Changes in morphological traits of the cabbage aphid (<Emphasis Type="Italic">Brevicoryne brassicae</Emphasis>) associated with the use of different host plants

The genetic and morphological differentiation of insect populations in relation to the use of different host plants is an important phenomenon that predates ecological specialisation and speciation in sympatric conditions. In this study, we describe the morphological variation of populations of Brevicoryne brassicae (Homoptera: Aphididae) associated with two host species, Brassica oleracea and Brassica campestris, which occur sympatrically in the highlands of Chiapas, Mexico. The study is aimed at obtaining evidence regarding phenotypic differentiation induced by, or associated with, the use of distinct but closely related host species. Seven morphological characters were measured in 696 wingless aphids collected from plants of the two host species at four localities. Morphological variation was summarised through principal components analysis (PCA). Sixty-two percent of morphological variation was explained by the first two PCs. The first component (PC1) was related to the general size of appendages, and PC2 was interpreted as the relationship between body size (body and leg size) and antenna length. Aphids growing on B. campestris were bigger than those collected from B. oleraceae. Significant differences between hosts were detected for PC1, whereas a significant effect of locality, host, and the interaction locality × host was detected for PC2. These results indicate that the average phenotype of B. brassicae individuals inhabiting different host-plant species differs as a consequence of the contrasting feeding environments the host species provide.     

Host preference of plant genotypes is altered by intraspecific competition in a phytophagous insect

Variation among aphid genotypes leads them to preferentially colonize different host-plant genotypes. In a natural community, different genotypes within a species are expected to coexist on a single host plant, and these aphids can interact, potentially, altering host-plant preferences. Using a model aphid (Sitobion avenae) and barley (Hordeum vulgare) system, we compared aphid preference and performance in one- or two-genotype colonies in pots with genetically diverse host plants (6 genotypes) or genetically uniform host plants (1 genotype per pot). Aphid host preference was shown to differ when a second aphid genotype was present, with one aphid genotype exhibiting a preference change due to the genotypic identity of the second aphid. The population growth rate of the aphids was not influenced by the competitor, and thus, we conclude that these effects are due to aphid distribution (preference) rather than effects through performance. Our work demonstrates that within a complex ecological community, an individual’s behavior can be influenced by interactions with other genotypes within the same species, as well as interactions with genotypes of other species.     

Survival to Parasitoids in an Insect Hosting Defensive Symbionts: A Multivariate Approach to Polymorphic Traits Affecting Host Use by Its Natural Enemy

Insect parasitoids and their insect hosts represent a wide range of parasitic trophic relations that can be used to understand the evolution of biotic diversity on earth. Testing theories of coevolution between hosts and parasites is based on factors directly involved in host susceptibility and parasitoid virulence. We used controlled encounters with potential hosts of the Aphidius ervi wasp to elucidate behavioral and other phenotypic traits of host Acyrthosiphon pisum that most contribute to success or failure of parasitism. The host aphid is at an advanced stage of specialization on different crop plants, and exhibits intra-population polymorphism for traits of parasitoid avoidance and resistance based on clonal variation of color morph and anti-parasitoid bacterial symbionts. Randomly selected aphid clones from alfalfa and clover were matched in 5 minute encounters with wasps of two parasitoid lineages deriving from hosts of each plant biotype in a replicated transplant experimental design. In addition to crop plant affiliation (alfalfa, clover), aphid clones were characterized for color morph (green, pink), Hamiltonella defensa and Regiella insecticola symbionts, and frequently used behaviors in encounters with A. ervi wasps. A total of 12 explanatory variables were examined using redundancy analysis (RDA) to predict host survival or failure to A. ervi parasitism. Aphid color was the best univariate predictor, but was poorly predictive in the RDA model. In contrast, aphid host plant and symbionts were not significant univariate predictors, but significant predictors in the multivariate model. Aphid susceptibility to wasp acceptance as reflected in host attacks and oviposition clearly differed from its suitability to parasitism and progeny development. Parasitoid progeny were three times more likely to survive on clover than alfalfa host aphids, which was compensated by behaviorally adjusting eggs invested per host. Strong variation of the predictive power of intrinsic (body color) and extrinsic traits (symbionts, host plant), indicate that host variables considered as key predictors of outcomes strongly interact and cannot be considered in isolation.     

