Electrophysiological studies of gustation in lepidopterous larvae

Summary Comparisons were made of the eleotrophysiological responses of the maxillary gustatory receptors of the following categories of caterpillars: (1) three closely related species (Papilio polyxenes L.,P. troilus L., andP. glaucus L.) each of which feeds on a different group of plants; (2) two unrelated oligophagous species (P. glaucus L. andMalacosoma americana Fabr.) that have one preferred food plant in common; (3) three unrelated monophagous species (Danaus plexippus L.,Euchaetias egle Drury, andPygarctia eglenensis Clemens) that share the same plant. Materials tested included sodium chloride, carbohydrates, amino acids, glycosides, and the saps ofDaucus carota L. andFoeniculum vulgare Mill. (the food plants ofP. polyxenes),Sassafras albidum (Nutt.) andLindera Benzoin (L.) (the food plants ofP. troilus),Prunus virginiana L. (a favored food ofP. glaucus andM. americana),Asclepias syriaca L. andApocynum androsaemifolium L. (eaten byD. plexippus,E. egle, andP. eglenensis), andBrassica oleraceae L. (food plant ofPieris rapae).The following conclusions were drawn: (1) no species of caterpillar gives a single standard electrophysiological response to all of the plants it rejects; that is, rejection is not a unitary modality; (2) a plant that is unacceptable to several species of caterpillars does not elicit the same pattern of response from each; (3) a food plant that is shared by several species of caterpillars does not elicit the same pattern of response from each; (4) a species of caterpillar that has more than one food plant does not generate the same sensory pattern to each; (5) there is no universal difference between sensory patterns for acceptance and those for rejection.A model based upon the hypothesis of across-fiber patterning is proposed to explain these results. The essence of this model is that the receptors have unique but overlapping action spectra and that each compound or mixture of compounds in leaf saps that can be discriminated generates a unique total pattern of response. Whether or not a plant is ingested depends, therefore, not on the presence or absence of a single stimulant or deterrent but upon the total sensory impression derived from integrated response to multiple plant components. Prior to the first bite a caterpillar makes its first discrimination on the basis of olfactory clues.This work was supported by National Science Foundation Grant GB 1472 and the Class of 1877 Professorship of Princeton University.     

Host plant direct defence against eggs of its specialist herbivore,Heliothis subflexa

1. The insect Heliothis subflexa Guenée is a specialist on plants in the genus Physalis. In the present study, the physical response of Physalis leaves to egg deposition by H. subflexa is described. 2. It was observed that the leaves of Physalis plants respond to the eggs of H. subflexa, while co‐occurring non‐host plants do not. Leaves of Physalis angulata L. and Physalis pubescens L. respond to H. subflexa eggs by the formation of (i) necrotic tissue, (ii) undifferentiated cells that form a bump (neoplasm) under the eggs of this herbivore, or (iii) both types of responses. 3. Greenhouse experiments showed that 64% of eggs laid on P. angulata elicited a response, and that a response to an egg decreased the probability of hatching. Further experiments in the field with P. angulata showed that the mean response to eggs by leaves was 31%, and that this response increased as temperature increased. Field experiments also confirmed that a plant response to an egg decreased the probability of hatching and increased the probability of removal from the plant by physical dislodgement or predation. 4. Eggs that elicited a response had a 25% lower probability of hatching and a 28% lower probability of remaining on the plant, resulting in an average fitness cost of 19.3% for H. subflexa. This is the first study to show an induced direct physical defence of a plant against eggs of a noctuid moth.     

The genetic basis of host range inHeliothis virescens: larval survival and growth

