Glandular trichome exudate is the critical factor in geranium resistance to foxglove aphid

Resistance to the foxglove-aphid (Acyrthosiphon solani Kaltenbach) has been demonstrated in some inbred geranium lines (Pelargonium Xhortorum Bailey). To establish more definitively the cause/effect relationship between tall glandular trichome exudate and resistance, an intact plant bioassay was performed comparing a resistant plant line, a resistant plant line from which the tall glandular trichome exudate had been removed using a basic buffer solution, a susceptible line and a susceptible line treated with the buffer wash. After 5 days of isolation on the respective surfaces, the number of surviving adult aphids as well as the number of nymphs produced and remaining alive were determined. Aphids on the buffer washed, resistant line exhibited mortality and fecundity which was not significantly different from that produced by the susceptible line. In contrast, the untreated resistance line was clearly resistant with lower adult survival and fewer living nymphs. The tall glandular trichome exudate must therefore be a critical factor in geranium resistance to the foxglove aphid.

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Zusammenfassung Widerstandsfähigkeit dem Fingerhut-Blattlaus (Acyrthosiphon solani Kaltenbach) gegenüber wurde in einigen durch Inzucht erzeugten Pelargonie-Linien (Pelargonium Xhortorum Bailey) gezeigt. Um das Verhältnis von Ursache und Wirkung zwischen dem hochgewachsenen glandulären Trichom-Exudat und Widerstandsfähigkeit genauer zu bestimmen, wurde eine Bio-Untersuchung an intakten Pflanzen unternommen. Dabei wurden eine widerstandsfähige Pflanzenlinie, eine widerstandsfähige Pflanzenlinie, von der das hochgewachsene glanduläre Trichom-Exudat durch eine basische Pufferlösung entfernt worden war, eine anfällige Linie und eine mit Pufferlösung behandelte Linie verglichen. Zwei erwachsene weibliche Blattläuse wurden fünf Tage durch ein engmaschiges Netz auf den zu untersuchenden dritten und vierten Knotenblättern eingesperrt. Bei jeder Pflanze wurde die Untersuchung an einem nichtbehandelten Blatt und an einem Blat, von dem das Exudat durch Waschen mit der Pufferlösung entfernt worden war, durchgeführt. Für jede Linie wurden fünf Pflanzen gebraucht, und der ganze Versuch wurde sechsmal wiederholt. Nach einer fünftägigen Isolierung auf den jeweiligen Oberflächen wurden die Blätter von der Pflanze entfernt, und sowohl die Zahl der überlebenden erwachsenen Blattläuse wie auch die der produzierten und noch am Leben gebliebenen Nymphen festgestellt. Mit einer niedrigeren Überlebensrate der Erwachsenen und weniger noch lebenden Nymphen war die nichtbehandelte widerstandsfähige Linie deutlich widerstandsfähig. Im Gegensatz dazu zeigten die Blattläuse auf der mit Puffer gewaschenen widerstandsfähigen Linie eine Sterblichkeit und Fruchtbarkeit, die nicht erheblich höher waren, als die auf der anfälligen Linie, was beweist, daß das Waschen mit der Pufferlösung den Widerstandsfaktor entfernt hatte. Das hochgewachsene glanduläre Trichom-Exudat muß deshalb ein kritischer Faktor in der Widerstandsfähigkeit gegen Pelargonie-Schädlinge sein.

     

Transgenic Nicotiana TabacumL. with enhanced trichome exudate cembratrieneols has reduced aphid infestation in the field

Glandular trichomes of many plants secrete natural products that influence plant/insect interactions. For example, tobacco varieties having relatively high cembratrieneols (CBTols) are known to have enhanced aphid resistance in the field. CBTols and corresponding CBTdiols comprise 1.4 and 62%, respectively, of trichome exudate in the aphid susceptible tobacco variety, T.I. 1068. Using this cultivar we suppressed the CYP71D16 gene that encodes the enzyme that converts CBTols to CBTdiols. In suppressed plants CBTols and CBTdiols accounted for about 27 and 35% of exudate weight, respectively. Total CBTols plus CBTdiols was not changed substantially. Here we studied the relationship between aphid infestation and increased exudate CBTols in the field using self progeny derived from 5 independent primary transgenic T. I. 1068 plants having suppressed CYP71D16 activity. Two hundred individual plants were scored for aphid infestation, and their trichome exudate compositions were determined by gas chromatography. A significant negative correlation was found between high CBTols levels in trichome exudates and aphid infestation. No aphid infestation was observed on the majority of plants with CBTols/CBTdiols ratios of >1.49, which represents a >20-fold increase in CBTols, and a >40% decrease in CBTdiols over control T.I. 1068 exudate. In contrast, aphid infestation occurred on most plants with CBTols/CBTdiols ratios <0.201, which is similar to that of the untransformed control T.I. 1068. These results demonstrate the feasibility of using metabolic engineering of glandular trichomes to enhance natural product-based pest resistance.     

