Interactions of elevated carbon dioxide and temperature with aphid feeding on transgenic oilseed rape: Are Bacillus thuringiensis (Bt) plants more susceptible to nontarget herbivores in future climate?

Climate change factors such as elevated carbon dioxide (CO₂) and temperature typically affect carbon (C) and nitrogen (N) dynamics of crop plants and the performance of insect herbivores. Insect‐resistant transgenic plants invest some nutrients to the production of specific toxic proteins [i.e. endotoxins from Bacillus thuringiensis (Bt)], which could alter the C–N balance of these plants, especially under changed abiotic conditions. Aphids are nonsusceptible to Lepidoptera‐targeted Bt Cry1Ac toxin and they typically show response to abiotic conditions, and here we sought to discover whether they might perform differently on compositionally changed Bt oilseed rape. Bt oilseed rape had increased N content in the leaves coupled with reduced total C compared with its nontransgenic counterpart, but in general the C : N responses of both plant types to elevated CO₂ and temperature were similar. Elevated CO₂ decreased N content and increased C : N ratio of both plant types. Elevated temperature increased C and N contents, total chlorophyll and carotenoid concentrations under ambient CO₂, but decreased these under elevated CO₂. In addition, soluble sugars were increased and starch decreased by elevated temperature under ambient but not under elevated CO₂, whereas photosynthesis was decreased in plants grown under elevated temperature in both CO₂ levels. Myzus persicae, a generalist aphid species, responded directly to elevated temperature with reduced developmental time and decreased adult and progeny weights, whereas the development of the Brassica specialist Brevicoryne brassicae was less affected. Feeding by M. persicae resulted in an increase in the N content of oilseed rape leaves under ambient CO₂, indicating the potential of herbivore feeding itself to cause allocation changes. The aphids performed equally well on both plant types despite the differences between C–N ratios of Bt and non‐Bt oilseed rape, revealing the absence of plant composition‐related effects on these pests under elevated CO₂, elevated temperature or combined elevated CO₂ and temperature conditions.     

Low parasitism by Diaeretiella rapae (Hym.: Braconidae) of Lipaphis pseudobrassicae (Hemip.: Aphididae): pre- or post-ovipositional host resistance?

While the aphid Lipaphis pseudobrassicae (Davis) (Hemip.: Aphididae: Macrosiphini) is considered one of the preferred hosts of Diaeretiella rapae (McIntosh) (Hym.: Braconidae: Aphidiinae) in several parts of the world, field surveys in Uberlandia (Brazil) found parasitism of this aphid to not exceed 10%. This study sought to determine the cause of this low parasitism, as well as the effects of parasitism on the intrinsic growth rate of the aphid population. We evaluated parasitism, percentage emergence, developmental time, longevity, number of attacks and number of parasitoid larvae in L. pseudobrassicae and compared these to the same characteristics in Brevicoryne brassicae (L.) and Myzus persicae (Sulzer). The lowest percentage of parasitism was found in L. pseudobrassicae, followed by M. persicae and B. brassicae. The ratio between the number of parasitoid larvae and the number of ovipositions in L. pseudobrassicae ranged from 0.02 to 0.03, while, in B. brassicae, it was between 0.41 and 0.44 and, in M. persicae, between 0.62 and 0.80, indicating high mortality rates of early stages of D. rapae in L. pseudobrassicae. Parasitism by D. rapae reduced the r_m of L. pseudobrassicae. The r_m for parasitised aphids was only 63% of that for unparasitised aphids. However, no hosts died before reaching adulthood, and 83% of parasitised aphids were still able to reproduce. As a result, the r_m of the aphid was positive, resulting in population growth of L. pseudobrassicae, even among individuals parasitised during the second instar. Our results indicate the existence of L. pseudobrassicae genotypes that are completely resistant to D. rapae.     

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.     