Larval feeding was assayed in a generalist caterpillar (Heliothis virescens (F.)), a specialist caterpillar (Heliothis subflexa (Gn.)), their F₁ hybrids and a backcross withH. s. to gain a preliminary understanding of the genetic basis of host use inH. v. Plants used in these experiments were tobacco (Nicotiana tabaccum), cotton (Gossypium hirsutum), soybean (Glycine max) (hosts ofH. v.) and ground cherry (Physalis pubescens) (host ofH.s.). A feeding study ofH.v., H.s. and their reciprocal hybrids showed that, after the first eight days of feeding,H.v. had its highest survival and weight gain on soybean and had lowest survival and weight gain on its non-host,Physalis. H.s. had its highest survival and weight gain on its host plant,Physalis, and performed very poorly on the three non-host plants. The two F₁ hybrids were SV₀ (offspring ofH.s. female andH.v. male) and VS₀ (offspring ofH.v. female andH.s. male). The hybrids did not differ from each other, indicating no sex linkage or maternal effects, except that VS₀ had greater weight gain on tobacco than did SV₀. The hybrids, unlike their parents, survived well on all four host plants and their weight gain was intermediate on all four host plants. In a separate experiment the VS₀ hybrid was mated to the specialist to produce the VS₁ backcross. In contrast to the F₁, the backcross had significantly lower survival than the generalist on soybean, cotton, tobacco and weight gain was lower on soybean, cotton and tobacco but higher onPhysalis. Survival and weight gain on cotton and tobacco were inherited as partially dominant traits; onPhysalis there was overdominance for survival and complete dominance for weight gain; on soybean both survival and weight gain were additive. Survival on cotton,Physalis, soybean and tobacco and weight gain onPhysalis could not be completely explained by a model that included only dominance and additive effects. These traits may be influenced by epistatic and/or environmental effects.     

Plant Finding and Acceptance Behaviors of Anasa tristis (DeGeer)

Finding and acceptance behaviors of Anasa tristis were investigated using plants that are suitable (pumpkin) or unsuitable (cucumber, coleus) for development. A. tristis found plants in the absence of non-contact cues. Plant finding is enhanced by olfaction and vision. A. tristis found suitable and unsuitable plants equally well. The importance of olfaction in plant acceptance is suggested by intense antennation while on plant and change in behavior on pumpkin and coleus after removal of antennae. Insects on suitable and unsuitable plants dab the surface, contact the plant surface with the labium and insert stylets. Post ingestive effects are important for plant acceptance and rejection. Ingesting water and some nutrients from plants unsuitable for development can be important in extending the time available to move between resource patches.     

Chemotaxis ofAeromonas hydrophila to the surface mucus of fish

Isolates ofAeromonas hydrophila from various sources show different chemotactic responses to mucus from the surface of freshwater fish. Some isolates were nonchemotactic to fish surface mucus. Isolates ofA. hydrophila from fish lesions had a significantly higher chemotactic index than isolates ofA. hydrophila from water. Maximum chemotactic responses occurred more often to diluted fish mucus than to undiluted samples. Fish which were experimentally stressed did not produce mucus that was more or less chemotactic than that of unstressed fish. Fish with red-sore lesions produced surface mucus which was not chemotactic toA. hydrophila. Differences between fish, for any isolate, were also not significant. The chemotactic substance(s) in fish mucus has a molecular weight of approximately 100,000 and did not appear to be labile when heated to 56°C.     

Deterrence coding by a larval Manduca chemosensory neurone mediating rejection of a non-host plant, Canna generalis L.

Abstract. The physiological basis of phagodeterrence was studied electrophy-siologically and behaviourally in the phytophagous caterpillars Manduca sexta 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 was 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 moderately polar and separable into two fractions. Previous ablation experiments had shown that the medial maxillary styloconica and epipharyngeal 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 neural responses of the medial styloconica and their contribution to the sensory coding responsible for this phagodeterrence. The active fractions of the deterrent ethanolic extract elicited a vigorous response from one chemosensory neurone in the medial styloconica. This neurone is distinguishable from others in the medial styloconica by its unique temporal response parameters and the characteristic shape changes of its action potentials. The response frequency of this neurone correlates with the degree of phagodeterrence in a dose-dependent manner. Threshold 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 Is. We conclude that this is a ’deterrent neurone’ in the sense that vigorous response from this neurone is a sufficient sensory code for behavioural rejection of canna. Thus input from a single sensory neurone is capable of blocking feeding, since only one (unilateral) medial styloconicum is needed to mediate this rejection.     

Chemically mediated associative learning in the host foraging behavior of the aphid parasitoidAphidius ervi (Hymenoptera: Braconidae)

