Host feeding experience affects host plant odour preference of the polyphagous leafminer Liriomyza bryoniae

Herbivorous insects use highly specific volatiles or blends of volatiles characteristic to particular plant species to locate their host plants. Thus, data on olfactory preferences can be valuable in developing integrated pest management tools that deal with manipulation of pest insect behaviour. We examined host plant odour preferences of the tomato leafminer, Liriomyza bryoniae (Kaltenbach) (Diptera: Agromyzidae), which is an economically important agricultural pest widespread throughout Europe. The odour preferences of leafminers were tested in dependence of feeding experiences. We ranked host plant odours by their appeal to L. bryoniae based on two‐choice tests using a Y‐tube olfactometer with five host plants: tomato, Solanum lycopersicum Mill.; bittersweet, Solanum dulcamara L.; downy ground‐cherry, Physalis pubescens L. (all Solanaceae); white goosefoot, Chenopodium album L. (Chenopodiaceae); and dead nettle, Lamium album L. (Lamiaceae). The results imply that ranking of host plant odours by their attractiveness to L. bryoniae is complicated due to the influence of larval and adult feeding experiences. Without any feeding experience as an adult, L. bryoniae males showed a preference for the airflow with host plant odour vs. pure air, whereas females did not display a preference. Further tests revealed that adult feeding experience can alter the odour choice of L. bryoniae females. After feeding experience, females showed a preference for host plant odour vs. pure air. Feeding experience in the larval stage influenced the choice by adults of both sexes: for males as well as females reared on bittersweet the odour of that plant was the most attractive. Thus, host feeding experience both in larval and/or adult stage of polyphagous tomato leafminer L. bryoniae influences host plant odour preference by adults.     

Temporal changes in olfactory and oviposition responses of the diamondback moth to herbivore‐induced host plants

Temporal changes in the pre‐ and post‐alighting responses of mated female diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), to two species of Brassica (Brassicaceae) host plants induced by larval feeding were studied using olfactometer and oviposition assays. Females displayed strong olfactory and oviposition preferences for herbivore‐induced common cabbage (Brassica oleracea var. capitata L. cv. sugarloaf) plants over intact plants; these preferences decreased with time and disappeared by the 7th day after induction. In herbivore‐induced common cabbage plants, eggs were clustered near feeding damage on the younger leaves (leaves 5–7), whereas in intact plants, eggs were clustered on the stem and lower leaves (leaves 1–4) . However, as the time interval between larval feeding and oviposition increased, more eggs were laid on the lower leaves of induced plants. This demonstrates a change in egg distribution from the pattern associated with induced plants to that associated with intact plants. In contrast, females displayed strong olfactory and oviposition preferences for intact Chinese cabbage [Brassica rapa ssp. pekinensis (Lour.) Hanelt cv. Wombok] plants over induced plants; these preferences decreased with time and disappeared by the 5th day after induction. More eggs were laid on the upper leaves (leaves 4–6) than on the lower leaves (leaves 1–3) of intact Chinese cabbage plants at first, but the distribution changed over time until there were no significant differences in the egg count between upper and lower leaves by the 4th day post induction. For both host plant species, pre‐alighting responses of moths were reliable indicators of post‐alighting responses on the first 2 days post induction. The results suggest that temporal changes in a plant's profile (chemical or otherwise) following herbivory may influence attractiveness to an insect herbivore and be accompanied by changes in olfactory and oviposition preferences.     

Population genetic structure,genetic diversity,and natural history of the South American species of Nothofagus subgenus Lophozonia (Nothofagaceae) inferred from nuclear microsatellite data