The effect of rearing history and aphid density on volatile‐mediated foraging behaviour of Diaeretiella rapae

1. Parasitoid females foraging for hosts rely on cues derived from the insect host, the host plant and/or their interaction, and all of these can be learned during the immature and adult stages. 2. The present study investigated the importance of rearing history on foraging behaviour of Diaeretiella rapae, an endoparasitoid often associated with aphids feeding on brassicaceous plant species. 3. Parasitoids were reared on one of the four possible combinations, comprising two brassicaceous host plant species, Brassica nigra or Raphanus sativus, and two aphid species Brevicoryne brassicae or Myzus persicae. These parasitoids were tested in a Y‐tube olfactometer and given the choice between volatiles emitted by an aphid‐infested plant (25 or 100 aphids per plant) and an uninfested control plant. The parasitoid's responses were compared when offered: (i) the same plant–aphid combination as the one on which it had been reared; (ii) the same host plant infested with the alternative aphid species; or (iii) an alternative plant with the alternative aphid species. 4. Aphid density did affect the behavioural responses to the various odour sources, but rearing history did not. Diaeretiella rapae only preferred aphid‐induced to non‐induced plant volatiles at low aphid infestation level, whereas they did not discriminate between volatiles at high aphid infestation level. 5. It is concluded that aphid‐induced volatiles of brassicaceous plants play an important role during host habitat location, but seem less important for parasitoids to locate the aphid host itself. The data are discussed in the light of manipulation of host plant defences by aphids.     

Maternal host plant effects on aphid performance: contrasts between a generalist and a specialist species on Brussels sprout cultivars

• 1 The performance of the second generation (G₂) of alates and apterae of a generalist, Myzus persicae, and a specialist, Brevicoryne brassicae, aphid species reared on Chinese cabbage or cabbage was evaluated on five cultivars of Brussels sprout.
• 2 Aphid performance was influenced both by the type of host on which the parent aphid had been reared and by the host on which it was feeding when reproducing.
• 3 The fecundity of the G₂ of alates of both aphid species reared on Chinese cabbage differed significantly between all the cultivars of Brussels sprout and, on average, was 25% higher than those reared on cabbage. These differences were also apparent for the intrinsic rate of increase of B. brassicae but not for M. persicae.
• 4 There was a trend for the G₂ of alates from Chinese cabbage to have greater fecundity compared with aphids from cabbage. These differences were significant for the fecundity of the G₂ of alates of both aphid species on Brussels sprout cultivars Fillbasket (30% higher), Red Delicious (35% higher) and Winter Harvest (25% higher) than those reared on cabbage.
• 5 The intrinsic rate of increase for the G₂ of alates of B. brassicae from Chinese cabbage was significantly different on all Brussels sprout cultivars tested. The intrinsic rate of increase differed significantly between aphids reared on either Chinese cabbage or cabbage on cultivars Oliver and Darkmar‐21 (M. persicae) and Red Delicious and Winter Harvest (B. brassicae). The cv. Oliver appeared to be the most consistently good host; Red Delicious was the poorest host overall.

     