Glandular trichomes of Solanum berthaultii and its hybrids with Solanum tuberosum affect nymphal emergence, development, and survival of Empoasca fabae (Homoptera: Cicadellidae)

The influence of foliar glandular trichomes on developmental biology of the potato leafhopper, Empoasca fabae (Harris) (Homoptera: Cicadellidae), was examined by selective removal of trichomes on resistant and susceptible potato germplasm comprising two glandular trichome-bearing genotypes (PI 473331 and PI 473334) of wild potato, Solanum berthaultii (Hawkes), two interspecific hybrids of S. berthaultii with Solanum tuberosum L. (Q174-2 and NY123), and two commercial S. tuberosum cultivars ('Elba' and 'Allegany'). Adult mortality was affected, whereas nymphal emergence was unaffected by removal of glandular trichome exudates. No nymphs were observed emerging from leaves of PI 473331 or PI 473334. Regardless of the trichome removal treatment, no nymphs completed development on PI 473331, and only a small percentage of nymphs survived to adulthood on NY123 and PI 473334. PI 473331 had the greatest frequency of adults and nymphs with trichome exudate deposition on their bodies, followed by PI 473334. Nymphs, but not adults, had exudate accumulation when caged on foliage of Q174-2. Removal of type A trichome heads and type B droplets eliminated exudate deposition. The results of these studies provide evidence that phenolic oxidation chemistry and the physical barrier imposed by trichomes of S. berthaultii are responsible for much of the observed resistance but that other factors possibly linked to the presence of trichomes aid in the expression on resistance.     

Evaluation of natural and engineered resistance mechanisms in potato against Colorado potato beetle in a no-choice field study

The Colorado potato beetle, Leptinotarsa decemlineata Say, is the major insect pest of potato, Solanum tuberosum L., in eastern North America and is renowned for resistance development, currently resistant to >40 insecticides worldwide. Host plant resistance may assist in delaying in resistance development to insecticides. We evaluated natural host plant resistance mechanisms (glandular trichomes and Solanum chacoense Bitter-derived resistance) and engineered resistance mechanisms (Bacillus thuringiensis [Bt] Berliner cry3A and cry1Ia1) in a no-choice cage study. Six different potato lines representing four host plant resistance mechanisms were evaluated over 2 yr. Egg masses were placed in each cage (one egg mass per plant). Almost no feeding was observed in the Bt-cry3A lines, and only minor feeding was observed in the Bt-cry1Ia1 lines in either year. On the S. chacoense-derived line, there was significantly less defoliation than on either the susceptible line or the glandular trichome line in 2003. In 2004, there was significantly higher defoliation on the S. chacoense-derived line than on the susceptible line or glandular trichome line. The defoliation of the Solanum chacoense-derived line was largely due to larvae clipping the petioles, rather than consumption of the leaves. Defoliation on the glandular trichome line did not differ significantly from the defoliation of the susceptible line, suggesting glandular trichomes may not be effective in controlling larvae and preventing defoliation. This study suggested that Bt can provide high levels of resistance, but the natural resistance mechanisms tested here are variable for control of Colorado potato beetle larvae in no-choice situations.     