The effect of glaciation on the levels and patterns of genetic variation has been well studied in the Northern Hemisphere. However, although glaciation has undoubtedly shaped the genetic structure of plants in the Southern Hemisphere, fewer studies have characterized the effect, and almost none of them using microsatellites. Particularly, complex patterns of genetic structure might be expected in areas such as the Andes, where both latitudinal and altitudinal glacial advance and retreat have molded modern plant communities. We therefore studied the population genetics of three closely related, hybridizing species of Nothofagus (N. obliqua, N. alpina, and N. glauca, all of subgenus Lophozonia; Nothofagaceae) from Chile. To estimate population genetic parameters and infer the influence of the last ice age on the spatial and genetic distribution of these species, we examined and analyzed genetic variability at seven polymorphic microsatellite DNA loci in 640 individuals from 40 populations covering most of the ranges of these species in Chile. Populations showed no significant inbreeding and exhibited relatively high levels of genetic diversity (H_E = 0.502–0.662) and slight, but significant, genetic structure (R_ST = 8.7–16.0%). However, in N. obliqua, the small amount of genetic structure was spatially organized into three well‐defined latitudinal groups. Our data may also suggest some introgression of N. alpina genes into N. obliqua in the northern populations. These results allowed us to reconstruct the influence of the last ice age on the genetic structure of these species, suggesting several centers of genetic diversity for N. obliqua and N. alpina, in agreement with the multiple refugia hypothesis.     

Oviposition site preference and developmental performance of a gall‐inducing leafhopper on galled and non‐galled host plants

Oviposition preferences of herbivorous insects affect offspring performance. Both positive and negative links between oviposition preference and offspring performance have been reported for many species. A gall‐inducing leafhopper, Cicadulina bipunctata Melichar (Hemiptera: Cicadellidae), feeds on various Poaceae plants and induces galls of enhanced nutritional value for their offspring. Although gall induction by C. bipunctata improves nymphal performance, the oviposition preference of females between galled and non‐galled host plants is still unclear. In this paper, the nymphal performance and oviposition and feeding‐site preference of C. bipunctata were investigated using galled wheat, Triticum aestivum L., and non‐galled barley, Hordeum vulgare L., as host plants. The survival rate of C. bipunctata on wheat was significantly higher than on barley. In the choice test, significantly more eggs were laid into barley, whereas the number of eggs deposited on both hosts was not significantly different in the no‐choice test. The number of settling individuals per leaf area was not significantly different between wheat and barley, suggesting no clear preference for oviposition between these plants. Experience as a nymph with a growing host did not affect oviposition preference as adult female. The inconsistent correspondence between offspring performance and oviposition preference of C. bipunctata may reflect the high mobility of nymphs and/or differences in leaf area between host plants. The results indicate that the previous finding that oviposition preference and offspring performance are not always positively correlated in herbivorous insects is applicable to gall‐inducing insects.     

Preference–performance relationship in the gall midge Rabdophaga rosaria: insights from a common‐garden experiment with nine willow clones

1. Oviposition preferences of herbivorous insects are predicted to match offspring performance on different host taxa or on conspecific host genotypes. In gall‐inducing insects, host‐plant properties such as growth rate and gall size, which are determined by plant genotype and growing conditions, may have a significant impact on offspring performance and, hence, should influence oviposition site selection. 2. The present study investigated host preference of the European rosette willow gall midge Rabdophaga rosaria (Loew) in relation to offspring success on seven clones of Salix myrsinifolia Salisb. and two naturally hybridised S. myrsinifolia × phylicifolia L. clones growing in a replicated design in an experimental field under two fertilisation regimes. For each clone, the average growth rate, number of shoot tips, and leaf and gall size were determined, and their effects on midge preference and larval survival were examined. 3. Main shoot height, number of shoot tips, and gall size were significantly affected by clone. The midges clearly preferred certain clones over the others, but preferences were not related to willow growth traits or to gall size. Survival probability was higher in large than in small galls, but females did not prefer large‐leaved clones that produced the biggest rosette galls. Midge oviposition was also uncorrelated with prior rates of leaf‐rust infection and with feeding preferences of voles and folivorous insects. 4. The weak preference–performance relationship of R. rosaria within S. myrsinifolia is probably explained by evolutionary constraints that prevent generalist insects from achieving an ability to discriminate among conspecific hosts of variable quality.     

Leaf trenching by Indian red admiral caterpillars for feeding and shelter construction

Many folivorous insects sever leaf veins, trench across leaf blades, or girdle petioles prior to feeding on leaves. The purpose of these behaviours is generally thought to be sabotage of the anti-herbivore chemical defences of host plants. For insects that construct leaf shelters, these behaviours may make leaves easier to roll or fold. Larvae of the Indian red admiral butterfly, Vanessa indica (Herbst), cut trenches at the base of host plant leaves and construct leaf-fold shelters. I attempted to determine which of the above functions the trenching behaviour of V. indica larvae serves. If the function of trenching is to facilitate leaf-folding, then larvae should cut trenches before folding leaves. When the larvae used larger leaves, they tended to trench them prior to folding them. This suggests that the function of trenching behaviour is to make leaves easier to fold. However, larvae sometimes cut trenches after folding the leaves, suggesting that this behaviour has additional functions. When the larvae were about to moult or pupate, they did not cut trenches, suggesting that trenching behaviour may also be involved in feeding.     