Effects of elevated CO2 on five plant-aphid interactions

We investigated interactions between five species of phloem-feeding aphids (Homoptera: Aphididae) and their host plants at elevated CO₂; Acyrthosiphon pisum (Harris) on Vicia faba L., Aphis nerii Boyer de Fonscolombe on Asclepias syriaca L., Aphis oenotherae Oestlund on Oenothera biennis L., Aulacorthum solani (Kaltenbach) on Nicotiana sylvestris Speg. & Comes and Myzus persicae (Sulzer) on Solanum dulcamara L. Host plants grown at elevated CO₂ generally had greater biomass, leaf area and C:N ratios than those grown at ambient CO₂, while plants with aphids had lower biomass and leaf area than those without aphids.The responses of aphid populations to elevated CO₂ were species-specific with one species increasing (M. persicae), one decreasing (A. pisum), and the other three being unaffected. CO₂ treatment did not affect the proportion of alate individuals produced. In general, aphid abundance was not significantly related to foliar nitrogen concentration.We performed separate analyses to test whether either aphid presence or aphid abundance modified the response of host plants to elevated CO₂. In terms of aphid presence, only three of the potential 15 interactions (five aphid species x three plant traits) were significant; A. solani slightly modified the response of the plant biomass to elevated CO₂ and M. persicae affected the response of leaf area and allocation. In terms of aphid abundance, only two of the potential 15 interactions were significant with A. nerii modifying the plant response to CO₂ in terms of total leaf area and allocation.We conclude that, in contrast to other insect groups such as leaf chewers, populations of most phloem-feeders may not be negatively affected by increased CO₂ concentrations in the future. The reasons for this difference include the possibility that aphids may be able to compensate for changes in host plant quality by altering feeding behaviour or by synthesizing amino acids. In addition, there is little evidence that aphid herbivory, even at high levels, will substantially modify the response of plants to elevated CO₂.     

Hoverfly oviposition response to aphids in broccoli fields

The oviposition response of predacious hoverflies (Diptera: Syrphidae) to Brevicoryne brassicae L. and Myzus persicae (Sulzer) (Homoptera: Aphididae) in commercial broccoli, Brassica oleracea var. italica L., Plenck (Brassicaceae), fields was investigated at two sites over the course of a growing season. The hoverfly oviposition responses to these aphid species on different parts of the broccoli plant canopy were also examined. There were no hoverfly eggs on broccoli plants without aphids, egg numbers were very low on plants with fewer than 50 aphids, and no peak in oviposition relative to aphid numbers was observed. Within individual plants that were colonized by aphids, there was some oviposition on individual leaves without aphids, and no hoverfly eggs were seen on leaves that had more than 400 aphids. Leaves in the broccoli plant canopy, and the datasets associated with them, were divided into three sections vertically, ‘upper’, ‘middle’, and ‘lower’. Brevicoryne brassicae was more abundant in the upper and middle canopy sections, while M. persicae was found mostly in the lower section. The rate of hoverfly oviposition per aphid was higher in the upper section than in the two other sections. Modeling of the oviposition response using logistic regression showed that the presence of hoverfly eggs was positively correlated with numbers of each aphid species and sampling date.     

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.     

Host-specific aphid population responses to elevated CO2 and increased N availability

Sap-feeding insects such as aphids are the only insect herbivores that show positive responses to elevated CO₂. Recent models predict that increased nitrogen will increase aphid population size under elevated CO₂, but few experiments have tested this idea empirically. To determine whether soil nitrogen (N) availability modifies aphid responses to elevated CO₂, we tested the performance of Macrosiphum euphorbiae feeding on two host plants; a C₃ plant (Solanum dulcamara), and a C₄ plant (Amaranthus viridis). We expected aphid population size to increase on plants in elevated CO₂, with the degree of increase depending on the N availability. We found a significant CO₂× N interaction for the response of population size for M. euphorbiae feeding on S. dulcamara: aphids feeding on plants grown in ambient CO₂, low N conditions increased in response to either high N availability or elevated CO₂. No population size responses were observed for aphids infesting A. viridis. Elevated CO₂ increased plant biomass, specific leaf weight, and C : N ratios of the C₃ plant, S. dulcamara but did not affect the C₄ plant, A. viridis. Increased N fertilization significantly increased plant biomass, leaf area, and the weight : height ratio in both experiments. Elevated CO₂ decreased leaf N in S. dulcamara and had no effect on A. viridis, while higher N availability increased leaf N in A. viridis and had no effect in S. dulcamara. Aphid infestation only affected the weight : height ratio of S. dulcamara. We only observed an increase in aphid population size in response to elevated CO₂ or increased N availability for aphids feeding on S. dulcamara grown under low N conditions. There appears to be a maximum population growth rate that M. euphorbiae aphids can attain, and we suggest that this response is because of intrinsic limits on development time and fecundity.     