Trichomes of Lycopersicon spp. and their effect on Myzus persicae (Sulzer) (Hemiptera: Aphididae)

Abstract The green peach aphid ( Myzus persicae ) is becoming resistant to many of the pesticides used in the tomato ( Lycopersicon esculentum ) industry. A potential alternative method of control is host-plant resistance, which may be mediated by trichomes. Resistance to the green peach aphid was assessed for six accessions of L. hirsutum, three of L. pennellii and a L. esculentum control. Mortality, entrapment and number of aphids unaffected by trichomes were recorded 24, 48 and 72 h after placement. Using multiple regression against counts of glandular and non-glandular trichomes, the role of foliar pubescence in resistance to the aphid was established. Lycopersicon pennellii possessed the highest level of resistance after 72 h and type IV trichome density accounted for 26.5% of variance in mortality ( P  < 0.001). Low densities of type IV and high densities of type VII trichomes increased the number of aphids 'unaffected' by trichomes, a relationship accounting for 29% of variance ( P  < 0.001). Information on which types of trichomes most strongly affect resistance to insect pests will aid future breeding and L. pennellii shows potential for use in breeding programs to increase levels of host-plant resistance in L. esculentum.     

Feeding behavior of potato aphid affected by glandular trichomes of wild tomato

Mortality of the potato aphid, Macrosiphum euphorbiae (Thomas), on Lycopersicon pennellii (Corr.) D'Arcy and its F₁ hybrid with Lycopersicon esculentum Mill. was significantly greater than that on L. esculentum. Physical entrapment was not the sole mechanism of resistance in L. pennellii since few late instar aphids were found trapped in the sticky glandular exudate of the type IV trichomes; entrapment could, however, affect survival of early instars. Aphid settling on L. pennellii was dramatically less than that on L. esculentum, suggesting that starvation may have contributed to high mortality. Compared to L. esculentum, aphid feeding behavior on L. pennellii and the F₁ was characterized by a delay in the time to first probe, a reduction in the number of probes, and a decrease in the total proportion of time spent feeding. Removal of the glandular exudate of the type IV trichomes from L. pennellii resulted in a decrease in preprobe time and an increase in both the number of probes and the percent of time spent probing. Transfer of glandular trichome exudate of L. pennellii to leaflets of L. esculentum resulted in an increase in resistance as measured by these three parameters.

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Zusammenfassung Die Absterberate der Kartoffellaus, Macrosiphum euphorbiae Thomas, auf Lycopersicon pennellii (Corr.) D'Arcy, sowie auf der Kreuzung L. esculentum Mill. und L. pennellii, war deutlich grösser als auf L. esculentum. Das mechanische Verfangen der Läuse war nicht der Hauptgrund der Resistenz von L. pennellii. Wenige tote Läuse wurden in dem klebrigen Sekret der Typus IV Trichome gefunden. Auf L. pennellii siedelten sich die Läuse in viel geringerer Zahl an als auf L. esculentum. Dies führte zum Schluss, dass Verhungern eine der Ursachen der hohen Mortalität der Läuse war. Im Vergleich zum Saugverhalten auf L. esculentum war das Saugverhalten auf L. pennellii, wie auch auf F₁, durch Folgendes gekennzeichnet 1) Verspätung des ersten Stichversuchs, 2) Verminderung der Stichversuche pro Zeiteinheit und 3) Verminderung des Zeitanteils, der zum Saugen verwendet wurde. Die Entfernung des Sekrets der Typus IV Trichomen auf L. pennellii verursachte 1) eine kürzere Zeitspanne vor dem ersten Stichversuch, 2) eine Vergrösserung der Anzahl Stichversuche pro Zeiteinheit, 3) eine Verlängerung der Saugzeit. Die Uebertragung des Sekretes von L. pennellii auf Blätter von L. esculentum verbesserte deren Resistenz gegen Blattläuse gemessen mit den genannten drei Kriterien.

     

Elevated CO2 alters the feeding behaviour of the pea aphid by modifying the physical and chemical resistance of Medicago truncatula