Intra-specific variation in wild Brassica oleracea for aphid-induced plant responses and consequences for caterpillar–parasitoid interactions

Herbivore-induced plant responses not only influence the initiating attackers, but also other herbivores feeding on the same host plant simultaneously or at a different time. Insects belonging to different feeding guilds are known to induce different responses in the host plant. Changes in a plant’s phenotype not only affect its interactions with herbivores but also with organisms higher in the food chain. Previous work has shown that feeding by a phloem-feeding aphid on a cabbage cultivar facilitates the interaction with a chewing herbivore and its endoparasitoid. Here we study genetic variation in a plant’s response to aphid feeding using plants originating from three wild Brassica oleracea populations that are known to differ in constitutive and inducible secondary chemistry. We compared the performance of two different chewing herbivore species, Plutella xylostella and M. brassicae, and their larval endoparasitoids Diadegma semiclausum and M. mediator, respectively, on plants that had been infested with aphids (Brevicoryne brassicae) for 1 week. Remarkably, early infestation with B. brassicae enhanced the performance of the specialist P. xylostella and its parasitoid D. semiclausum, but did not affect that of the generalist M. brassicae, nor its parasitoid M. mediator. Performance of the two herbivore–parasitoid interactions also varied among the cabbage populations and the effect of aphid infestation marginally differed among the three populations. Thus, the effect of aphid infestation on the performance of subsequent attackers is species specific, which may have concomitant consequences for the assembly of insect communities that are naturally associated with these plants.     

Phenotypic plasticity of Brevicoryne brassicae in responses to nutritional quality of two related host plants

Abstract 1. The host species used by a herbivorous insect may impose different selective pressures promoting host race formation, yet the presence of plasticity can potentially constrain host race formation. 2. The goal of this study is to determine if there is phenotypic plasticity in life history traits of Brevicoryne brassicae in response to host and nutritional quality of two host species, Brassica oleraceae and Brassica campestris, and to what extent there are genetic differences among genotypes in plasticity. 3. Plants of B. oleraceae and B. campestris were fertilised with three different nitrogen doses (with nutritive solutions of 50, 200 and 400 ppm of soluble nitrogen) to produce plants with different nutritional qualities. Eight clones of B. brassicae were reared on those plants, and days to reproduction (DTR), number of nymphs, and fitness (r_m) were recorded. 4. A significant genotype × host interaction was detected in days to reproduction. Genotype × nitrogen interaction (plasticity) was detected in the number of nymphs when aphids were raised on B. campestris. Aphids showed plasticity in DTR and marginal plasticity in r_m in reaction to the varying nitrogen content of B. oleraceae. 5. The phenotypic plasticity to fine‐scale variation of host (nutritional quality) documented here may be an important source of phenotypic variation and may potentially constrain host race formation.     

Host location and parasitism of Brevicoryne brassicae in diversified broccoli cropping systems

Host location is a crucial step in the life cycle of the cabbage aphid, Brevicoryne brassicae (L.) (Homoptera: Aphididae). Diversified cropping systems have the potential to inhibit or interfere with host location processes to reduce the severity of aphid outbreaks. We altered the vegetation mix of a broccoli [Brassica oleracea var. italica (Plenck) (Brassicaceae)] cropping system by substituting broccoli with strips of potatoes [Solanum tuberosum (L.) (Solanaceae)], planting broccoli into a cereal rye [Secale cereale (L.) (Poaceae)] cover crop, or both. The probability of aphid infestation was significantly reduced by the presence of the cover crop, whereas strips of potatoes slightly increased initial numbers. The effectiveness of the cover crop treatments was primarily due to fewer alate aphids initially colonizing broccoli plants. Aphid parasitism by Diaeretiella rapae (McIntosh) (Hymenoptera: Braconidae) did not explain the observed differences in the number of aphid colonies present in each treatment.     