Cascading effects of N input on tritrophic (plant–aphid–parasitoid) interactions

Because N is frequently the most limiting mineral macronutrient for plants in terrestrial ecosystems, modulating N input may have ecological consequences through trophic levels. Thus, in agro‐ecosystems, the success of natural enemies may depend not only from their herbivorous hosts but also from the host plant whose qualities may be modulated by N input. We manipulated foliar N concentrations by providing to Camelina sativa plants three different nitrogen rates (control, optimal, and excessive). We examined how the altered host‐plant nutritional quality influenced the performances of two aphid species, the generalist green peach aphid, Myzus persicae, and the specialist cabbage aphid, Brevicoryne brassicae, and their common parasitoid Diaeretiella rapae. Both N inputs led to increased N concentrations in the plants but induced contrasted concentrations within aphid bodies depending on the species. Compared to the control, plant biomass increased when receiving the optimal N treatment but decreased under the excessive treatment. Performances of M. persicae improved under the optimal treatment compared to the control and excessive treatments whereas B. brassicae parameters declined following the excessive N treatment. In no‐choice trials, emergence rates of D. rapae developing in M. persicae were higher on both optimum and excessive N treatments, whereas they remained stable whatever the treatment when developing in B. brassicae. Size of emerging D. rapae females was positively affected by the treatment only when it developed in M. persicae on the excessive N treatment. This work showed that contrary to an optimal N treatment, when N was delivered in excess, plant suitability was reduced and consequently affected negatively aphid parameters. Surprisingly, these negative effects resulted in no or positive consequences on parasitoid parameters, suggesting a buffered effect at the third trophic level. Host N content, host suitability, and dietary specialization appear to be major factors explaining the functioning of our studied system.     

Elevated CO2 shifts the focus of tobacco plant defences from cucumber mosaic virus to the green peach aphid

Elevation in CO₂ concentration broadly impacts plant physiological characteristics, which influences herbivores and biotrophic pathogens, which in turn regulate the plant defensive response. In this study, responses of tobacco plants to stress in the form of the green peach aphid, Myzus persicae (Sulzer), or cucumber mosaic virus (CMV), or both aphid and CMV combined were investigated in open‐top chambers under ambient and elevated CO₂ concentrations. We measured aboveground biomass and foliar chlorophyll, nitrogen, non‐structural carbohydrates, soluble protein, total amino acid and nicotine content in tobacco plants and also measured aphid population dynamics, body weight, honeydew production and anti‐oxidative enzyme activities in individual aphids. Plants produced more secondary metabolites for defence in both CO₂ treatments when treated with aphid and CMV combined than with either alone. Aphid density significantly increased on CMV‐infected tobacco plants (relative to uninfected plants) under ambient CO₂ but not under elevated CO₂. This suggests that plant defences against virus and aphid would be more efficient under elevated CO₂. Plant defence appears to shift from plant virus to aphid under increasing CO₂ levels, which highlights the potential influences of multiple biotic stressors on plants under elevated CO₂.     

Influence of water stress on the glucosinolate profile of Brassica oleracea var. italica and the performance of Brevicoryne brassicae and Myzus persicae

