The role of the antennae and maxillary palps in mediating food preference by larvae of the tobacco hornworm, Manduca sexta

The role of antennae and maxillary palps in mediating food preferences by Manduca sexta (Johan.) (Lepidoptera: Sphingidae) was examined. Fifth‐instar larvae, having different chemosensory organs remaining after microsurgery, were tested in two‐choice feeding assays using three solanaceous host species Lycopersicon esculentum (Mill.), Datura innoxia (Mill.), and Solanum pseudocapsicum (L.), an acceptable non‐host Raphanus sativus (L.) (Cruciferae), and an unacceptable non‐host Pelargonium hortorum (L.H. Bailey) (Geraniaceae). Larvae had a choice between leaf discs of two plant species or between one species and moist filter paper discs (water). The antennae are fully competent in mediating normal (unimpaired) food preferences for S. pseudocapsicum vs. water and P. hortorum vs. water. Thus, the antennae alone can mediate both acceptance and rejection behaviour. The latter is the first report of such a function in M. sexta. The antennae are partly competent (reduced preference) for S. pseudocapsicum vs. P. hortorum. No antennal competence could be demonstrated using the other plant species tested. The antennae alone are either partly needed (R. sativus vs. water) or not necessary to elicit normal food preferences for the plant species tested. The maxillary palps are fully competent in mediating normal food preference for S. pseudocapsicum vs. water, and D. innoxia vs. water. The palps are partly competent for S. pseudocapsicum vs. P. hortorum and incompetent for the other plant species tested. Thus, the maxillary palps alone could only mediate acceptance behaviour in this study. The need of the maxillary palps in normal food preferences could not be demonstrated. Interestingly, the maxillary palps alone can mediate food preferences for two normally rejected plants, R. sativus and P. hortorum. Similar results are reported for two other non‐host plant species, Vigna unguiculata (Walp.) (Leguminosae) and Vinca rosea (L.) (Apocynaceae). Perhaps, the maxillary palps inform M. sexta mostly about feeding stimulatory chemicals common to most plant species.     

Diet-induced food preference by Manduca sexta larvae: acceptable non-host plants elicit a stronger induction than host plants

Diet-induced changes in food preference by fifth instar larvae of the tobacco hornworm, Manduca sexta (Johan.) (Lepidoptera, Sphingidae), were examined. Two groups of larvae with different diet experience were used: larvae reared on a host or on an acceptable non-host plant species. Each group of larvae was offered a choice between leaf discs from each rearing plant species (2-plant choice test) and food preference was measured as the consumption of one plant species relative to that of the other plant species. Diet-induced changes in preferences were larger with the host versus acceptable non-host plant pairs Solanum pseudocapsicum (L.) versus Raphanus sativus (L.), Lycopersicon esculentum (Mill.) versus Vigna sinensis (Savi), and Datura innoxia (L.) versus V. sinensis than with the host versus host plant pairs L. esculentum versus Capsicum annuum (L.) and L. esculentum versus D. innoxia. To examine how much the food preference had been altered for each test plant species alone, two other groups of larvae were offered a choice between leaf discs from a single plant species and filter paper discs laced with distilled water (1-plant choice test). Larvae preferred the dietary plant species more strongly than the non-dietary plant species in tests using the following plant species: for C. annuum with C. annuum and L. esculentum as diets, for V. sinensis with V. sinensis and L. esculentum or D. innoxia as diets, and for R. sativus with R. sativus and S. pseudocapsicum as diets. The preference for the hosts L. esculentum and D. innoxia did not change significantly after rearing larvae on different hosts or on an acceptable non-host. Thus, diet-induction by M. sexta larvae results in an enhancement of preference for the dietary plant species which is much stronger with acceptable non-hosts than with hosts.     