Elevated CO₂ compromises the resistance of leguminous plants against chewing insects, but little is known about whether elevated CO₂ modifies the resistance against phloem‐sucking insects or whether it has contrasting effects on the resistance of legumes that differ in biological nitrogen fixation. We tested the hypothesis that the physical and chemical resistance against aphids would be increased in Jemalong (a wild type of Medicago truncatula) but would be decreased in dnf1 (a mutant without biological nitrogen fixation) by elevated CO₂. The non‐glandular and glandular trichome density of Jemalong plants increased under elevated CO₂, resulting in prolonged aphid probing. In contrast, dnf1 plants tended to decrease foliar trichome density under elevated CO₂, resulting in less surface and epidermal resistance to aphids. Elevated CO₂ enhanced the ineffective salicylic acid‐dependent defence pathway but decreased the effective jasmonic acid/ethylene‐dependent defence pathway in aphid‐infested Jemalong plants. Therefore, aphid probing time decreased and the duration of phloem sap ingestion increased on Jemalong under elevated CO₂, which, in turn, increased aphid growth rate. Overall, our results suggest that elevated CO₂ decreases the chemical resistance of wild‐type M. truncatula against aphids, and that the host's biological nitrogen fixation ability is central to this effect.     

Inhibition of lipoxygenase and prostaglandin endoperoxide synthase by anacardic acids

C22:1 omega 5-anacardic acid was found to be a good inhibitor of both potato lipoxygenase and ovine prostaglandin endoperoxide synthase with approximate IC50's of 6 and 27 microM, respectively. Very similar inhibition was seen with the crude exudate, rich in omega 5-anacardic acids, from glandular trichomes of an arthropod-resistant strain of geranium, Pelargonium xhortorum. The saturated anacardic acid (C22:0 sat), abundant in the trichome exudate of susceptible strains, was nearly as inhibitory toward both prostaglandin endoperoxide synthase and lipoxygenase as the omega 5-unsaturated compound. However, the dimethyl derivative of C22:1 omega 5-anacardic acid was a poor inhibitor of prostaglandin endoperoxide synthase and caused only moderate (32%) inhibition of lipoxygenase even at 135 microM. The possible role of prostaglandin endoperoxide synthase and lipoxygenase inhibition in the enhanced pest resistance of geraniums which produce the omega 5-AnAs is discussed.     

Development of a mite bioassay to evaluate plant resistance and its use in determining regeneration of spider mite resistance

The zonal geranium (Pelargonium xhortorum) possesses tall glandular trichomes that secrete anacardic acids, a viscous, sticky exudate which has been suggested as the primary mechanism in two-spotted spider mite (Tetranychus urticae Koch) resistance. A new bioassay was developed using small Plexiglas^® cylinders as chambers for evaluating the resistance of geranium leaves to the two-spotted spider mite. This bioassay was easy to prepare, required only 24 hours to conduct, exhibited no problems with desiccation, condensation, or mite accountability, and yielded reproducible results. This bioassay was then used to study the regeneration of resistance of attached geranium leaves after they were made mite-susceptible by removing the excreted anacardic acids with water. Washed leaves regained full resistance after 14 days.     

Recombinant and classically selected factors of potato plant resistance to the Colorado potato beetle, Leptinotarsa decemlineata, variously affect the potato aphid parasitoid Aphidius nigripes

Different forms of crop resistance developed against majorpotato pests such as the Colorado potato beetle (CPB), Leptinotarsadecemlineata, may be variously compatible with biological controlof secondary pests such as aphids. We compared the performance of theparasitoid Aphidius nigripes developing in the aphid hostMacrosiphum euphorbiae, on five potato lines, including atransgenic `Superior-BT' line expressing the CryIIIA toxin ofBacillus thuringiensis, which is specific to Coleoptera; andthe `NYL 235-4' line derived from Solanum berthaultii,characterized by relatively unspecific resistance to herbivores based onmoderately-high density of glandular trichomes. The other lines testedwere a `Kennebec-OCI' transgenic line expressing the protease inhibitorrice cystatin I (OCI), a potential resistance factor against coleopteranpests; and the commercial cultivars `Superior' and `Kennebec' used ascontrols. Parasitoid immature survival and adult size were reducedcompared to controls when the wasps developed on aphids fed the`Superior-BT' potato. In contrast, parasitoid size and fecundityincreased above control when the wasps developed on aphids fed the`Kennebec-OCI' potato. Parasitoids reached the adult stage faster andwere more fecund on `NYL 235-4' than control lines. The results indicatethat the different forms of potato resistance currently developed mainlyagainst the CPB had various unexpected effects on aphid parasitoidfitness. These effects on the parasitoid were complex but were generallyinterpretable in terms of host aphid quality variation among potatolines used as food by the aphids during parasitoiddevelopment.     