Drought stress alters the chemical composition of plants, which can influence their tolerance to insect herbivory. To evaluate plant chemical responses to drought stress, broccoli, Brassica oleracea L. var. italica Plenck (Brassicaceae), was grown under well‐watered, drought, and water‐logged conditions. The glucosinolate (GS) levels and the performance of two aphid species, the specialist Brevicoryne brassicae (L.) and the generalist Myzus persicae (Sulzer) (both Hemiptera: Aphididae), in relation to water stress conditions were studied. High Performance Liquid Chromatography analysis showed that water stress changed the levels of GS in broccoli plants. Plants grown for 2 weeks under drought stress were significantly smaller and showed decreased levels of total GS when compared with GS contents of well‐watered plants, whereas water‐logged conditions led to a slight increase in the GS contents. A substantial decrease in indolyl GS was detected in water‐deficient plants, whereas aliphatic GS decreased slightly. Analysis of sugar levels in phloem sap of broccoli plants revealed that plants under water‐logged conditions contained the highest amounts of sugars followed by drought‐stressed and well‐watered plants. The two aphid species responded differently to water stress‐induced changes in their host plants. Significantly larger populations of M. persicae were recorded on plants with a limited water supply than on plants grown under well‐watered or water‐logged conditions. Brevicoryne brassicae was less affected by water stress, and similar population sizes were found on plants that were subject to different treatments. Analysis of covariance showed a significant effect of the plants’ water condition but no significant effect of GS content on the performance of M. persicae. However, the specialist B. brassicae remained unaffected by changes induced under water stress conditions.     

A comparison of the reproduction of Brevicoryne brassicae and Myzus persicae in relation to soluble nitrogen concentration and leaf age (leaf position) in the brussels sprout plant

Wide variations in total soluble nitrogen occurred in different leaves of Brussels sprout plants. These variations were partly the result of leaf age (leaf position) and partly the result of different fertiliser application to the plants. The fecundity of Brevicoryne brassicae and Myzus persicae caged on such leaves also varied greatly and was correlated with total soluble nitrogen within leaves of the same age, but not between leaves of different ages. The value of total soluble nitrogen concentrations, based on analysis of leaf tissue, for indicating plant suitability for aphids is discussed as well as the difference in response of the two aphid species to experimental conditions favouring proteolysis in the leaf.

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Ein vergleich der vermehrung von Brevicoryne brassicae und Myzus persicae in beziehung zum gehalt löslichen stickstoffs und dem blattalter von rosenkohlpflanzen

Zusammenfassung Brevicoryne brassicae (L.) und Myzus persicae (Sulz.) wurden an Kohlblättern gekäfigt, die infolge verschiedener physiologischer Ursachen sehr unterschiedliche Mengen löslichen Stickstoffs enthielten. Den Pflanzen wurden zwei Düngerstoffe ((NH₄)₂SO₄ und KCL) verabreicht und die Blattläuse auf jungen, mittelalten und alten Blättern gekäfigt. Dadurch wurde der Gehalt löslichen Stickstoffs in den Blättern zwischen 0,543% und 1,286% variiert. Die Nachkommenzahl der Blattläuse bei diesen Düngungen und auf verschiedenaltrigen Blättern war sehr unterschiedlich (zwischen 7,3 und 30,2 bei B. brassicae; 17,6 und 59,4 bei M. persicae) und konnte in jedem Blattalter mit dem Stickstoffgehalt korreliert werden. Der Befund, daß die Korrelationen zwischen Blattlausnachkommenzahl und Stickstoffgehalt nicht unabhängig vom Blattalter waren, deutet darauf hin, daß der Stickstoffgehalt in physiologisch aktiven Blattgeweben nicht relativ mit dem der Siebröhren übereinstimmt. M. persicae vermochte in höherem Maße als B. brassicae Blätter auszunützen, in denen der Eiweißabbau durch Düngung oder Alter begünstigt war. Dieser Unterschied zwischen den beiden Arten steht möglicherweise mit ihren verschiedenen Nährstoffbedürfnissen in Zusammenhang.