Rôle of gustation and olfaction in food plant discrimination in the tobacco hornworm, Manduca sexta

The rôle of the various chemoreceptors in behavioural discrimination among closely related food plants was studied in Manduca sexta larvae. Amputation of the three types of receptors in various combinations showed that: (1) removal of the maxillary styloconica (gustatory) causes a drastic loss of discrimination; (2) ablation of either the maxillary palpi or antennae (olfactory) also reduces discrimination, but to a lesser degree; and (3) amputation of both palpi and antennae simultaneously causes a severe discriminatory loss comparable to that of gustatory loss. We conclude that both gustation and olfaction are important for host plant discrimination. These chemosensory organs were also shown to play a strong role in the induction of preference. Unilateral extirpations of all three sense organs caused no detectable loss, demonstrating their redundancy in normal animals.     

Differentiation of roles of chemosensory organs in food discrimination among host and non-host plants by larvae of the tobacco hornworm, Manduca sexta

ABSTRACT. The contributions of olfactory and gustatory organs in food plant discrimination were examined in larvae of Manduca sexta (Johan.) (Lepidoptera, Sphingidae). Larvae, from which various chemosensory organs had been removed surgically, were tested for feeding preferences for a host, tomato ( Lycopersicon esculentum ); a weakly acceptable non-host, rape ( Brassica napus ); and an unacceptable non-host canna ( Canna generalis ), using a two-choice disc bioassay.
Removal of all known chemosensory organs resulted in failure to show discriminatory behaviour in a strictly chemosensory bioassay, indicating that all external chemosensory organs have been accounted for. The involvement of non-chemosensory organs results in residual discrimination for leaves by individuals with total chemosensory ablations.
Larvae possessing either olfactory or gustatory organs still exhibit normal preferences for tomato over rape. Gustatory (but not olfactory) organs are required for larvae to show normal preferences for tomato over canna; in fact, olfactory organs do not appear to participate in this decision.
To examine which if any of the plant species is being selected in two-choice tests, larvae were given a choice between each leaf species and a 'neutral' substance (wet filter paper). Both olfactory and gustatory organs are required for normal preferences for tomato, but either alone will suffice for rape. Only gustation is needed to select canna, and participation of either the epipharyngeal sensilla or a single medial sensillum styloconicum is sufficient to elicit complete rejection behaviour.
We conclude that, in larvae of M. sexta , the complement of chemosensory organs needed for food plant discrimination varies with the plant species sampled. Evidence is presented exposing a potential artefact of ablation experiments; extirpation of one sensory organ may affect the functioning of others nearby, even though they may appear normal by visual inspection.     

Innate and acquired components of oligophagy in the herbivorous lepidopteran,Manduca sexta

Innate and acquired aspects of oligophagy were investigated in the tobacco hornworm, Manduca sexta L. (Lepidoptera: Sphingidae), a species normally restricted to members of the family Solanaceae. Larvae were reared in the laboratory on solanaceous species tobacco (Nicotiana tabacum L.), strawberry groundcherry (Physalis pruinosa L.), and potato (Solanum tuberosum L.), as well as the non‐solanaceous foods cowpea [Vigna sinensis (L.) (Fabaceae)], rapeseed [Brassica napus L. (Brassicaceae)], and artificial diet. Feeding assays were conducted using the above plants plus corkscrew vine [Vigna caracalla (L.) Verdc. (Fabaceae)], lettuce [Lactuca sativa L. (Asteraceae)], and moist glass‐fiber filter paper. Larval feeding was characterized using two camera‐monitored assays: (1) a quantitative no‐choice disc test that determines consumption over time, delays in initiation of feeding, and % of larvae rejecting the food, and (2) a novel no‐choice emigration test that measures how soon a larva abandons a whole plant. Experimental results from both assays revealed that larvae displayed high levels of acceptance of Solanaceae regardless of whether they had been reared on solanaceous or non‐solanaceous food. We conclude that solanaceous oligophagy in M. sexta is primarily innate and does not require (but may be strengthened by) previous feeding experience on Solanaceae. In contrast, larvae tested on non‐solanaceous plants or moist filter paper showed large variation in both acceptance of foods and emigration times that were strongly dependent on the food on which they had been reared (analogous to the ‘induction of preference’ of earlier literature). Two types of induction were identified: an increase in acceptability of a plant of (1) the same species as the one on which the larva was reared, or (2) a related species. These discoveries both challenge and expand our current understanding of oligophagy in the tobacco hornworm.     