An ecogenomic analysis of herbivore‐induced plant volatiles in Brassica juncea

Upon herbivore feeding, plants emit complex bouquets of induced volatiles that may repel insect herbivores as well as attract parasitoids or predators. Due to differences in the temporal dynamics of individual components, the composition of the herbivore‐induced plant volatile (HIPV) blend changes with time. Consequently, the response of insects associated with plants is not constant either. Using Brassica juncea as the model plant and generalist Spodoptera spp. larvae as the inducing herbivore, we investigated herbivore and parasitoid preference as well as the molecular mechanisms behind the temporal dynamics in HIPV emissions at 24, 48 and 72 h after damage. In choice tests, Spodoptera litura moth preferred undamaged plants, whereas its parasitoid Cotesia marginiventris favoured plants induced for 48 h. In contrast, the specialist Plutella xylostella and its parasitoid C. vestalis preferred plants induced for 72 h. These preferences matched the dynamic changes in HIPV blends over time. Gene expression analysis suggested that the induced response after Spodoptera feeding is mainly controlled by the jasmonic acid pathway in both damaged and systemic leaves. Several genes involved in sulphide and green leaf volatile synthesis were clearly up‐regulated. This study thus shows that HIPV blends vary considerably over a short period of time, and these changes are actively regulated at the gene expression level. Moreover, temporal changes in HIPVs elicit differential preferences of herbivores and their natural enemies. We argue that the temporal dynamics of HIPVs may play a key role in shaping the response of insects associated with plants.     

Lack of evolution in a leaf beetle that lives on two contrasting host plants

The interactions between plant‐eating insects and their hosts have shaped both the insects and the plants, driving evolution of plant defenses and insect specialization. The leaf beetle Trirhabda eriodictyonis (Chrysomelidae) lives on two shrubs with differing defenses: Eriodictyon crassifolium has hairy leaves, whereas E. trichocalyx has resinous leaves. We tested whether these beetles have differentiated onto the two host plants, and if not, whether the beetles prefer the better host plant and prefer mates who are from that host plant. In feeding tests, adult beetles strongly preferred eating E. trichocalyx regardless of which host they came from. In addition, females laid more eggs if they ate E. trichocalyx than E. crassifolium. So, E. trichocalyx is generally the better host. However, beetle mate preference was not in line with food choice. Males did not prefer to mate with females from E. trichocalyx. Females from E. crassifolium did prefer males from E. trichocalyx over males from E. crassifolium, but did not lay more eggs as a result of these matings. We conclude that the beetle populations we studied have not differentiated based on their host plants and may not have even adapted to the better host. Although to humans these host plant defenses differ dramatically, signs that they have caused evolution in the beetles are lacking. The case of T. eriodictyonis stands counter to many other studies that have seen the differentiation of ecotypes and/or adaptive coordination of an herbivore's life cycle based on host plant differences.     

Feeding preferences of Melanoplus femurrubrum grasshoppers on native and exotic grasses: behavioral and molecular approaches