     

Behavioural responses of the aphid parasitoid Diaeretiella rapae to volatiles from Arabidopsis thaliana induced by Myzus persicae

In response to herbivory by insects, several plant species have been shown to produce volatiles that attract the natural enemies of those herbivores. Using a Y‐tube olfactometer, we investigated responses of the aphid parasitoid Diaeretiella rapae MacIntosh (Hymenoptera: Aphidiidae) to volatiles from Arabidopsis thaliana Columbia (Brassicaceae) plants that were either undamaged, infested by the peach‐potato aphid, Myzus persicae Sulzer (Homoptera: Aphididae), or mechanically damaged, as well as to volatiles from just the aphid or its honeydew. In dual‐choice experiments, female D. rapae given oviposition experience on A. thaliana infested with M. persicae were significantly attracted to volatiles from A. thaliana infested with M. persicae over volatiles from undamaged A. thaliana and similarly were significantly attracted to plants that had been previously infested by M. persicae, but from which the aphids were removed, over undamaged plants. Diaeretiella rapae did not respond to volatiles from M. persicae alone, their honeydew, or plants mechanically damaged with either a pin or scissors. We conclude that an interaction between the plant and the aphid induces A. thaliana to produce volatiles, which D. rapae can learn and respond to. Poor responses of D. rapae to volatiles from an A. thaliana plant that had two leaves infested with M. persicae, with the two infested leaves being removed before testing, suggested the possibility that, at this stage of infestation, the majority of volatile production induced by M. persicae may be localized to the infested tissues of the plant. We conclude that this tritrophic interaction is a suitable model system for future investigations of the biochemical pathways involved in the production of aphid‐induced volatiles attractive to natural enemies.     

Effects of infection with the cabbage black ringspot strain of turnip mosaic virus on turnip as a host to Myzus persicae and Brevicoryne brassicae

When turnip plants with 3–7 leaves were inoculated with cabbage black ringspot virus (CBRSV) on the 3rd rough-leaf, symptoms only appeared on leaves that had been less than 15 mm long at the time of inoculation, although infection decreased the area and both fresh and dry weight of all leaves. Leaves were ‘aged’ by their appearance and placed in Leaf Age Categories (LACs). Leaves with symptoms senesced (‘aged’) prematurely. CBRSV-infection of cv. Green Top White did not change the distribution of populations of Myzus persicae between LACs, but increased the proportion of the plant suitable for colonisation. All suitable LACs were quickly colonised by adult apterae and nymphs. On CBRSV-infected plants the nymphal period was shorter, F₁ adults deposited larvae more frequently and the live body weight and tibial length of the F₂ generation was greater, than on healthy plants. The distribution of Brevicoryne brassicae populations on cv. Green Top White differed from that of M. persicae but was also unchanged by CBRSV-infection. On healthy plants the largest colonies were on mature leaves, so that on virus-infected plants premature senescence shortened the life of the colony. On CBRSV-infected plants the nymphal period was prolonged and the live weight of F₁ and F₂ adult apterae was less than on healthy plants. The differences between the biology of M. persicae and B. brassicae on CBRSV-infected cv. Green Top White were associated with the accelerated senescence of CBRSV-infected leaves. The possibility that CBRSV-infection might reduce the resistance of turnips to aphid infestation was tested. M. persicae and B. brassicae were cultured on two favourable and two less favourable cultivars. No improvement in population growth rate was found when the less favourable host cultivars were infected with CBRSV, but both aphid species weighed less and/or had smaller nymphal populations on cultivars showing the severest symptoms. These results are discussed in relation to the evolution of non-persistent virus transmission by aphids.     

Aphid response to elevated ozone and CO2

Numerous reports indicate that pollution stress caused by sulphur dioxide (SO₂), oxies of nitrogen or fluorides promote aphid growth on herbaceous and woody plants. At SO₂ exposures, the response curve of aphids is bell-shaped having the peak at 100 ppb. This curvilinear response is related to physiological stress responses of host plants exposed to pollutants. On the other hand, observations of aphid performance on ozone-exposed (O₃) or elevated carbon dioxide-exposed (CO₂) plants have given very variable results. Depending on the duration and concentration of O₃ or elevated CO₂ exposure or the age of the exposed plants, aphid growth on the same plants either decreased or increased in comparison to growth on control plants grown in filtered air. The results of these studies suggest that there is no general air pollution-induced plant stress that triggers aphid outbreaks on plants. Plants grown in elevated CO₂ usually have higher C/N ratios than plants grown in current ambient CO₂ atmosphere. A reduced proportion of nitrogen in the plant foliage decreases growth of chewing herbivorous insects, but the few studies of elevated CO₂ effects on sucking insects such as aphids have not yielded similar consistent effects. The present paper reviews recent studies of elevated CO₂ effects on aphids and discusses the effects of combined elevated O₃ and CO₂ exposures on aphid performance on woody plants using pine and birch aphids as examples.     