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.     

Feeding responses to solanaceous allelochemicals by larvae of the tobacco hornworm, Manduca sexta

Feeding responses of the oligophagous tobacco hornworm to allelochemicals prevalent in their host plants were determined in food choice-tests using filter paper discs laced with a test solution or water (control). Six solanaceous alkaloids, tomatine, tomatidine, solanine, solanocapsine, atropine and nicotine, were tested and only tomatine and solanocapsine were found to influence preference behavior. Solanocapsine (5 mM) deters feeding whereas tomatine (1 mM) stimulates feeding slightly. No synergistic effect of either tomatine or tomatidine with sucrose was found.The responses to tomatine are affected by previous feeding experience. Tomatine slightly stimulates feeding in larvae reared on tomato (Lycopersicon esculentum), but slightly deters feeding in larvae reared on Jerusalem cherry (Solanum pseudocapsicum). Such induced preference is absent for the other alkaloids tested, which indicates that these alkaloids do not by themselves induce preferences for the plants containing them.The non-alkaloid allelochemicals, chlorogenic acid, rutin, and 2-tridecanone also influenced food choice behavior. Chlorogenic acid is slightly stimulatory at its natural concentration (1mM), but strongly deterrent at higher concentrations. Rutin stimulates feeding in a concentration-dependent manner. Its activity must be due to the glycosylated structure, because both the aglycone (quercetin) and the sugar moiety (rutinose) are neutral. Removal of the glucose part of rutin, as in quercitrin, results in feeding deterrent activity. 2-Tridecanone is neutral at its concentration in cultivated tomato (1 mM), but strongly deterrent and toxic at higher concentrations. Preference behavior is not affected by solanesol, GABA, and a mixture of host plant compounds stimulatory for anothe solanaceous-specific feeder, the Colorado potato beetle (Leptinotarsa decemlineata).We conclude that the prevalent solanaceous alkaloids and other allelochemicals tested do not play important roles in food selection of the tobacco hornworm, although some may make small contributions.

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Résumé Des experiences de choix de chenilles oligophages de M. sexta ont été réalisees avec des disques de papier filtre imbiles d'eau ou de solutions des substances allélochimiques dominantes dans les plantes consommées. Sur les six alcaloïdes de solanées examinés: tomatine, tomatidine, solanine, solanocapsine, atropine et nicotine, seuls la tomatine et la solanocapsine ont influé sur le choix; la solanocapsine (5 mM) empêche la prise de nourriture, tandis que la tomatine (1 mM) la stimule légèrement. Aucun effet synergique de la tomatine ou de la tomatidine n'a été observé en présence de sucrose.La réponse à la tomatine est modifiée par la prise de nourriture antérieure. Elle stimule légèrement l'alimentation de chenilles élevées sur tomates (Lycopersicon esculentum), mais dissuade légèrement les chenilles élevées sur Solanum pseudocapsicum. II n'y a pas d'action induite semblable avec les autres alcalïdes examinés, ce qui indique que ces alcaloïdes ne peuvent pas induire par eux-mêmes de préférences pour les plantes qui les contiennent.Des substances allélochimiques non-alcaloïdes: acide chlorogénique, rutine, et 2-tridécanone, influent aussi sur le comportement de choix alimentaire. L'acide chlorogénique est légèrement stimulant à sa concentration naturelle (1 mM), mais fortement dissuasif aux concentrations supérieures. La rutine stimule la prise de nourriture en fonction de sa concentration. Son activité doit être due à sa structure glucosylate, puisqu'aussi bien l'aglycone (quercitine) que la moiteé sucrée (rutinose) sont neutres. La suppression de la partie glucose de la rutine, comme dans le cas de la quercitine, a un effet dissuasif. A sa concentration dans la tomate cultivée (1 mM), le 2-tridécanone est neutre, mais il est fortement dissuasif et toxique à des concentrations supérieures.Le comportement de choix n'est pas modifié par le solanésol, le GABA, et par un mélange de composés végétaux stimulant un consommateur spécifique de solanées, comme le doryphore (Leptinotarsa decemlineata).Nous pouvons conclure que les principaux alcaloïdes et autres substances allélochimiques des solanées que nous avons examinés n'interviennent pas d'une façon importante, mais peuvent avoir une influence secondaire, dans les choix alimentaires de Manduca sexta.

