Root and shoot jasmonic acid induction differently affects the foraging behavior of <Emphasis Type="Italic">Cotesia glomerata</Emphasis> under semi-field conditions

Plants can accumulate and release defensive chemicals by activating various signaling pathways when they are damaged by herbivores or pathogens. The jasmonic acid pathway is activated after damage by chewing herbivores. Here we used jasmonic acid (JA) as an exogenous elicitor to induce feral cabbage plants. In this study, the effects of root JA (RJA) and shoot JA (SJA) induction on the foraging behavior of Cotesia glomerata, a parasitoid of the large cabbage white butterfly Pieris brassicae, was investigated under semi-field conditions. In all combinations of differently induced plants (RJA, SJA and control plants), the percentages of shoot induced plants that were visited by at least one wasp were significantly higher than those of controls or root induced plants during 3 h of foraging. Consequently, parasitism rates of P. brassicae on shoot-JA induced plants were significantly higher than on plants induced with JA to the roots or control plants in all tests. However, this behavioral preference was not reflected in the allocation of offspring. The clutch sizes of C. glomerata eggs on control, root induced and shoot induced plants were not significantly different from each other in two-choice or three-choice experiments, but did differ with clutch size in the two-choice experiment of uninduced control plants versus SJA. This semi-field study helps to further understand the choice behavior and preferences of parasitoids in natural multitrophic communities in which plants induced with root or shoot herbivores occur together.     

Leaf Colour as a Signal of Chemical Defence to Insect Herbivores in Wild Cabbage (Brassica oleracea)

Leaf colour has been proposed to signal levels of host defence to insect herbivores, but we lack data on herbivory, leaf colour and levels of defence for wild host populations necessary to test this hypothesis. Such a test requires measurements of leaf spectra as they would be sensed by herbivore visual systems, as well as simultaneous measurements of chemical defences and herbivore responses to leaf colour in natural host-herbivore populations. In a large-scale field survey of wild cabbage (Brassica oleracea) populations, we show that variation in leaf colour and brightness, measured according to herbivore spectral sensitivities, predicts both levels of chemical defences (glucosinolates) and abundance of specialist lepidopteran (Pieris rapae) and hemipteran (Brevicoryne brassicae) herbivores. In subsequent experiments, P. rapae larvae achieved faster growth and greater pupal mass when feeding on plants with bluer leaves, which contained lower levels of aliphatic glucosinolates. Glucosinolate-mediated effects on larval performance may thus contribute to the association between P. rapae herbivory and leaf colour observed in the field. However, preference tests found no evidence that adult butterflies selected host plants based on leaf coloration. In the field, B. brassicae abundance varied with leaf brightness but greenhouse experiments were unable to identify any effects of brightness on aphid preference or performance. Our findings suggest that although leaf colour reflects both levels of host defences and herbivore abundance in the field, the ability of herbivores to respond to colour signals may be limited, even in species where performance is correlated with leaf colour.     

Herbivore and phytohormone induced defensive response in kale against cabbage butterfly, Pieris brassicae Linn

The constitutive and induced resistance were studied in two varieties (Khanyari and Kawdari) of kale, Brassica oleracea var. acephala in response to cabbage butterfly, Pieris brassicae infestation and exogenous application of jasmonic acid (JA) and salicylic acid (SA). Phenols, condensed tannins, flavonoids and proteins were measured at six days after JA (1?mM) and SA (1?mM) application and/or insect infestation. Plant damage and larval weights were also recorded. Khanyari variety showed highest amounts of phenols (208.23?μg/g FW), condensed tannin (347.76?μg/g FW), flavonoids (175.61?μg/g FW) and proteins (0.71?mg/g FW) in plants pre-treated with JA and infested with insects. The PAL activity was high in response to JA application followed by insect infestation. Insects reared on Khanyari and Kawdari plants pre-treated with JA prior showed significantly reduced larval weights (97.88 and 102.46?mg, respectively). Damage was low in plants pre-treated with JA in Khanyari at 3, 6 and 9?days after treatment (10.52%, 8.52%, and 5.30%, respectively). Thus, JA can play an important role in plant defense in kale against P. brassicae.     