Infection of Arabidopsis by cucumber mosaic virus triggers jasmonate-dependent resistance to aphids that relies partly on the pattern-triggered immunity factor BAK1

Many aphid-vectored viruses are transmitted nonpersistently via transient attachment of virus particles to aphid mouthparts and are most effectively acquired or transmitted during brief stylet punctures of epidermal cells. In Arabidopsis thaliana, the aphid-transmitted virus cucumber mosaic virus (CMV) induces feeding deterrence against the polyphagous aphid Myzus persicae. This form of resistance inhibits prolonged phloem feeding but promotes virus acquisition by aphids because it encourages probing of plant epidermal cells. When aphids are confined on CMV-infected plants, feeding deterrence reduces their growth and reproduction. We found that CMV-induced inhibition of growth as well as CMV-induced inhibition of reproduction of M. persicae are dependent upon jasmonate-mediated signalling. BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1) is a co-receptor enabling detection of microbe-associated molecular patterns and induction of pattern-triggered immunity (PTI). In plants carrying the mutant bak1-5 allele, CMV induced inhibition of M. persicae reproduction but not inhibition of aphid growth. We conclude that in wildtype plants CMV induces two mechanisms that diminish performance of M. persicae: a jasmonate-dependent and PTI-dependent mechanism that inhibits aphid growth, and a jasmonate-dependent, PTI-independent mechanism that inhibits reproduction. The growth of two crucifer specialist aphids, Lipaphis erysimi and Brevicoryne brassicae, was not affected when confined on CMV-infected A. thaliana. However, B. brassicae reproduction was inhibited on CMV-infected plants. This suggests that in A. thaliana CMV-induced resistance to aphids, which is thought to incentivize virus vectoring, has greater effects on polyphagous than on crucifer specialist aphids.     

Bottom‐up effects mediated by an organic soil amendment on the cabbage aphid pests Myzus persicae and Brevicoryne brassicae

Earthworm‐produced compost or vermicompost has been shown to increase resistance of plants to a variety of insect pests, but it is still unclear whether this resistance is dose dependent and whether the mechanisms responsible are the same for insect species with differing feeding habits and preferences. Therefore, we tested the effects of plants grown in various vermicompost concentrations (0, 20, 40, and 60%) on the preference and performance of generalist, Myzus persicae L., and specialist, Brevicoryne brassicae L. (both Hemiptera: Aphididae), aphid pests. Preference was evaluated with leaf disk (apterous) and whole plant (alate) choice assays. After 24 h of feeding, there was no significant negative effect on the feeding preference noted for apterae of either species of any of the treatments tested. To the contrary, apterae B. brassicae showed a significant preference for vermicompost treatments over control leaf disks. Alate M. persicae preferred alighting on control plants over vermicompost‐grown plants, but B. brassicae showed no preference toward any of the treatments tested. Both aphid species deposited significantly more nymphs on control plants than on those grown in 20% vermicompost. Furthermore, plants grown in soil amended with 20% vermicompost significantly suppressed mass accumulation, as well as numbers of adults and nymphs of both aphid species compared to controls. These data clearly show that vermicompost soil amendments can significantly influence pest aphid preference and performance on plants and that these effects are not dose dependent, but rather species and morph dependent.     

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.