The effect of experience on the responsiveness of the braconid parasitoidAphidius ervi to host(Acyrthosiphon pisum)-associated cues was investigated on bean plants(Vicia faba) using a wind tunnel bioassay. Oviposition experience on the plant-host complex significantly increased the oriented flight and landing responses ofA. ervi females to an undamaged plant and to a plant-host(A. pisum) complex. However, oviposition experience onA. pisum aphids when isolated from the plant did not change their responses to the intact plant and the complex. Searching on an unwashed plant which had been previously damaged byA. pisum also increased their response to an undamaged plant and a host-damaged plant, whereas the experience of searching on an undamaged plant did not significantly change their responses to undamaged plants. However, when parasitoids were allowed to search on an undamaged plant which had been sprayed withA. pisum honeydew, this significantly increased their response to an undamaged plant. Oviposition experience on the plant-host complex and foraging experience on a host-damaged plant or an undamaged plant sprayed with honeydew also significantly reduced the mean time taken by the parasitoids to respond in the wind tunnel. The behavioral changes associated with such experience were acquired within 30 min and persisted for at least 3 days. The results demonstrate the capacity ofA. ervi to learn associatively olfactory cues from plants, host-damaged plants, and plant-host complexes and confirm the role of aphid honeydew as a host recognition kairomone forA. ervi.     

Hair morphology and systematics ofPhysalis (Solanaceae)

Hair morphology was exammed on developing seedlings and mature plants of ten species from five of seven series inPhysalis. The taxonomic importance of hair development and the distribution of hair types then was evaluated with respect to other comparative data. Two classes of hair types were observed, as found in earlier investigations ofSolanum andRhododendron. The first hair class comprises a sequence from unbranched hairs to dendroid-stelliform hairs. Hairs of this class show considerable variability among taxa in size, presence or absence of glandular tips, and presence and degree of branching, and so were useful in making taxonomic comparisons. The second hair class consists of short hairs, each tipped with a multicellular gland. This second hair class was not useful taxonomically because of its lack of variability in morphology and its often rare occurrence on a plant. The taxa studied exhibit distinctive trichome features which serve to distinguish series and, in some cases together with other data, provided new insight into previously enigmatic species relationships.     

Intraspecific Variation in Chemical Attraction of Rice to Insect Predators

The olfactory response of predators of the brown planthopper,Nilaparvata lugensStål, to different genotypes of rice (14 cultivars and breeding lines ofOryza sativaL. and 1 wild species,Oryza nivaraSharma et Shastry) was measured in an airflow olfactometer. Odor from rice plants attracted more females of the mirid predatorCyrtorhinus lividipennisReuter than plain air (control) on only 6 of the 15 rice genotypes. Orientation ofC. lividipennistoward volatiles of certain rice genotypes was apparent even when the plants were free of the brown planthopper. However, the predator distinguished between prey-infested and uninfested plants and preferred plants with eggs over plants with nymphs. The predator did not distinguish different stages of plant growth (vegetative, booting, or flowering). Plants artificially injured to simulate brown planthopper oviposition wounds were not as attractive to the predator as plants on which the planthopper had oviposited. The preassay preconditioning on the cultivar TN1 did not produce a predator bias for this genotype. This suggests that rearing effects or chemically mediated associative learning reported for some natural enemies did not influenceC. lividipennis'host response. Results with another predator, the coccinellidMicraspis hirashimaiSasaji, produced less consistent behavior. Planthopper-infested plants attracted more females ofM. hirashimaithan unifested plants in only 1 of the 12 rice genotypes evaluated. Implications for augmenting predators by rice cultivar selection and modification are discussed.     

Localization of the chemoreceptive properties of the surface membrane ofParamecium tetraurelia

Summary The plasma membrane ofParamecium tetraurelia comprises two morphologically distinct components; a membrane that encloses the cell body and a ciliary membrane. In order to investigate the relative contributions of the two membranes to attractant-induced membrane potential changes, cells were deciliated with ethanol and their subsequent responses to attractants examined.Deciliation did not significantly affect the magnitude of the hyperpolarizations evoked by acetic or lactic acids, and had no effect on the concentration dependence of responses to folic acid. We conclude that the components necessary for detection and response to attractants are not exclusive to the ciliary membrane ofP. tetraurelia. Deciliation ofParamecium concomitantly permits localized chemical stimuli to be applied directly to the cell surface in the absence of strong fluid currents that are generated by the activity of the locomotory organelles. By systematically applying K₂ folate to a number of sites on the cell surface, it has been possible to demonstrate an anterior-posterior gradient of chemosensitivity on the cell body ofP. tetraurelia.     