Plant‐mediated species networks: the modulating role of herbivore density

1. When herbivores of distinct feeding guilds, such as phloem feeders and leaf chewers, interact, the outcome of these interactions often shows facilitation. However, whether this facilitation turns into competition at stronger herbivory pressure remains unknown. 2. Using an integrative approach that links ecological processes (behavioural choices of insects) with physiological plant mechanisms (nutrient and phytohormone levels) for the wild crucifer Brassica nigra (L.) Koch., this study evaluates preferences of leaf chewers for plants previously infested with several densities of the specialist aphid Brevicoryne brassicae L. (Hemiptera, Aphididae). As leaf chewers, four species of caterpillars (Lepidoptera) were selected that differ in their degree of specialisation in crucifers. 3. These results show that, whereas at low and medium aphid densities caterpillars displayed a preference for aphid‐infested plants or no preference, at high aphid infestation density, all four species of caterpillar preferred uninfested plants, with a significant difference for Pieris rapae and Mamestra brassicae. 4. In contrast to our expectation, the consistent preference for uninfested plants at a high aphid density could not be associated with a decrease in plant nutrition. However, while jasmonate concentrations [i.e. 12‐oxo‐phytodienoic acid and jasmonic acid (JA)] at medium aphid‐density infestation decreased compared with low levels of infestation, at high infestation level, the jasmonates JA as well as JA conjugated with the amino acid isoleucine were present at higher levels compared with low‐infestation treatments. 5. This work provides evidence that positive interactions observed in herbivore communities can be transient, leading to negative interactions mediated by changes in plant defences rather than in plant nutrition.     

Bottom-up and top-down herbivore regulation mediated by glucosinolates in Brassica oleracea var. acephala

Quantitative differences in plant defence metabolites, such as glucosinolates, may directly affect herbivore preference and performance, and indirectly affect natural enemy pressure. By assessing insect abundance and leaf damage rate, we studied the responses of insect herbivores to six genotypes of Brassica oleracea var. acephala, selected from the same cultivar for having high or low foliar content of sinigrin, glucoiberin and glucobrassicin. We also investigated whether the natural parasitism rate was affected by glucosinolates. Finally, we assessed the relative importance of plant chemistry (bottom-up control) and natural enemy performance (top-down control) in shaping insect abundance, the ratio of generalist/specialist herbivores and levels of leaf damage. We found that high sinigrin content decreased the abundance of the generalist Mamestra brassicae (Lepidoptera, Noctuidae) and the specialist Plutella xylostella (Lepidoptera, Yponomeutidae), but increased the load of the specialist Eurydema ornatum (Hemiptera, Pentatomidae). Plants with high sinigrin content suffered less leaf injury. The specialist Brevicoryne brassicae (Hemiptera, Aphididae) increased in plants with low glucobrassicin content, whereas the specialists Pieris rapae (Lepidoptera, Pieridae), Aleyrodes brassicae (Hemiptera, Aleyrodidae) and Phyllotreta cruciferae (Coleoptera, Chrysomelidae) were not affected by the plant genotype. Parasitism rates of M. brassicae larvae and E. ornatum eggs were affected by plant genotype. The ratio of generalist/specialist herbivores was positively correlated with parasitism rate. Although both top-down and bottom-up forces were seen to be contributing, the key factor in shaping both herbivore performance and parasitism rate was the glucosinolate concentration, which highlights the impact of bottom-up forces on the trophic cascades in crop habitats.     

A common pathway for metabolism of 4-hydroxybenzylglucosinolate in Pieris and Anthocaris (Lepidoptera: Pieridae)