Role of leaf glandular trichomes of melon plants in deterrence of Aphis gossypii Glover

External characteristics of the leaf epidermis and their effects on behaviour of Aphis gossypii Glover were evaluated in two Cucumis melo L. genotypes, ‘Bola de Oro’ (aphid susceptible) and TGR‐1551 (aphid resistant) in order to explore their role in the early rejection of TGR‐1551 by this aphid. No differential effects of epicuticular waxes on aphid behaviour were observed. The type, distribution and number of trichomes on melon leaves were also studied. Pubescence in melon, measured as the number of non‐glandular trichomes per cm², was not sufficient to prevent aphid settling. However, there was a high density of type I glandular trichomes on leaves of the aphid‐resistant genotype. According to microscopic observations and stain testing, these trichomes store and secrete phenols and flavonoids. Free‐choice tests were conducted to determine the effect of these glandular trichomes on A. gossypii preference, revealing that aphids reject leaf disks of TGR‐1551 from the onset of the experiment. Additional experiments after removal of leaf type I glandular trichome exudates showed that A. gossypii preferred washed TGR‐1551 leaf disks over unwashed disks, while this effect was not observed in experiments using washed and unwashed ‘Bola de Oro’ leaf disks. These results suggest that a high density of glandular trichomes and chemicals secreted by them deter A. gossypii and disturb aphid settling on TGR‐1551.     

Aphid repellent sesquiterpenes in glandular trichomes of Solanum berthaultii and S. tuberosum

Exudate from glandular trichomes of the wild potato species, Solanum berthaultii Hawkes, and the cultivated potato, S. tuberosum L., contained volatile substances including sesquiterpenes when examined with gas chromatography/mass spectrometry procedures. Sesquiterpenes were the major volatile constituents of Type A trichome exudate but were a minor constituent of Type B exudate. Sesquiterpene mixtures of the two potato species differed qualitatively and quantitatively. -Caryophyllene and E--Farnesene were major components identified in Type A trichome exudate of the cultivated potato. Sesquiterpene mixtures of both potato species were repellent to the green peach aphid, Myzus persicae (Sulzer). The role of trichome sesquiterpenes as a determinant of aphid behavior on the two potato species and in the expression of host resistance is discussed.

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Résumé L'examen en chromatographie gazeuse et en spectométrie de masse a révélé la présence de substances volatiles contenant des sesquiterpènes dans l'exsudat des trichômes glandulaires des pommes de terre sauvage (Solanum berthaultii) et cultivée (S. tuberosum). Les sesquiterpènes étaient le principal composé volatile de l'exsudat de type A, tandis qu'ils n'étaient que faiblement représentés dans l'exsudat de type B. Les mélanges de sesquiterpènes des deux espèces différaient qualitativement et quantitativement. Le -Caryophyllène et le E--Farnesène constituaient les principaux composés identifiés dans l'exsudat de trichôme de type A de S. tuberosum. Les mélanges sesquiterpéniques des deux espèces ont repousée Myzus persicae Sulzer. La discussion a porté sur l'influence des sesquiterpènes des trichômes dans la définition du comportement des pucerons sur ces deux plantes et sur la manifestation de la résistance de l'hôte.

     

Glandular Trichomes on Alfalfa Impede Searching Behavior of the Potato Leafhopper Parasitoid

New cultivars of alfalfa, Medicago sativa L., have been released with glandular trichomes for resistance to potato leafhopper, Empoasca fabae (Harris). Yet, the impact of the glandular trichomes on the primary natural enemy of the leafhopper, Anagrus nigriventris Girault, is unknown. We compared the host searching behavior of the egg parasitoid on four alfalfa clones varying in trichome characters. Female wasps were videotaped on Ranger, a susceptible clone with relatively sparse trichomes, B14, a resistant clone with dense but nonglandular trichomes, and FG12 and FG18, two resistant clones with glandular trichomes. Although the number of leafhopper eggs per stem exposed to wasps did not significantly differ among the four clones, the frequency of foraging and total foraging time were less on the two clones with glandular trichomes than on the two clones with nonglandular trichomes. In addition, an analysis of covariance demonstrated that, although the number of ovipositional probes increased with egg density on a stem, the number of probes on stems with glandular trichomes was significantly less than that on stems without glandular trichomes. The allocation of time by wasps among drumming, probing, and grooming behaviors was similar among the clones. Wasps tended to fly off of clones with glandular trichomes more often than off of clones with nonglandular trichomes. This study suggests that cultivars with glandular trichomes may interfere with host searching by A. nigriventris.     