     

Maxillary palps can mediate taste rejection of plant allelochemicals by caterpillars

All caterpillars possess a pair of maxillary palps that “drum” the surface of foods during feeding. These chemosensory organs contain over 65% of a caterpillar's taste receptor cells, but their functional significance remains largely unknown. We examined their role in rejection of plant allelochemicals, using the tobacco hornworm (Manduca sexta) as a model insect and an extract from a plant species (Grindeliaglutinosa) as a model stimulus. We selected this system because hornworms reject foods containing Grindelia extract, and because preliminary studies indicated that their maxillary palps respond to this extract. We hypothesized that Grindelia extract elicits rejection through stimulating: (1) olfactory receptor cells, (2) taste receptor cells, (3) oral mechanoreceptors, and/or (4) a postingestive response mechanism. Our results were consistent only with hypothesis 2: caterpillars approached Grindelia-treated diets without apparent hesitation, but rejected it within 6 s of initiating biting; Grindelia-treated solutions stimulated taste receptor cells in the maxillary palp, but not the other gustatory chemosensilla; and ablating the maxillary palps eliminated rejection of Grindelia-treated diets. Our results demonstrate that taste receptor cells in the maxillary palps mediate rejection of Grindelia extract, and provide the first direct evidence for a role of maxillary palps in rejection of plant allelochemicals. Accepted: 25 January 1998     

Interaction of fertilizer regime with host-plant resistance in tomato

The effect of fertilizer regime on trichome- and lamellar-based resistance in the wild tomato species, Lycopersicon hirsutum f. glabratum C.H. Mull accession PI 134417, to three insect pests of tomato, the tobacco hornworm, Manduca sexta (L.), the colorado potato beetle, Leptinotarsa decemlineata (Say), and the tomato fruitworm, Heliocoverpa zea (Boddie), was examined. Increasing the rate at which NPK fertilizer was applied, from 1.8 to 19.6 g/plant/week, reduced the trichome-based resistance of PI 134417 to M. sexta and L. decemlineata by lowering both the density of type VI (sensu Luckwill, 1943) glandular trichomes and the amount of 2-tridecanone contained in the tips of these trichomes. 2-Tridecanone is a toxic methyl-ketone responsible for glandular trichome-mediated resistance in PI 134417 to M. sexta and L. decemlineata. A similar increase in the application rate of NPK fertilizer reduced the lamellarbased resistance of PI 134417 to L. decemlineata and H. zea. The meachanisms for this reduction of resistance are unknown, but may be related to improved nutritional quality of hosts at higher fertilizer regimes.     

Maintenance of narrow diet breadth in the monarch butterfly caterpillar: response to various plant species and chemicals