The effect of host plants on genotype variability in fitness and honeydew composition of Aphis fabae

Aphid species can be polyphagous, feeding on multiple host plants across genera. As host plant species can have large variation in their phloem composition, this can affect aphid fitness and honeydew composition. Previous research showed significant intraspecific genotype variation in the composition of the honeydew carbohydrates of the black bean aphid Aphis fabae, with the ant attractant trisaccharide melezitose showing especially large variation across different genotypes. In this study, we test if variation in melezitose and carbohydrate composition of aphid honeydew could be linked to the adaptation of specific aphid genotypes to particular host plants. To this end, 4 high and 5 low melezitose secreting genotypes of the black bean aphid Aphis fabae were reared on 4 common host plants: broad bean, goosefoot, beet, and poppy. The carbohydrate composition, and in particular melezitose secretion, showed important aphid genotype and host plant interactions, with some genotypes being high melezitose secreting on 1 host plant but not on another. However, the interaction effects were not paralleled in the fitness measurements, even though there were significant differences in the average fitness across the different host plants. On the whole, this study demonstrates that aphid honeydew composition is influenced by complex herbivore–plant interactions. We discuss the relevance of these findings in the context of ant–aphid mutualisms and adaptive specialization in aphids.     

Community structure and abundance of insects in response to early‐season aphid infestation in wild cabbage populations

1. Changes in the arthropod community structure can be attributed to differences in constitutively expressed plant traits or those that change depending on environmental conditions such as herbivory. Early‐season herbivory may have community‐wide effects on successive insect colonisation of host plants and the identity of the initially inducing insect may determine the direction and strength of the effects on the dynamics and composition of the associated insect community. 2. Previous studies have addressed the effect of early infestation with a chewing herbivore. In the present study, the effect of early infestation was investigated with a phloem‐feeding aphid [Brevicoryne brassicae L. (Hemiptera, Aphididae)] on the insect community associated with three wild cabbage (Brassica oleracea L.) populations, which are known to differ in defence chemistry, throughout the season in field experiments. 3. Aphid infestation had asymmetric effects on the associated insect community and only influenced the abundance of the natural enemies of aphids, but not that of chewing herbivores and their natural enemies. The effect size of aphid infestation further depended on the cabbage population. 4. Aphid feeding has been previously reported to promote host‐plant quality for chewing herbivores, which has been attributed to antagonism between the two major defence signalling pathways controlled by the hormones salicylic acid (SA) and jasmonic acid (JA), respectively. Our results show no effects of early infestation by aphids on chewing herbivores, suggesting the absence of long‐term JA–SA antagonism. 5. Investigating the effects of the identity of an early‐season coloniser and genotypic variation among plant populations on insect community dynamics are important in understanding insect–plant community ecology.     

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.     

Phenotypic plasticity in host choice behavior in black bean aphid, Aphis fabae (Homoptera: Aphididae)

The study of phenotypic plasticity in host choice behavior is crucial to predict evolutionary patterns of insect–plant interactions. The presence of sufficient variation in plasticity may facilitate host race formation and sympatric speciation. In this study, 13 Aphis fabae Scopoli genotypes reared both on broad bean and nasturtium exhibited statistically significant genotypic variability in host selection behavior. Some genotypes displayed increase in preference and acceptance in a novel host plant through generations. There are also strong conditioning effects of nasturtium as nasturtium reared genotypes are more willing to choose nasturtium over broad bean while broad bean reared genotypes do not show differences in choosing between the two host plants. There are also positive relationships between fitness and host choice behavior particularly for nasturtium. Results of the study supported the hypothesis that phenotypic plasticity in host choice behavior may be one of the major determinants of the evolutionary trajectory of a parasitic species, such as aphids.     

The influence of genetic background on resistance to the cabbage aphid (Brevicoryne brassicae) in Kale (Brassica oleracea var. acephala)

Resistance to Brevicoryne brassicae has been identified in the progeny of two selected kale (B. oleracea var. acephala) plants, one from the F1 hybrid cultivar ‘Arsis RS’ and one from the landrace ‘Butzo’. These plants were crossed with susceptible B. oleracea morphotypes that have different periods to flowering. The type of susceptible plant line used had an effect on the resistance phenotypc of the progeny. Tested F2 populations derived from these crosses show that resistance is not under simple genetic control. This, in addition to variation in aphid numbers within accessions, suggests that separation of genetic components of control from environmental ‘noise’ for any accession may only be possible by the production of double haploid plant lines.     