Generalist insect herbivores, such as grasshoppers, may either avoid feeding on exotic plants, potentially enabling these plants to become invasive in the introduced range, or insects may incorporate exotic plants into their diet, contributing to the biotic resistance of native communities and potentially preventing plant invasions. Accurate determination of insect diet preferences with regard to native and exotic plants can be challenging, but this information is critical for understanding the interaction between native herbivores and exotic plants, and ultimately the mechanisms underlying plant invasions. To address this, we combined behavioral and molecular approaches to accurately compare food consumption of the polyphagous red‐legged grasshopper, Melanoplus femurrubrum (De Geer) (Orthoptera: Acrididae), on native [Andropogon gerardii Vitman and Bouteloua curtipendula (Michx.) Torr.] and exotic, potentially invasive grasses [Miscanthus sinensis Andersson and Bothriochloa ischaemum (L.) Keng] (all Poaceae). We found that M. femurrubrum grasshoppers demonstrated strong feeding preferences toward exotic grasses in experiments with intact plants under both field and greenhouse conditions, but they showed no preference in experiments with clipped leaves. Additionally, we sampled the gut contents of M. femurrubrum collected in the field and identified the ingested plant species based on DNA sequences for the non‐coding region of the chloroplast trnL (UAA) gene. We found that exotic plants were prevalent in the gut contents of grasshoppers collected at study sites in Ohio and Maryland, USA. These results suggest that the generalist herbivore M. femurrubrum does not avoid feeding on exotic grasses with which they do not share coevolutionary history. In addition, by demonstrating greater food consumption of exotic plants, these grasshoppers potentially provide biotic resistance should these grasses escape cultivation and become invasive in the introduced range.     

Influence of presence and spatial arrangement of belowground insects on host‐plant selection of aboveground insects: a field study

Abstract 1. Several studies have shown that above‐ and belowground insects can interact by influencing each others growth, development, and survival when they feed on the same host‐plant. In natural systems, however, insects can make choices on which plants to oviposit and feed. A field experiment was carried out to determine if root‐feeding insects can influence feeding and oviposition preferences and decisions of naturally colonising foliar‐feeding insects. 2. Using the wild cruciferous plant Brassica nigra and larvae of the cabbage root fly Delia radicum as the belowground root‐feeding insect, naturally colonising populations of foliar‐feeding insects were monitored over the course of a summer season. 3. Groups of root‐infested and root‐uninfested B. nigra plants were placed in a meadow during June, July, and August of 2006 for periods of 3 days. The root‐infested and the root‐uninfested plants were either dispersed evenly or placed in clusters. Once daily, all leaves of each plant were carefully inspected and insects were removed and collected for identification. 4. The flea beetles Phyllotreta spp. and the aphid Brevicoryne brassicae were significantly more abundant on root‐uninfested (control) than on root‐infested plants. However, for B. brassicae this was only apparent when the plants were placed in clusters. Host‐plant selection by the generalist aphid M. persicae and oviposition preference by the specialist butterfly P. rapae, however, were not significantly influenced by root herbivory. 5. The results of this study show that the presence of root‐feeding insects can affect feeding and oviposition preferences of foliar‐feeding insects, even under natural conditions where many other interactions occur simultaneously. The results suggest that root‐feeding insects play a role in the structuring of aboveground communities of insects, but these effects depend on the insect species as well as on the spatial distribution of the root‐feeding insects.     

Insect oviposition preference between Epichloë‐symbiotic and Epichloë‐free grasses does not necessarily reflect larval performance

Variation in plant communities is likely to modulate the feeding and oviposition behavior of herbivorous insects, and plant‐associated microbes are largely ignored in this context. Here, we take into account that insects feeding on grasses commonly encounter systemic and vertically transmitted (via seeds) fungal Epichloë endophytes, which are regarded as defensive grass mutualists. Defensive mutualism is primarily attributable to alkaloids of fungal origin. To study the effects of Epichloë on insect behavior and performance, we selected wild tall fescue (Festuca arundinacea) and red fescue (Festuca rubra) as grass–endophyte models. The plants used either harbored the systemic endophyte (E+) or were endophyte‐free (E?). As a model herbivore, we selected the Coenonympha hero butterfly feeding on grasses as larvae. We examined both oviposition and feeding preferences of the herbivore as well as larval performance in relation to the presence of Epichloë endophytes in the plants. Our findings did not clearly support the female's oviposition preference to reflect the performance of her offspring. First, the preference responses depended greatly on the grass–endophyte symbiotum. In F. arundinacea, C. hero females preferred E+ individuals in oviposition‐choice tests, whereas in F. rubra, the endophytes may decrease exploitation, as both C. hero adults and larvae preferred E? grasses. Second, the endophytes had no effect on larval performance. Overall, F. arundinacea was an inferior host for C. hero larvae. However, the attraction of C. hero females to E+ may not be maladaptive if these plants constitute a favorable oviposition substrate for reasons other than the plants' nutritional quality. For example, rougher surface of E+ plant may physically facilitate the attachment of eggs, or the plants offer greater protection from natural enemies. Our results highlight the importance of considering the preference of herbivorous insects in studies involving the endophyte‐symbiotic grasses as host plants.     