In order to better understand the maintenance of a fairly narrow diet breadth in monarch butterfly larvae, Danaus plexippus L. (Lepidoptera: Nymphalidae: Danainae), we measured feeding preference and survival on host and non-host plant species, and sensitivity to host and non-host plant chemicals. For the plant species tested, a hierarchy of feeding preferences was observed; only plants from the Asclepiadaceae were more or equally preferred to Asclepias curassavica, the common control. The feeding preferences among plant species within the Asclepiadaceae are similar to published mean cardenolide concentrations. However, since cardenolide data were not collected from individual plants tested, definitive conclusions regarding cardenolide concentrations and plant acceptability cannot be made. Although several non-Asclepiadaceae were eaten in small quantities, all were less preferred to A. curassavica. Additionally, these non-Asclepiadaceae do not support continued feeding, development, and survival of first and fifth-instar larvae. Preference for a host versus a non-host (A. curassavica versus Vinca rosea) increased for A. curassavica reared larvae as compared to diet-reared larvae suggesting plasticity in larval food preferences. Furthermore, host species were significantly preferred over non-host plant species in bioassays using a host plant or sucrose as a common control. Larval responses to pure chemicals were examined in order to determine if host and non-host chemicals stimulate or deter feeding in monarch larvae. We found that larvae were stimulated to feed by some ubiquitous plant chemicals, such as sucrose, inositol, and rutin. In contrast, several non-host plant chemicals deterred feeding: caffeine, apocynin, gossypol, tomatine, atropine, quercitrin, and sinigrin. Additionally the cardenolides digitoxin and ouabain, which are not in milkweed plants, were neutral in their influence on feeding. Another non-milkweed cardenolide, cymarin, significantly deterred feeding. Extracts of A. curassavica leaves were tested in bioassays to determine which components of the leaf stimulate feeding. Both an ethanol extract of whole leaves and a hexane leaf-surface extract are phagostimulatory, suggesting the involvement of both polar and non-polar plant compounds. These data suggest that the host range of D. plexippus larvae is maintained by both feeding stimulatory and deterrent chemicals in host and non-host plants.     

The effect of plant chemistry on the acceptability of caterpillar prey to the argentine antIridomyrmex humilils (Hymenoptera: Formicidae)

Experiments were performed to test the acceptability of two palatable, cryptic caterpillars, the tobacco hornworm,Manduca sexta, and the cabbage looper,Trichoplusia ni, reared on different diets, to the Argentine ant,Iridomyrmex humilis. Ants preferred larvae reared on artificial diet, groundcherry, or cowpea to tobacco-reared larvae. Ants also preferred larvae reared on artificial diet without nicotine to larvae reared on diet containing nicotine (5% dry wt). Experiments were also performed to test the response of ants to larval extracts and chemicals applied to the surface of palatable prey. Ants did not respond differently to larvae of the potato tuber moth,Phthorimaea operculella, treated with larval extracts or regurgitate from tobacco-reared larvae compared to artificialdiet-reared larvae, but ants were deterred byP. operculella larvae treated with nicotine compared to untreated larvae. The results of this study indicate that caterpillars can derive at least some degree of chemical protection from their food plant without sequestering and storing plant compounds and without the development of elaborate aposematic characteristics.     

Lack of physiological improvement in performance of Callosamia promethea larvae on local host plant favorites

Summary As a species, the promethea silkmoth, Callosamia promethea (Saturniidae: Lepidoptera) exhibits a wide host range on 6–10 families of plants, although specific populations are known to have local foodplant favorites. We tested the hypothesis that larvae from a particular host plant lineage would show physiological adaptations to this host compared with larvae from other host plant lineages. We found no evidence that larval survival and growth was any better for larvae fed the natural plant of the parental population than for larvae from other host lineages. These natural host lineages include: black cherry (Prunus serotina Ehrh.), tuliptree (Liriodendron tulipifera L.), sassafras (Sassafras albidum (Nutt.) Nees) and spicebush (Lindera benzoin (L.) Blume). The only apparent manifestation of physiological specialization was the inability of tuliptree lineages of C. promethea to survive on paper birch (Betula papyrifera Marsh), although this may reflect the geographical pattern of adaptation to birch, rather than a negative correlation with adaptation to tuliptree. These results suggest that for C. promethea larvae, growth performance and survival is primarily influenced by plant nutritional quality, rather than physiological adaptations to the locally preferred host plant.     

Modulation of somatic embryogenesis in hypocotyl-derived cultures of geranium (Pelargonium X hortorum bailey) CV ringo rose by a bacterium

Summary The Ringo Rose cultivar of zonal geranium (Pelargonium x hortorum Bailey) has been shown to be morphogenetically unresponsive. Attempts to improve somatic embryogenesis using various seed stress treatments before germination proved ineffective. However, bacterial contamination of one of the seed-stress treatments led to infected explants that had a significant increase in frequency of high-quality somatic embryos. The co-cultivation of explants with the isolated bacterium (tentatively identified asBacillus sp.) was found to be repeatable, and potentially represents a novel way to improve morphogenesis in geranium and possibly other species.     