Growth inhibition of suspension cultured plant cells by ribonucleases

Extracellular, stylar RNases (S-RNases) are produced by self-incompatible, solanaceous plants, such asNicotiana alata, and are thought to be involved in selfpollen rejection by acting selectively as toxins to selfpollen. In this study, the toxicity of RNases to other plant cells was tested by culturing cells ofN. alata andN. plumbaginifolia in the presence ofS-RNases fromN. alata. The growth of cultured cells ofN. plumbaginifolia was inhibited by theS-RNases, but viability was not affected. Growth of cultured cells of oneN. alata selfincompatibility genotype was inhibited by twoS-RNases, indicating that inhibition was not allele specific. Comparisons with the effects of inactivated RNase and other proteins, suggest that the inhibition of growth byS ₂-RNase was partly, but not wholly, due to RNase activity. Heat-denaturedS ₂-RNase was a very effective inhibitor of cell growth, but this inhibitory activity may be a cell surface phenomenon.     

Oviposition of potato tuber moth,Phthorimaea operculella Zell. (Lepidoptera: Gelechiidae); identification of host-plant factors influencing oviposition response

Abstract

A series of laboratory experiments was conducted to investigate the nature of the factors which determine acceptance or rejection of plants for oviposition by potato tuber moth. Bare leaves of only two plant species (potato and egg-plant) were accepted for oviposition, but when covered with a layer of muslin, leaves of all plants examined—including several known non-host plants—were rendered acceptable. Thus, volatile repellent factors are not possessed by unacceptable plants; rather, they must be rejected either because of chemical deterrents detectable only on contact or because of unsuitably textured surfaces. Plant juices expressed on to a covering layer of muslin increased the acceptance of potato and egg-plant, had no effect with tobacco, and decreased acceptance of all known non-host plants and of tomato. Brushing the leaf surface without releasing plant juices induced a detectable deterrent effect with some plant species. There was no stimulation or deterrence when the oviposition surface was separated from expressed plant juices by about 2 mm. Tomato reacted like known non-host plants in all tests. Potato and egg-plant stimulated greater total egg deposition in most experiments, but strongly deterrent treatments did not reduce the total egg lay. Antennectomy reduced but did not eliminate the stimulatory effect of potato and the deterrent effect of bean plant juices. Virgin moths were not stimulated to oviposit by potato peel. It is concluded that selection of plants for oviposition is determined both by the physical nature of their surfaces and by chemical factors which are detected only on contact. The effects of stimulative chemical factors in host plants and deterrent factors in non-host plants reinforce each other in the plant selection process.     

SEASONAL GROWTH,BIOMASS, AND REPRODUCTIVE BEHAVIOR OF THREE SPECIES OF RED ALGAE IN GODAVARI ESTUARY,INDIA1

Growth and different phases in the life histories of Bos-trychia tenella (Vohl.) J. Ag., Caloglossa leprieurii (Mont.) J. Ag., and Catenella impudica (Mont.) J. Ag. were estimated for 23 months from January 1986 to December 1987 in the Gautami Godavari estuary of lndia. Seasonal data on hydrographical conditions, biomass, and plant length were collected from three stations in this estuary. Biomass was minimum in August and September and maximum in January and February, as was frond length of tetrasporic and vegetative plants. Temperatures of 24°–27°C and salinities of 5–20 ppt coincided with optimal growth for all three algae. In all three species, tetrasporophytes were present in all months of the year without any seasonal periodicity, and nearly 50% of the plants were tetrasporophytes. The gametophytes of B. tenella and C. leprieurii and cystocarpic plants of C. impudica occurred from October to May, with greatest abundance in January. The abundance of spermatangial and cystocarpic plants in the populations of B. tenella and C. leprieurii ranged from 3 to 15%. Spermatangial plants of Catenella impudica could not be identified, and the abundance of cystocarpic plants was very low.     

The relationship between plant acceptability and suitability for survival and development of the polyphagous grasshopper,Schistocerca americana (Orthoptera: Acrididae)

The feeding behavior of final-instar nymphs ofSchistocerca americana was observed when they first encountered plants that ranged in acceptability from being eaten in large amounts to not being eaten at all. Growth and survival on the same plants through the last stadium were also studied and the results combined as a suitability index to facilitate comparison with the behavior. Although the plants that were eaten most gave the highest suitability index and those that were not eaten permitted no survival, there was no simple relationship between the amounts eaten and the suitability for growth and survival. The possibility that the insects might become habituated to plants that were initially unacceptable was investigated, but no habituation was found over a 3-day period. It is suggested that food intake is largely determined by the presence of deterrent compounds in the less acceptable foods and that nutritional differences between the plants are likely to have been of minor importance. The behavior on some foods suggests that food aversion learning may be involved. It is concluded that the variability of the insects' behavior makes it impossible to predict the suitability of a plant from their immediate behavioral responses. In the field, insects may sometimes reject foods that would be suitable for survival and development, and feed on plants that are nutritionally deficient or even toxic.     