Caterpillars of Pieris rapae L. (Lepidoptera: Pieridae) convert 4-hydroxybenzylglucosinolate (sinalbin) in brassicaceous plants into 4-hydroxybenzylcyanide sulfate (HBC sulfate), with 4-hydroxybenzylcyanide (HBC) as intermediate. This apparently serves as a detoxification, because alternative formation of a mustard oil is avoided. We confirmed the capacity of P. rapae to convert the intermediate HBC into HBC sulfate. Four additional Pieridae – Anthocaris cardamines L., Pieris virginiensis Harris, Pieris napi oleracea Edwards and Pieris brassicae L., likewise excreted HBC sulfate after ingesting leaves with topically added HBC or leaves naturally containing sinalbin and myrosinase, but not after ingesting control leaves devoid of HBC and sinalbin. We confirmed the capacity of the most distantly related pierid species (A. cardamines) for converting ingested (topically added) sinalbin into HBC sulfate. Larvae of two non-pierid Brassicaceae-feeding insects, the oligophagous sawfly Athalia rosae L. (Hymenoptera: Tenthrenidae) and the polyphagous moth Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae), did not excrete HBC sulfate after ingesting sinalbin-containing leaves or topically added HBC.     

Ecological fits, mis-fits and lotteries involving insect herbivores on the invasive plant, Bunias orientalis

Exotic plants bring with them traits that evolved elsewhere into their new ranges. These traits may make them unattractive or even toxic to native herbivores, or vice versa. Here, interactions between two species of specialist (Pieris rapae and P. brassicae) and two species of generalist (Spodoptera exigua and Mamestra brassicae) insect herbivores were examined on two native crucifer species in the Netherlands, Brassica nigra and Sinapis arvensis, and an exotic, Bunias orientalis. Bu. orientalis originates in eastern Europe and western Asia but is now an invasive pest in many countries in central Europe. P. rapae, P. brassicae and S. exigua performed very poorly on Bu. orientalis, with close to 100% of larvae failing to pupate, whereas survival was much higher on the native plants. In choice experiments, the pierid butterflies preferred to oviposit on the native plants. Alternatively, M. brassicae developed very poorly on the native plants but thrived on Bu. orientalis. Further assays with a German Bu. orientalis population also showed that several specialist and generalist herbivores performed very poorly on this plant, with the exception of Spodoptera littoralis and M. brassicae. Bu. orientalis produced higher levels of secondary plant compounds (glucosinolates) than B. nigra but not S. arvensis but these do not appear to be important factors for herbivore development. Our results suggest that Bu. orientalis is a potential demographic ‘trap’ for some herbivores, such as pierid butterflies. However, through the effects of an evolutionary ‘lottery’, M. brassicae has found its way through the plant’s chemical ‘minefield’.     

Plants Know Where It Hurts: Root and Shoot Jasmonic Acid Induction Elicit Differential Responses in Brassica oleracea

Plants respond to herbivore attack by rapidly inducing defenses that are mainly regulated by jasmonic acid (JA). Due to the systemic nature of induced defenses, attack by root herbivores can also result in a shoot response and vice versa, causing interactions between above- and belowground herbivores. However, little is known about the molecular mechanisms underlying these interactions. We investigated whether plants respond differently when roots or shoots are induced. We mimicked herbivore attack by applying JA to the roots or shoots of Brassica oleracea and analyzed molecular and chemical responses in both organs. In shoots, an immediate and massive change in primary and secondary metabolism was observed. In roots, the JA-induced response was less extensive and qualitatively different from that in the shoots. Strikingly, in both roots and shoots we also observed differential responses in primary metabolism, development as well as defense specific traits depending on whether the JA induction had been below- or aboveground. We conclude that the JA response is not only tissue-specific but also dependent on the organ that was induced. Already very early in the JA signaling pathway the differential response was observed. This indicates that both organs have a different JA signaling cascade, and that the signal eliciting systemic responses contains information about the site of induction, thus providing plants with a mechanism to tailor their responses specifically to the organ that is damaged.     

Scale-dependent responses in cabbage herbivores affect attack rates in spatially heterogeneous systems

Herbivorous insects face a dilemma when selecting suitable hosts in a complex environment, and their sensory capability may often reduce the female capacity for proper selection. As a consequence, eggs are often deposited on inferior hosts, affecting both insect and host plant fitness. We examined the attack rates of three cabbage herbivores in monocultures and biculture plots of different Brassica oleracea genotypes, with different spatial heterogeneity. The main goals of the study were to improve our understanding of the spatial scales involved in herbivore search processes and to examine the possibility of using spatial heterogeneity for manipulating pest attack rates in cabbage cropping systems. The results showed that the host selection behaviour of the small white butterfly (Pieris rapae) was strongly dependent on spatial heterogeneity. The difference in egg density between plant genotypes was larger when contrasting plants were growing in close proximity than in monoculture. This suggests that P. rapae is able to differentiate among genotypes from a small distance, while selection is compromised at larger spatial scales. The two other herbivores in the study (Mamestra brassicae and Delia radicum) did not respond to heterogeneity at any spatial scale, but showed a constant preference hierarchy. This suggests that host selection in these species occurs after direct plant contact. The difference in species’ responses to spatial heterogeneity has consequences both for selection gradients in natural communities and for the potential to reduce pest attack in polyculture systems.     