Instar-specific sensitivity of specialist Manduca sexta larvae to induced defences in their host plant Nicotiana attenuata

1. The time delay associated with the activation of induced defences is thought to be a liability for this type of defence because it allows herbivores to remove biomass before the defence is fully induced. When defences are costly and plants grow with competitors, however, it may be more advantageous not to induce defences too fast and motivate the herbivore to move to the neighbour when it is most voracious. 2. Such a strategy can only work when the costs for the herbivore of moving to a neighbouring plant are smaller than the costs of staying on a fully induced plant. For lepidopteran herbivores, both the sensitivity to induced defences and the costs of moving may vary considerably between instars and this variation may constrain the plant's defensive opportunities. 3. This study was designed to examine whether the cost of moving, mimicked by a starvation period of 8 h, was larger than the cost of staying on a fully induced plant for each larval instar of the specialist Manduca sexta feeding on induced and control tissues of Nicotiana attenuata. 4. For first‐ and second‐instar larvae, the costs of moving were larger than the costs of staying on a fully induced plant. In contrast, feeding on induced plant material retarded development in third‐instar larvae more than did starvation, indicating that in this instar the costs of leaving are smaller than the costs of staying on an induced plant. More than 98% of the lifetime leaf mass consumed by a M. sexta larva is consumed during the fourth and fifth instars, and during these instars larval development was not affected by either induced defences or starvation. Thus the third instar, the stage just before larvae cause the majority of damage, represents a window of sensitivity to induced defences during which larvae can be motivated to change plants. 5. These results suggest that N. attenuata plants, which commonly compete with conspecifics in nature, have the opportunity to manipulate the behaviour of the specialist herbivore M. sexta to minimise the fitness effects of inducing defences when these defences are most costly, i.e. when plants grow under intraspecific competition.     

Linkages between nectaring and oviposition preferences of Manduca sexta on two co‐blooming Datura species in the Sonoran Desert

1. The oviposition choices of phytophagous insects determine the environment that their offspring will experience, affecting both larval performance and host plant fitness. These choices, however, may be influenced by other activities, such as nectar foraging. 2. In the Sonoran Desert, Manduca sexta (Lepidoptera: Sphingidae) oviposits primarily on the perennial herb Datura wrightii. It has been reported to oviposit on the smaller‐flowered, co‐blooming, sympatric annual Datura discolor as well. Datura is also M. sexta's most important source of nectar in this region. Nectaring and oviposition decisions thus determine M. sexta's reproductive success, as well as the relative benefits (pollination) and costs (herbivory) that each Datura species derives from this interaction. 3. The nectaring and oviposition choices of adult M. sexta on these congeners were studied to investigate how nectar foraging influences oviposition. Larval performance on the two hosts was then assessed. 4. Nectaring and oviposition were behaviourally linked, with M. sexta preferring the perennial D. wrightii as both a nectar source and larval host when given a choice between the two species. This preference disappeared, however, when only D. discolor bore flowers. 5. In the laboratory, larvae developed at equal rates when fed D. wrightii or D. discolor leaves, but survival was higher on D. wrightii when larvae fed on intact plants. 6. These findings suggest that while female preferences match larval performance in most cases, the link between nectaring and oviposition may at times bias moths to lay eggs on inferior larval hosts.     