Temporal consistency in herbivore responses to glucosinolate polymorphism in populations of wild cabbage (Brassica oleracea)

Natural populations of wild cabbage (Brassica oleracea) show significant qualitative diversity in heritable aliphatic glucosinolates, a class of secondary metabolites involved in defence against herbivore attack. One candidate mechanism for the maintenance of this diversity is that differential responses among herbivore species result in a net fitness balance across plant chemotypes. Such top-down differential selection would be promoted by consistent responses of herbivores to glucosinolates, temporal variation in herbivore abundance, and fitness impacts of herbivore attack on plants varying in glucosinolate profile. A 1-year survey across 12 wild cabbage populations demonstrated differential responses of herbivores to glucosinolates. We extended this survey to investigate the temporal consistency of these responses, and the extent of variation in abundance of key herbivores. Within plant populations, the aphid Brevicoryne brassicae consistently preferred plants producing the glucosinolate progoitrin. Among populations, increasing frequencies of sinigrin production correlated positively with herbivory by whitefly Aleyrodes proletella and negatively with herbivory by snails. Two Pieris butterfly species showed no consistent response to glucosinolates among years. Rates of herbivory varied significantly among years within populations, but the frequency of herbivory at the population scale varied only for B. brassicae. B. brassicae emerges as a strong candidate herbivore to impose differential selection on glucosinolates, as it satisfies the key assumptions of consistent preferences and heterogeneity in abundance. We show that variation in plant secondary metabolites structures the local herbivore community and that, for some key species, this structuring is consistent over time. We discuss the implications of these patterns for the maintenance of diversity in plant defence chemistry.     

Host range limitation caused by incomplete host regulation in an aphid parasitoid

Defining host ranges in parasitoid insects is important both from a theoretical and an applied point of view. Based on the literature, some species seem able to use a wide range of hosts, while field studies indicate possible local host specialization. In koinobiont endoparasitoid species, such specialization could involve physiological processes. We tested the ability of two strains of the cosmopolitan and polyphagous parasitoid Diaeretiella rapae, to develop in three of its recorded aphid host species. Both strains produced high parasitism rates on the cabbage aphid Brevicoryne brassicae and the green peach aphid Myzus persicae but almost no progeny on the cherry-oat aphid Rhopalosiphum padi. This last species was less attacked by female parasitoids. Moreover, parasitoid eggs and larvae were smaller than in the two other host aphid species and their development was delayed. This abnormal development appeared to be due to an incomplete host regulation process, probably related to the low number and the size of teratocytes produced by D. rapae in R. padi individuals. Such a failure as far as gaining control of the host's metabolism is concerned could play an important role in shaping the host range of parasitoid insects, leading to local variation of the host spectrum in populations from various geographical areas.     

Neonates know better than their mothers when selecting a host plant

Evolutionary ecological theory predicts that among insect herbivores ‘mothers know best’ when selecting a plant to deposit their eggs. Host‐plant selection is usually studied for the adult stage exclusively, although mothers have not always been reported to know best. Here, we investigate the host‐plant selection behaviour of caterpillars, which are considered to be completely dependent on their mothers’ choices. We use a system that offers a biologically relevant framework to compare the degree of participation of adults and juveniles in host‐plant selection. Our results show that neonate Pieris brassicae caterpillars can actively discriminate between conspecific Brassica oleracea plants with or without aphid (Brevicoryne brassicae) infestation. The caterpillars prefer aphid‐infested plants on which their performance is significantly better, while their mothers, the female butterflies, did not discriminate. We compared caterpillar preferences of individuals released individually or in groups, because P. brassicae is a gregarious species. We found that the strength of the preference for aphid‐infested plants was not affected by the degree of grouping. Caterpillar choices were made before contact with the plants, indicating that plant odours were used for orientation. However, the composition of the volatile blends emitted by plants with and without aphids did not show strong differences. Similarly, like with aphid‐infested plants, plants treated with salicylic acid (SA) were also preferred by neonates over untreated control, indicating that the infestation by aphids may have rendered the plants more attractive to the neonates via changes related to interference with JA‐signaling. The main parasitoid of the caterpillars did not discriminate between plants with hosts in the presence or absence of aphids, showing that top–down forces do not influence the relative suitability of the different food sources for the caterpillars. These data are discussed in the context of mothers and offspring having both important, but different roles in the process of host‐plant selection. Butterflies may select the plant species patch, while their offspring adjust and/or update the choices of their mothers at the local scale, within the micro‐habitat selected by the adult.     