Genetic variation in a phylogenetic context: Responses of two specialized leaf beetles (Coleoptera: Chrysomelidae) to host plants of their congeners

This work explores the possibility that constraints on genetic variation guide host shifts and are responsible for the evolutionary conservatism of host affiliation in phytophagous insects. To this end, we used full- and half-sib breeding designs to screen two species of the North American bettle genus Ophraella for genetic variation in larval and adult feeding responses to several host plants of other species of Ophraella. All the plants are in the family Asteraceae. In O. conferta, we observed effectively no feeding response, and hence no genetic variation in response, to three of five test plant species; only those plants related to the species' natural hosts evoked genetically variable responses. In O. artemisiae, adults displayed genetic variation in response to a congener of the natural host, but not to two distantly related plants. However, significant variation among full-sib broods in larval feeding suggests the existence of nonadditive genetic variance in feeding response to all five species of test plants—although survival was very low on most of them. The results suggest that patterns of presence versus apparent absence of detectable genetic variation may be related to the chemical similarity of plants to the insects' natural hosts, but not evidently to the phylogenetic history of host affiliation within the genus. Almost all genetic correlations in responses to host plants were not significantly different from zero; the few significant correlations were positive, and negative correlations that might explain host specificity were not found. Our data do not explain why exclusive shifts to new hosts should occur, but the apparent lack of genetic variation in responses to some plants suggests that the direction of host shifts is genetically constrained.     

The impact of insecticide resistance in the currant-lettuce aphid, Nasonovia ribisnigri, on pest management in lettuce

Abstract 1 This paper reports on experiments to determine how two different insecticide resistance phenotypes in the aphid Nasonovia ribisnigri (Mosley), which is a major pest of lettuce, change its susceptibility to pyrethroid insecticides and the carbamate pirimicarb. 2 A novel statistical approach determined how the effectiveness of different insecticides was changed by the two resistance phenotypes. This compared the between‐plant distribution of aphid numbers, as opposed to the mean number of aphids per plant. 3 Results from field cage experiments showed that the effect of the resistances differed. Pyrethroid resistance resulted in lower mortality immediately after application of pyrethroids, whereas resistance to pirimicarb shortened the time over which the chemical was effective. 4 The results of laboratory bioassays suggested that these two resistances were not found together in N. ribisnigri. However, the results reported here contradict this assertion. 5 Experiments with insecticide residues showed that reproduction of resistant N. ribisnigri was greater than that of susceptible N. ribisnigri on plants with ageing insecticide residues, even in circumstances where mortality of resistant and susceptible clones of N. ribisnigri were similar. 6 If more than a few aphids are found on a plant then a whole consignment can be rejected for processing. The results reported here suggest that the effect of both insecticide resistances in N. ribsinigri will be to increase the proportion of lettuce heads with an unacceptable number of aphids on them, leading to increased rejection of plants for processing.     

Active attachment ofAzospirillum brasilense to root surface of non-cereal plants and to sand particles

The rhizosphere bacteriumAzospirillum brasilense Cd adsorbed strongly to light-textured and heavy-textured soils, but only slightly to quartz sand. Bacterial attachment to sand particles was mediated by a network made up of various sizes and shapes of fibrillar material. Inoculation of sand with an aggregate-deficient mutant resulted in no detectable fibrillar formation. Rinsing or agitating the sand, colonized by the wild-type and the mutant, had a greater effect on the mutant than on the parental strain. We propose that bacterial fibrils are essential for anchoring ofA. brasilense to sand. A. brasilense Cd was capable of efficiently colonizing the elongation and root-hair zones of tomato, pepper, cotton and soybean plants as well as of wheat plants. All inoculated plants demonstrated: (i) larger amounts of a mucigel-like substance on the root surface than non-inoculated plants, and (ii) fibrillar material which anchored the bacterial cells to the root surface. These fibrils established also connections between cells within bacterial aggregates. On non-water stressed soybean roots, mostA. brasilense Cd cells occurred as vibroid forms. Whereas, those on roots of water-stressed plants.(wilting) were cyst-like. A lower rhizosphere bacterial population was observed on water-stressed plants. When water stress conditions were eliminated, cells reverted to the vibroid form. A concomitant increase in the bacterial population was observed. We suggest that cyst-like formation is a natural response forA. brasilense Cd in the rhizosphere of water-stressed plants.