蛋白在亚洲小车蝗中的功能奠定了基础。     

Insect feeding deterrent activity of bisabolangelone and of some sesquiterpenes of eremophilane type

The insect feeding deterrent activity of some sesquiterpenes of furanoeremophilane type and of the related eremophilanolide type were tested towards three selected storage pests: Sitophilus granarius, Tribolium confusum and Trogoderma granarium. The results were compared with the biogenetically related compound of bakkenolide type, known as a potent antifeedant. Bisabolangelone, tested on the same insects, exhibited the strongest activity, and may be included in the class of very good insect feeding deterrents. The antifeeding activity of bisabolangelone and bakkenolide A towards larvae of Leptinotarsa decemlineata was also observed.