Bottom‐up effects of glucosinolate variation on aphid colony dynamics in wild cabbage populations

1. There is an ongoing debate about the relative importance of top‐down and bottom‐up regulation of herbivore dynamics in the wild. Secondary metabolites, produced by plants, have negative effects on survival and growth of some herbivore species, causing bottom‐up regulation of population dynamics. Herbivore natural enemies may use plant secondary metabolites as cues to find their prey, but their survival and reproduction can also be influenced by the upward cascade of secondary metabolites through the food web. Thus plant chemistry might also affect herbivore populations by mediating top‐down regulation. 2. We investigated the influence of heritable variation in aliphatic glucosinolates, a class of secondary metabolites produced by Brassica plants, on the relative importance of top‐down and bottom‐up regulation of Brevicoryne brassicae (mealy cabbage aphid) colonies in natural Brassica oleracea (wild cabbage) populations. We manipulated natural enemy pressure on plants differing in their glucosinolate profiles, and monitored aphid colony growth and disperser production. 3. Aphid colony sizes were significantly smaller on plants producing sinigrin, compared with plants producing alternative aliphatic glucosinolates. Aphid natural enemy numbers correlated with aphid colony size, but there was no additional effect of the plants' chemical phenotype on natural enemy abundance. Furthermore, experimental reduction of natural enemy pressure had no effect on aphid colony size or production of winged dispersers. 4. Our results provide evidence for glucosinolate‐mediated, bottom‐up regulation of mealy cabbage aphid colonies in natural populations, but we found no indication of top‐down regulation. We emphasise that more studies of these processes should focus on tritrophic interactions in the wild.     

Prey‐mediated effects of glucosinolates on aphid predators

1. Plant resistance against herbivores can act directly (e.g. by producing toxins) and indirectly (e.g. by attracting natural enemies of herbivores). If plant secondary metabolites that cause direct resistance against herbivores, such as glucosinolates, negatively influence natural enemies, this may result in a conflict between direct and indirect plant resistance. 2. Our objectives were (i) to test herbivore‐mediated effects of glucosinolates on the performance of two generalist predators, the marmalade hoverfly (Episyrphus balteatus) and the common green lacewing (Chrysoperla carnea) and (ii) to test whether intraspecific plant variation affects predator performance. 3. Predators were fed either Brevicoryne brassicae, a glucosinolate‐sequestering specialist aphid that contains aphid‐specific myrosinases, or Myzus persicae, a non‐sequestering generalist aphid that excretes glucosinolates in the honeydew, reared on four different white cabbage cultivars. Predator performance and glucosinolate concentrations and profiles in B. brassicae and host‐plant phloem were measured, a novel approach as previous studies often measured glucosinolate concentrations only in total leaf material. 4. Interestingly, the specialist aphid B. brassicae selectively sequestered glucosinolates from its host plant. The performance of predators fed this aphid species was lower than when fed M. persicae. When fed B. brassicae reared on different cultivars, differences in predator performance matched differences in glucosinolate profiles among the aphids. 5. We show that not only the prey species, but also the plant cultivar can have an effect on the performance of predators. Our results suggest that in the tritrophic system tested, there might be a conflict between direct and indirect plant resistance.     