Settling preferences of the whitefly vector Bemisia tabaci on infected plants varies with virus family and transmission mode

Virus infection may change not only the host‐plant phenotypic (morphological and physiological) characteristics, but can also modify the behavior of their insect vector in a mutualistic or rather antagonistic manner, to promote their spread to new hosts. Viruses differ in their modes of transmission and depend on vector behavior for successful spread. Here, we investigated the effects of the semi‐persistently transmitted Tomato chlorosis virus (ToCV, Crinivirus) and the persistent circulative Tomato severe rugose virus (ToSRV, Begomovirus) on alighting preferences and arrestment behavior of their whitefly vector Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) Middle East Asia Minor 1 (MEAM1) on tomato plants (Solanum lycopersicum L. cv. Santa Clara, Solanaceae). The vector alighting preferences between infected and uninfected plants in choice assays were apparently influenced by the presence of ToCV and ToSRV in the whiteflies or by their previous exposure to infected plants. The observed changes in vector behavior do not seem to benefit the spread of ToCV: non‐viruliferous insects clearly preferred mock‐inoculated plants, whereas ToCV‐viruliferous insects landed on mock‐inoculated and ToCV‐infected plants, indicating a partial change in insect behavior – ToCV was able to directly affect the preference of its vector B. tabaci, but this change in insect behavior did not affect the virus spread because viruliferous insects landed on mock‐inoculated and infected plants indistinctly. In contrast, ToSRV‐viruliferous insects preferred to land on mock‐inoculated plants, a behavior that increases the probability of spread to new host plants. In the arresting behavior assay, the majority of the insects remained on mock‐inoculated plants when released on them. A greater number of insects moved toward mock‐inoculated plants when initially released on ToCV‐ or ToSRV‐infected plants, suggesting that these viruses may repel or reduce the nutritional quality of the host plants for B. tabaci MEAM1.     

Host‐plant use by two Orthomeria (Phasmida: Aschiphasmatini) species feeding on Macaranga myrmecophytes

Myrmecophytes depend on symbiotic ants (plant‐ants) to defend against herbivores. Although these defensive mechanisms are highly effective, some herbivorous insects can use myrmecophytes as their host‐plants. The feeding habits of these phytophages on myrmecophytes and the impacts of the plant‐ants on their feeding behavior have been poorly studied. We examined two phasmid species, Orthomeria alexis and O. cuprinus, which are known to feed on Macaranga (Euphorbiaceae) myrmecophytes in a Bornean primary forest. Our observations revealed that: (i) each phasmid species relied on two closely‐related myrmecophytic Macaranga species for its host‐plants in spite of their normal plant‐ant symbioses; and (ii) there was little overlap between their host‐plant preferences. More O. cuprinus adults and nymphs were found on new leaves, which were attended by more plant‐ants than mature leaves, while most adults and nymphs of O. alexis tended to avoid new leaves. In a feeding choice experiment under ant‐excluded conditions, O. alexis adults chose a non‐host Macaranga myrmecophyte that was more intensively defended by plant‐ants and was more palatable than their usual host‐plants almost as frequently as their usual host‐plant, suggesting that the host‐plant range of O. alexis was restricted by the presence of plant‐ants on non‐host‐plants. Phasmid behavior that appeared to minimize plant‐ant attacks is described.     

Indirect effect of elevated CO2 concentration on Bemisia tabaci MEAM1 feeding on Bt soybean plants