Molecular markers locate genes for resistance to the Colorado potato beetle,Leptinotarsa decemlineata,in hybrid Solanum tuberosum x S. berthaultii potato progenies

The wild Bolivian potato, Solanum berthaultii Hawkes, has been used as a source of resistance to the Colorado potato beetle (CPB), Leptinotarsa decemlineata Say, one of the most significant pests of potato. In this study, two reciprocal backcross S. tuberosum x S. berthaultii potato progenies, BCB and BCT, were mapped with RFLP markers and screened for resistance to CPB consumption, oviposition and defoliation. The genotypic and phenotypic data were combined and analysed to locate quantitative trait loci (QTLs) for resistance to CPB. Three QTLs on three chromosomes in BCB, and two QTLs on two chromosomes in BCT influenced resistance. The QTLs were generally additive but one instance of epistasis was noted. Each QTL accounted for 4–12% of the phenotypic variation observed in resistance. In the more resistant BCB population, a three QTL model explained ca. 20% of the variation in CPB oviposition. When alleles at the three QTLs were homozygous S. berthaultii, oviposition was reduced ca. 60% compared to the heterozygotes. The QTLs for resistance to CPB were compared to those previously identified for the type A and B glandular trichomes, which have been implicated in resistance in the same progenies. Generally, the QTLs for resistance to CPB coincided with loci associated with the glandular trichomes confirming the importance of the glandular trichomes in mediating resistance. However, a relatively strong and consistent QTL for insect resistance in both BCB and BCT on chromosome 1 was observed that was not associated with any trichome traits, suggesting the trichomes may not account for all of the resistance observed in these progenies.     

Effects of Aphis fabae and Uromyces viciae-favae on the growth of a susceptible and an aphid resistant cultivar of Vicia faba

Damaging effects of either black bean aphid (Aphis fabae), broad bean rust (Uromyces viciae-fabae), or the combination of both were investigated on a susceptible (cv. Diana) and an aphid resistant (cv. Bolero) cultivar of Vicia faba. When compared with rust, aphids caused greater reductions of root dry weight, shoot dry weight, leaf area, and mean relative growth rate. The mean unit leaf rate was also reduced whereas the leaf area ratio was not affected. The damage caused per aphid was highest on the susceptible cultivar. Rust induced damage did not differ between the cultivars. Concomitant infestation with both pests only resulted in additive damage. The population development of aphids was delayed on partially resistant plants. High temperature and rust infection reduced the total number of aphids the plants were able to support but not the level of resistance. Thus the specific damaging effect per aphid was increased.     

Trichome-borne and artificially applied acylsugars of wild tomato deter feeding and oviposition of the leafminer Liriomyza trifolii

Oviposition and adult feeding of the leafminer Liriomyza trifollii (Burgess) (Diptera, Agromyzidae) on Lycopersicon pennellii (Corr.) D'Arcy and its F₁ hybrid with Lycopersicon esculentum (Mill.) was significantly less than that on the cultivated tomato, L. esculentum. The resistance of L. pennellii and the F₁ was reduced following rinsing of foliage with ethanol. Resistant attributes of L. pennellii were transferred to L. esculentum through appression of L. pennellii foliage to L. esculentum leaflets. Application of purified 2,3,4-tri-O-acylglucoses (the principal component of type IV glandular trichome exudate of L. pennellii) to L. esculentum significantly decreased feeding and oviposition on L. esculentum leaflets by 61–99%. Therefore the principal mechanism of resistance to this leafminer by L. pennellii is the secretion of these acylglucoses. Dose response analysis of acylglucoses applied to L. esculentum shows that dosages as low as 10% those found on L. pennellii provide large reductions (91%) in leaf punctures and mines.     