Effects of organic and conventional fertilizer treatments on host selection by the aphid parasitoid Diaeretiella rapae

The type and quantity of fertilizer supplied to a crop will differ between organic and conventional farming practices. Altering the type of fertilizer a plant is provided with can influence a plant’s foliar nitrogen levels, as well as the composition and concentration of defence compounds, such as glucosinolates. Many natural enemies of insect herbivores can respond to headspace volatiles emitted by the herbivores’ host plant in response to herbivory. We propose that manipulating fertilizer type may also influence the headspace volatile profiles of plants, and as a result, the tritrophic interactions that occur between plants, their insect pests and those pests’ natural enemies. Here, we investigate a tritrophic system consisting of cabbage plants, Brassica oleracea, a parasitoid, Diaeretiella rapae, and one of its hosts, the specialist cabbage aphid Brevicoryne brassicae. Brassica oleracea plants were provided with either no additional fertilization or one of three types of fertilizer: Nitram (ammonium nitrate), John Innes base or organic chicken manure. We investigated whether these changes would alter the rate of parasitism of aphids on those plants and whether any differences in parasitism could be explained by differences in attractivity of the plants to D. rapae or attack rate of aphids by D. rapae. In free‐choice experiments, there were significant differences in the percentage of B. brassicae parasitized by D. rapae between B. oleracea plants grown in different fertilizer treatments. In a series of dual‐choice Y‐tube olfactometry experiments, D. rapae females discriminated between B. brassicae‐infested and undamaged plants, but parasitoids did not discriminate between similarly infested plants grown in different fertilizer treatments. Correspondingly, in attack rate experiments, there were no differences in the rate that D. rapae attacked B. brassicae on B. oleracea plants grown in different fertilizer treatments. These findings are of direct relevance to sustainable and conventional farming practices.     

Response of the wasp (<Emphasis Type="Italic">Cotesia glomerata</Emphasis>) to larvae of the large white butterfly (<Emphasis Type="Italic">Pieris brassicae</Emphasis>)

The large white butterfly (Pieris brassicae L) first invaded northernmost Japan from Siberia around 1994, and after a few years, began to expand its range. The wasp, Cotesia glomerata (L) parasitizes larvae of the small white butterfly (Pieris rapae crucivora Boisduval), a usual host in the same geographic area. Some Pieris brassicae larvae in Hokkaido have been parasitized by Cotesia glomerata, but the parasitism rate of Pieris brassicae larvae tends to be lower than that of Pieris rapae. To examine the process of parasitizing Pieris brassicae larvae, we observed how the parasitoid wasp responded to the host larvae on damaged leaves. Cotesia glomerata females tended to avoid Pieris brassicae larvae, and even when female wasps inserted their ovipositors into Pieris brassicae larvae, none laid eggs. The parasitoids obtained from Pieris rapae larvae failed to parasitize Pieris brassicae during the host-acceptance step.     

Differential selection of host plants by twoPieris species: the role of oviposition stimulants and deterrents

Oviposition responses ofPieris rapae L. andP. napi oleracea Harris to nine crucifers, one Capparidaceae and one Tropaeolaceae were directly compared under controlled conditions. Chemical fractions from these plants were also tested on both insects for the presence of oviposition stimulants or deterrents. The results showed that plant chemistry is a key factor in differential selection of potential hosts by thesePieris species. Some plant species were equally acceptable to bothPieris species. However,P. rapae preferred cabbage over most test plants whereasP. napi oleracea strongly preferred plant species that were avoided byP. rapae. The observed preferences were explained in most cases by the presence of stimulants and deterrents in extracts of the plants. The twoPieris species have apparently evolved differential sensitivities to the chemical stimuli that trigger or deter oviposition. The balance of positively and negatively interpreted sensory signals evoked by plant chemicals obviously plays an important role in acceptance or rejection of a plant by both species. The role of specific glucosinolates and differing structure-activity relationships is suggested.     