The development of herbivore insects is influenced by the quality of their host plants. Elevated CO₂ alters plant metabolism, which may change the nutritional quality of the plant, modifying the life history and feeding behaviour of herbivore insects. Understanding how insect pests respond to increasing CO₂ concentration is essential for predicting the impact of the pest on food security. In this study, we investigated the effects of elevated CO₂ (eCO₂) on the life history and feeding behaviour of the MEAM1 species of Bemisia tabaci on a Bt soybean cultivar. We found that eCO₂ increased the egg to adult development time and reduced the reproductive responses (fecundity and fertility) of B. tabaci. The whitefly B. tabaci that fed on the soybean plants grown under eCO₂ conditions was negatively influenced by several traits related to the host plant resistance, such as the time spent on phloem sap ingestion. Furthermore, we evaluated the changes in the C:N concentration and plant morphology of the Bt plants. The biomass (weight of leaves and stems) of the Bt soybean plants grown under eCO₂ conditions was significantly increased, and the elevated C:N ratio in the phenological stage V6 (i.e. when the plants had six trifoliate leaves developed) was the most pronounced difference in the Bt soybean plants subjected to eCO₂ treatment. Taken together, our results indicate that Bt plants cultivated under eCO₂ inhibit B. tabaci feeding, which can reduce whitefly infestations of the soybean fields.     

You stay,but I Hop: Host shifting near and far co‐dominated the evolution of Enchenopa treehoppers

The importance and prevalence of phylogenetic tracking between hosts and dependent organisms caused by co‐evolution and shifting between closely related host species have been debated for decades. Most studies of phylogenetic tracking among phytophagous insects and their host plants have been limited to insects feeding on a narrow range of host species. However, narrow host ranges can confound phylogenetic tracking (phylogenetic tracking hypothesis) with host shifting between hosts of intermediate relationship (intermediate hypothesis). Here, we investigated the evolutionary history of the Enchenopa binotata complex of treehoppers. Each species in this complex has high host fidelity, but the entire complex uses hosts across eight plant orders. The phylogenies of E. binotata were reconstructed to evaluate whether (1) tracking host phylogeny; or (2) shifting between intermediately related host plants better explains the evolutionary history of E. binotata. Our results suggest that E. binotata primarily shifted between both distant and intermediate host plants regardless of host phylogeny and less frequently tracked the phylogeny of their hosts. These findings indicate that phytophagous insects with high host fidelity, such as E. binotata, are capable of adaptation not only to closely related host plants but also to novel hosts, likely with diverse phenology and defense mechanisms.     

Proteomic analysis of Arabidopsis thaliana (L.) Heynh responses to a generalist sucking pest (Myzus persicae Sulzer)

Herbivorous insects can cause severe cellular changes to plant foliage following infestations, depending on feeding behaviour. Here, a proteomic study was conducted to investigate the influence of green peach aphid (Myzus persicae Sulzer) as a polyphagous pest on the defence response of Arabidopsis thaliana (L.) Heynh after aphid colony establishment on the host plant (3 days). Analysis of about 574 protein spots on 2‐DE gels revealed 31 differentially expressed protein spots. Twenty out of these 31 differential proteins were selected for analysis by mass spectrometry. In 12 of the 20 analysed spots, we identified seven and nine proteins using MALDI‐TOF‐MS and LC‐ESI‐MS/MS, respectively. Of the analysed spots, 25% contain two proteins. Different metabolic pathways were modulated in Arabidopsis leaves according to aphid feeding: most corresponded to carbohydrate, amino acid and energy metabolism, photosynthesis, defence response and translation. This paper has established a survey of early alterations induced in the proteome of Arabidopsis by M. persicae aphids. It provides valuable insights into the complex responses of plants to biological stress, particularly for herbivorous insects with sucking feeding behaviour.     

Divergent strategies of photoprotection in high-mountain plants

Leaves of high-mountain plants were highly resistant to photoinhibitory damage at low temperature. The roles of different photoprotective mechanisms were compared. Mainly, the alpine species Ranunculus glacialis (L.) and Soldanella alpina were investigated because they appeared to apply greatly divergent strategies of adaptation. The ratio of electron transport rates of photosystem II/photosystem I measured in thylakoids from R. glacialis did not indicate a specific acclimation to high irradiance. Low rates of a chloroplast-mediated inactivation of catalase (EC 1.11.1.6) in red light indicated, however, that less reactive oxygen was released by isolated chloroplasts from R. glacialis than by chloroplasts from lowland plants. Leaves of S. alpina and of Homogyne alpina (L.) Cass, but not those of R. glacialis, had a very high capacity for antioxidative protection, relative to lowland plants, as indicated by a much higher tolerance against paraquat-mediated photooxidative damage and a higher -tocopherol content. Accordingly, ascorbate and glutathione were strongly oxidized and already largely destroyed at low paraquat concentrations in leaves of R. glacialis, but were much less affected in leaves of  S. alpina. Non-radiative dissipation of excitation energy was essential for photoprotection of leaves of  S. alpina and depended on the operation of the xanthophyll cycle. Strong non-photochemical quenching of chlorophyll fluorescence occurred also in R. glacialis leaves at high irradiance, but was largely independent of the presence of zeaxanthin or antheraxanthin. For R. glacialis, photorespiration appeared to provide a strong electron sink and a most essential means of photoprotection, even at low temperature. Application of phosphinothricin, which interferes with photorespiration by inhibition of glutamine synthetase, caused a striking reduction of electron transport through photosystem II and induced marked photoinhibition at both ambient and low temperature in leaves of R. glacialis, while  S. alpina was less affected. Received: 18 March 1998 / Accepted: 7 August 1998     