Foodplant selection and induction of feeding preference among host and non-host plants in larvae of the tobacco hornworm Manduca sexta

Ten host plant (Solanaceae) and twelve non-host plant species were tested as foodplants for first instar larvae of the tobacco hornworm, Manduca sexta. Only nine host and three non-host plant species elicited feeding and supported growth up to fifth instar. The range of acceptability suggested that plants be divided into hosts, acceptable non-hosts, and unacceptable non-hosts. Using the two-choice feeding preference test we found that the initial preference for hosts was preserved when larvae were reared on hosts, but was less strong or absent for larvae reared on acceptable non-hosts. Thus oligophagy in the tobacco hornworm is not induced, but must be inherited.Newly-hatched first instar larvae and fifth instar larvae showed a preference hierarchy among both hosts and non-hosts. Fifth instar larvae reared separately on two different host species showed slightly different preference hierarchies among hosts. The preference for the rearing plant was increased and also two other host species changed positions in hierarchies.Feeding preferences of larvae reared on hosts or acceptable non-hosts were determined using plant combinations of host vs. host, host vs. acceptable non-host, and acceptable non-host vs. acceptable non-host. Induction of feeding preference was found in all three of these categories. This shows that induction of feeding preference in the tobacco hornworm is not restricted to host plant species.The degree to which feeding preferences were induced ranged from very strong to undetectable and dependend on the plant species paired. The strength of induction in the tobacco hornworm was found to correlate inversely with taxonomic relatedness of the plant species paired. Analysis of induction data from the literature revealed a similar correlation for other lepidopteran species.

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Résumé Vingt-deux espèces de plantes, dont 10 planteshôtes (Solanées), ont été testés comme plantes alimentaires pour des chenilles de ler stade de Manduca sexta. Sur cet ensemble, seulement 12 plantes (dont 9 plantes hôtes) induisaient la prise de nourriture et permettaient la croissance jusqu'au 5ème stade. La diversité des résultats suggère que les plantes pouvaient être classées en hôtes, non-hôtes acceptables et non-hôtes refusés. En utilisant le test du choix alimentaire préférentiel entre deux rondelles de feuilles, les chenilles néonates de ler stade ont préféré nettement les plantes-hôtes aux autres. Cette préférence initiale pour les plantes-hôtes était préservée quand les cheniles étaient élevées sur plantes-hôtes, mais devenait moins nette ou disparaissait pour des chenilles élevées sur d'autres plantes acceptées. Ainsi l'oligophagie ches M. sexta n'est pas induite, mais doit être héritée.Les chenilles néonates, aussi bien que celles de 5ème stade, présentent des préférences hiérarchisées parmi les plantes hôtes ou non. La seule frontiere nette observée était entre espèces de plantes acceptables ou non. Les hiérarchies préférentielles des chenilles du 5ème stade différaient légèrement lors-qu'elles avaient été élevées sur deux plantes-hôtes différentes. La différence essentielle était l'observation d'une préférence accrue pour l'espèce ayant servi à l'élevage, mais deux autres plantes-hôtes changaient aussi de position hiérarchique.La cause de ces changements de préférence a été approfondie, les chenilles étant élevées sur des feuilles de chaque espèce acceptable (hôte ou non). Leurs préférences alimentaires ont été définies en utilisant des combinaisons diverses (hôte x hôte, hôte x non-hôte acceptable, non-hôte acceptable x non-hôte acceptable). L'induction de la préférence alimentaires a été obtenue dans ces trois associations. Ceci montre que l'induction des choix alimentaires chez M. sexta n'est pas limitée aux plantes-hôtes. Le degré d'induction de la préférence alimentaire variait de très fort à indécelable; il dépendait de l'association examinée. La source de la variabilité de cette induction a été examinée en fonction de la relation entre la force de l'induction et les rapports taxonomiques des plantes associées. La relation obervée était inversée pour M. sexta. L'examen des données de la littérature ont révélé une relation du même type pour les autres espèces de Lépidoptères.

     

Different responses to UV-B enhanced solar radiation in radish and carrot

Radish (Raphanus sativus L.) and carrot (Daucus carota L.), plants with underground storage organs grown in the field, were exposed to either ambient (UVA) or 20 % UV-B (UVE) enhanced solar radiation till their root yield stage. In radish, UVE produced a significant increase in shoot and root fresh mass (FM), increase in the contents of chlorophyll, carotenoids, flavonoids, and total proteins per unit FM, Hill reaction rate, and root yield. In contrast, carrot responded negatively to UVE showing a loss in the above parameters.