Signal transduction downstream of salicylic and jasmonic acid in herbivory-induced parasitoid attraction by Arabidopsis is independent of JAR1 and NPR1

Plants can defend themselves indirectly against herbivores by emitting a volatile blend upon herbivory that attracts the natural enemies of these herbivores, either predators or parasitoids. Although signal transduction in plants from herbivory to induced volatile production depends on jasmonic acid (JA) and salicylic acid (SA), the pathways downstream of JA and SA are unknown. Use of Arabidopsis provides a unique possibility to study signal transduction by use of signalling mutants, which so far has not been exploited in studies on indirect plant defence. In the present study it was demonstrated that jar1‐1 and npr1‐1 mutants are not affected in caterpillar (Pieris rapae)‐induced attraction of the parasitoid Cotesia rubecula. Both JAR1 and NPR1 (also known as NIM1) are involved in signalling downstream of JA in induced defence against pathogens such as induced systemic resistance (ISR). NPR1 is also involved in signalling downstream of SA in defence against pathogens such as systemic acquired resistance (SAR). These results demonstrate that signalling downstream of JA and SA differs between induced indirect defence against herbivores and defence against pathogens such as SAR and ISR. Furthermore, it was demonstrated that herbivore‐derived elicitors are involved in induced attraction of the parasitoid Cotesia rubecula     

Comparative headspace analysis of cabbage plants damaged by two species ofPieris caterpillars: consequences for in-flight host location byCotesia parasitoids

Headspace composition, collected from intact cabbage plants and cabbage plants infested with eitherPieris brassicae L. orP. rapae L. (Lepidoptera: Pieridae) first instar larvae, was determined by GC-MS. Twenty-one volatiles were identified in the headspace of intact plants. Twenty-two volatiles were identified in the headspace of plants infested byP. brassicae larvae, 2 of which, Z-3-hexenyl butyrate and Z-3-hexenyl isovalerate, were not detected in the headspace of either intact orP. rapae damaged plants. In the headspace of the latter, 21 compounds were identified, all of which which were also produced by intact plants. No significant quantitative differences were found between headspace composition of the plants damaged by one or the other caterpillar species. Major differences between intact and caterpillar-damaged plants in contribution to the headspace profile were revealed for hexyl acetate, Z-3-hexenyl acetate, myrcene, sabinene and 1,8-cineole. The larval endoparasitoidCotesia glomerata L. was attracted by the volatiles emanating fromB. oleracea damaged byP. brassicae first instar larvae.C. rubecula L., a specialized larval endoparasitoids ofP. rapae, was attracted by the volatiles released from theB. oleracea-P. rapae plant-host complex. This shows that cabbage plants kept under the conditions of headspace collection produce attractive volatiles for both parasitoids.     

Interspecific egg load assessment of host plants by Pieris rapae butterflies

Egglaying responses of Pieris rapae L. butterflies to the oviposition deterring pheromone (ODP) of Pieris brassicae L. were studied in the laboratory. Choice experiments with ODP treated leaves and control leaves revealed that females perform a strong preference to lay their eggs on the control leaves. This preference is maintained even when during the experiment the control leaf becomes covered with a large number of conspecific eggs. Choice experiments with cabbage leaves with and without P. rapae eggs seem to indicate the absence of intraspecific egg load assessment of host plants in P. rapae. The deterrent effect of the ODP of P. brassicae to P. rapae females persists for at least 8 days. Behavioural observations suggest olfactory hairs as well as gustatory hairs to be involved in the perception of the ODP but electrophysiological recordings of the various chemoreceptors are necessary to confirm this. Finally the prospects of application of this pheromone/kairomone in cabbage pest control are discussed.