Intergeneric and interspecific hybrids through in vitro ovule culture in the Cruciferae

A procedure to obtain interspecific and intergeneric hybrids has been developed through culture in vitro of ovules of cruciferous crops using Brassica oleracea as the female plant. A modified medium of Murashige and Skoog supplemented with 10% (v/v) coconut milk, 300 mg/l casein hydrolysate, 0.1 mg/l 1-napthaleneacetic acid (NAA) and 0.1 mg/l kinein were effective for culturing hybrid ovules. All the interspecific and intergeneric hybrids between B. oleracea and Brassica campestris; and between B. oleracea and Raphanus sativus had an intermediate plant morphology in combination with their parental traits. Hybrids of cross combinations between B. oleracea and B. campestris, and between B. olecacea and R. sativus were amphihaploids. This was revealed through chromosome and isozyme analyses.     

Associational resistance or susceptibility: the indirect interaction between chemically‐defended and non‐defended herbivore prey via a shared predator

Many organisms possess chemical defences against their natural enemies, which render them unpalatable or toxic when attacked or consumed. These chemically‐defended organisms commonly occur in communities with non‐ or less‐defended prey, leading to indirect interactions between prey species, mediated by natural enemies. Although the importance of enemy‐mediated indirect interactions have been well documented (e.g. apparent competition), how the presence of prey chemical defences may affect predation of non‐defended prey in terrestrial communities remains unclear. Here, an experimental approach was used to study the predator‐mediated indirect interaction between a chemically‐defended and non‐defended pest aphid species. Using laboratory‐based mesocosms, aphid community composition was manipulated to include chemically‐defended (CD) aphids Brevicoryne brassicae, non‐defended (ND) aphids Myzus persicae or a mixed assemblage of both species, on Brassica oleracea cabbage plants, in the presence or absence of a shared predator (Chrysoperla carnea larvae). Aphid population growth rates, aphid distributions on host plants and predator growth rates were measured. In single‐species treatments, C. carnea reduced M. persicae population growth rate, but had no significant impact on B. brassicae population growth rate, suggesting B. brassicae chemical defences are effective against C. carnea. Chrysoperla carnea had no significant impact on either aphid species population growth rate in mixed‐species treatments. Myzus persicae (ND) therefore experienced reduced predation in the presence of B. brassicae (CD) through a predator‐mediated indirect effect. Moreover, predator growth rates were significantly higher in the M. persicae‐only treatments than in either the B. brassicae‐only or mixed‐species treatments, suggesting predation was impaired in the presence of B. brassicae (CD). A trait‐mediated indirect interaction is proposed, consistent with associational resistance, in which the predator, upon incidental consumption of chemically‐defended aphids is deterred from feeding, releasing non‐defended aphids from predatory control.     

UV‐B impact on aphid performance mediated by plant quality and plant changes induced by aphids

Plants face various abiotic and biotic environmental factors and therefore need to adjust their phenotypic traits on several levels. UV‐B radiation is believed to impact herbivorous insects via host plant changes. Plant responses to abiotic challenges (UV‐B radiation) and their interaction with two aphid species were explored in a multifactor approach. Broccoli plants [Brassica oleracea L. convar. botrytis (L.), Brassicaceae] were grown in two differently covered greenhouses, transmitting either 80% (high UV‐B) or 4% (low UV‐B) of ambient UV‐B. Three‐week‐old plants were infested with either specialist cabbage aphids [Brevicoryne brassicae (L.), Sternorrhyncha, Aphididae] or generalist green peach aphids [Myzus persicae (Sulzer), Sternorrhyncha, Aphididae]. Plants grown under high‐UV‐B intensities were smaller and had higher flavonoid concentrations. Furthermore, these plants had reduced cuticular wax coverage, whereas amino acid concentrations of the phloem sap were little influenced by different UV‐B intensities. Cabbage aphids reproduced less on plants grown under high UV‐B than on plants grown under low UV‐B, whereas reproduction of green peach aphids in both plant light sources was equally poor. These results are likely related to the different specialisation‐dependent sensitivities of the two species. The aphids also affected plant chemistry. High numbers of cabbage aphid progeny on low‐UV‐B plants led to decreased indolyl glucosinolate concentrations. The induced change in these glucosinolates may depend on an infestation threshold. UV‐B radiation considerably impacts plant traits and subsequently affects specialist phloem‐feeding aphids, whereas aphid growth forces broccoli to generate specific defence responses.