The mechanism of trichome resistance in Anaphalis margaritacea to the meadow spittlebug Philaenus spumarius

This study demonstrated how simple, lanate trichomes on the stem of Anaphalis margaritacea (D.C.) inhibit feeding by young nymphs of Philaenus spumarius (L.). Nymphs were caged on stems which were subsequently sectioned and examined for feeding marks. The lack of stylet punctures at sites where nymphs were unable to feed indicated that trichomes prevented the start of stylet penetration rather than interfering with the progress of the stylets after penetration had begun. Trichome resistance declined progressively with spittlebug instar from complete resistance to first instar nymphs through complete absence of resistance to fifth instar nymphs. Discriminant analysis of the effect of number of trichomes per mm², vertical density, and height on trichome resistance to third instar nymphs revealed that trichome height was the critical factor in resistance. Variation in the percent of nymphs in each instar able to feed when caged on the stem was (1) positively correlated with degree to which the average rostrum length of the instar exceeded the trichome height, (2) independent of trichome density, which was uniformly high on all stems. Trichomes prevented stylet penetration where trichome height exceeded the length of the nymph's rostrum, probably because the necessary stimuli were not received when the sensilla on the tip of the rostrum could not touch the surface of the stem. A similar mechanism may explain resistance of simple trichomes to crop pests in the Cicadellidae.

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Résumé Ce travail montre comment des trichomes simples et laineux sur les tiges d'Anaphalis margaritacea (D.C.) inhibent la prise d'aliments par les jeunes larves de Philaenus spumarius L. Les larves ont été encagées sur des tiges qui ont été ultérieurement coupées et les piqûres alimentaires dénombrées. L'absence de piqûres par les stylets aux lieux où les larves ont été incapables de s'alimenter a montré que les trichomes ont empêché le début de la pénétration des stylets plus qu'ils n'ont entravé leur progression après le début de la pénétration. La résistance des trichomes diminue progressivement: totale pour le premier stade larvaire, elle est devenue nulle au cinquième stade. Une analyse discriminante des effets de la densité des trichome par mm², de la densité verticale, de leur hauteur sur la résistance des trichomes aux larves de 3ème stade, a révélé que la hauteur du trichome est le facteur de résistance. La variation dans le pourcentage de larves de chaque stade encagées sur la tige, capables de consommer, présentait une relation positive avec le degré par lequel la longueur du rostre dépasse la hauteur du trichome, quelle que soit la densité des trichomes qui est toujours élevée sur toutes les tiges. Les trichomes ont empêché la pénétration des stylets quand leur hauteur a dépassé la longueur du rostre, probablement parce que les stimuli nécessaires n'ont pas été reçus quant l'extrémité du rostre ne pouvait pas atteindre la surface de la tige. Un mécanisme semblable peut expliquer la résistance de trichomes simples aux Cicadellidae.

     

Role of the glandular trichomes in resistance of perennial alfalfa to the potato leafhopper (Homoptera: Cicadellidae)

Experiments were performed to elucidate resistance of glandular-haired alfalfa, Medicago sativa L., to the potato leafhopper, Empoasca fabae (Harris). The primary objective was to examine the potential role of the glandular trichomes and stem lignification for imparting resistance to this pest. During free-choice foliar discoloration experiments, the resistant alfalfa clone FGplh13 expressed lower levels of injury than the susceptible P5373 after 13 d. No-choice tests performed using nymphal potato leafhoppers showed higher levels of mortality associated with the resistant glandular-haired clone FGplh13, and no nymphs survived after 48 h. Additional no-choice experiments using FGplh13 and P5373 alfalfa with the glandular and nonglandular trichomes intact or removed show that mortality of nymphs and adults decreased after removal of the glandular trichomes from FGplh13. Nymphal and adult mortality and nymphal development time did not differ on FGplh13 with the trichomes removed and P5373 with the trichomes intact or removed. Firstinstar, and possibly second-instar, potato leafhoppers were entrapped by an exudate produced by the glandular trichomes on the FGplh13 alfalfa. Stem lignification and the number of vascular bundles did not differ between FGplh13 and P5373 alfalfas. The glandular trichomes on FGplh13 alfalfa appeared to provide the major host resistance factor, with resistance to adults being chemically based and resistance to nymphs being chemically and mechanically based. To maintain levels of potato leafhopper resistance, breeders would appear to benefit by continuing to select for the expression of the glandular trichome phenotype.