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Appréciation de la charge interspécifique en oeufs sur la plante hôte par Pieris rapae

Résumé La réponse au laboratoire de P. rapae à la phéromone dissuadant la ponte (ODP) de P. brassicae a été étudiée par l'oviposition. Des expériences de choix entre des feuilles traitées à l'ODP et des témoins ont montré que les femelles préfèrent nettement les feuilles témoins. Cette préférence s'est maintenue même quand les feuilles témoins ont été recouvertes d'un grand nombre d'oeufs de P. rapae. Ceci peut indiquer l'absence chez P. rapae d'une évaluation de la charge de ses propres oeufs. L'effet dissuadant du ODP de P. brassicae sur les femelles de P. rapae persiste au moins 8 jours. Les observations comportementales suggèrent que des poils olfactifs aussi bien que des poils gustatifs sont impliqués dans la perception d'ODP mais une confirmation de cette hypothèse par enregistrements électrophysiologiques est nécessaire. Les perspectives d'utilisation de cette phéromone/kairomone dans la lutte contre les insectes du chou sont examinées.

     

Metabolomic analysis of the interaction between plants and herbivores

Insect herbivores by necessity have to deal with a large arsenal of plant defence metabolites. The levels of defence compounds may be increased by insect damage. These induced plant responses may also affect the metabolism and performance of successive insect herbivores. As the chemical nature of induced responses is largely unknown, global metabolomic analyses are a valuable tool to gain more insight into the metabolites possibly involved in such interactions. This study analyzed the interaction between feral cabbage (Brassica oleracea) and small cabbage white caterpillars (Pieris rapae) and how previous attacks to the plant affect the caterpillar metabolism. Because plants may be induced by shoot and root herbivory, we compared shoot and root induction by treating the plants on either plant part with jasmonic acid. Extracts of the plants and the caterpillars were chemically analysed using Ultra Performance Liquid Chromatography/Time of Flight Mass Spectrometry (UPLCT/MS). The study revealed that the levels of three structurally related coumaroylquinic acids were elevated in plants treated on the shoot. The levels of these compounds in plants and caterpillars were highly correlated: these compounds were defined as the ‘metabolic interface’. The role of these metabolites could only be discovered using simultaneous analysis of the plant and caterpillar metabolomes. We conclude that a metabolomics approach is useful in discovering unexpected bioactive compounds involved in ecological interactions between plants and their herbivores and higher trophic levels.     

Root and shoot jasmonic acid induced plants differently affect the performance of Bemisia tabaci and its parasitoid Encarsia formosa

Plants employ both direct and indirect defenses to protect themselves from attacks by herbivores and pathogens. To date most aboveground and belowground interaction studies have focused on interactions between plants and leaf-chewing herbivores, while the plant defence on the performance of phloem-feeding insects, induced by above- and belowground interaction, has been less explored. Here, jasmonic acid (JA) was used to mimic herbivore-induced responses in Chinese broccoli (Brassica oleracea var. alboglabra) roots (RJA) and shoots (SJA). The effects of JA-induced plant defenses on the performance of the phloem-feeding whitefly, Bemisia tabaci, and its aphelinid parasitoid Encarsia formosa were investigated. The results indicated that SJA induction has a much larger negative effect on B. tabaci than RJA: nymphs develop slower and have a lower survivorship. Also, females live shorter and have a lower fecundity on SJA plants compared to those on RJA and untreated control (CON) plants. The intrinsic rate of increase (r_m) of B. tabaci on SJA plants was 0.089, which was significantly lower than those on CON and RJA plants (0.115 and 0.104, respectively). The parasitoid E. formosa, on the other hand, shows a significantly faster development when parasitizing whitefly hosts feeding on SJA plants, whereas parasitism rate, longevity and fecundity were similar to those on RJA and CON plants. The current study reveals that plants induced with exogenous JA vary in both their resistance to whitefly and suitability for parasitoids, depending on the organ to which the JA was applied. Root and shoot JA applications also have contrasting effects on the phloem-feeding insect B. tabaci and its parasitoid, that is, SJA induction leads to more negative effects on whitefly performance than RJA induction, but its parasitoid performs better on hosts reared on SJA plants. These results show that top-down and bottom-up processes governing herbivore populations on Chinese broccoli are working in concert to increase plant resistance when plants are induced by SJA application.