Angiosperm to Gymnosperm host‐plant switch entails shifts in microbiota of the Welwitschia bug,Probergrothius angolensis (Distant, 1902)

The adaptation of herbivorous insects to new host plants is key to their evolutionary success in diverse environments. Many insects are associated with mutualistic gut bacteria that contribute to the host's nutrition and can thereby facilitate dietary switching in polyphagous insects. However, how gut microbial communities differ between populations of the same species that feed on different host plants remains poorly understood. Most species of Pyrrhocoridae (Hemiptera: Heteroptera) are specialist seed‐feeders on plants in the family Malvaceae, although populations of one species, Probergrothius angolensis, have switched to the very distantly related Welwitschia mirabilis plant in the Namib Desert. We first compared the development and survival of laboratory populations of Pr. angolensis with two other pyrrhocorids on seeds of Welwitschia and found only Pr. angolensis was capable of successfully completing its development. We then collected Pr. angolensis in Namibia from Malvaceae and Welwitschia host plants, respectively, to assess their bacterial and fungal community profiles using high‐throughput amplicon sequencing. Comparison with long‐term laboratory‐reared insects indicated stable associations of Pr. angolensis with core bacteria (Commensalibacter, Enterococcus, Bartonella and Klebsiella), but not with fungi or yeasts. Phylogenetic analyses of core bacteria revealed relationships to other insect‐associated bacteria, but also found new taxa indicating potential host‐specialized nutritional roles. Importantly, the microbial community profiles of bugs feeding on Welwitschia versus Malvaceae revealed stark and consistent differences in the relative abundance of core bacterial taxa that correlate with the host‐plant switch; we were able to reproduce this result through feeding experiments. Thus, a dynamic gut microbiota may provide a means for insect adaptation to new host plants in new environments when food plants are extremely divergent.     

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.     

Fitness of Bt‐resistant cabbage loopers on Bt cotton plants

Development of resistance to the insecticidal toxins from Bacillus thuringiensis (Bt) in insects is the major threat to the continued success of transgenic Bt crops in agriculture. The fitness of Bt‐resistant insects on Bt and non‐Bt plants is a key parameter that determines the development of Bt resistance in insect populations. In this study, a comprehensive analysis of the fitness of Bt‐resistant Trichoplusia ni strains on Bt cotton leaves was conducted. The Bt‐resistant T. ni strains carried two genetically independent mechanisms of resistance to Bt toxins Cry1Ac and Cry2Ab. The effects of the two resistance mechanisms, individually and in combination, on the fitness of the T. ni strains on conventional non‐Bt cotton and on transgenic Bt cotton leaves expressing a single‐toxin Cry1Ac (Bollgard I) or two Bt toxins Cry1Ac and Cry2Ab (Bollgard II) were examined. The presence of Bt toxins in plants reduced the fitness of resistant insects, indicated by decreased net reproductive rate (R₀) and intrinsic rate of increase (r). The reduction in fitness in resistant T. ni on Bollgard II leaves was greater than that on Bollgard I leaves. A 12.4‐day asynchrony of adult emergence between the susceptible T. ni grown on non‐Bt cotton leaves and the dual‐toxin‐resistant T. ni on Bollgard II leaves was observed. Therefore, multitoxin Bt plants not only reduce the probability for T. ni to develop resistance but also strongly reduce the fitness of resistant insects feeding on the plants.