Impact of plant and aphid stress history on infestation in arugula plants

Plants can activate inducible defence mechanisms against pests, pathogens, or chemical elicitors, such as ozone, mediated by reactive oxygen species (ROS), particularly hydrogen peroxide (H₂O₂). An unfavourable balance between ROS production and the plant antioxidant capacity seems to be responsible for the resulting susceptibility of the plant to insect attack. Arugula plants [Eruca sativa Mill. (Brassicaceae)] and green peach aphids, Myzus persicae (Sulzer) (Hemiptera: Aphididae), were used in this study to test the hypothesis that the growth of an aphid population depends on both plant and insect stress history. We investigated the impact of density and duration of a previous aphid infestation, and the time lag before re‐infestation, on aphid population growth. In a second experiment, we assessed the effect on aphid population growth of previous ozone exposure of arugula plants in open top chambers receiving a continuous O₃ fumigation of 100–120 p.p.b., 90 min per day during 3 days. A third experiment was conducted to study the effect of aphid density during a previous infestation on the population growth on an uninfested host. Both previous herbivory and ozone changed the oxidative status of plant tissues and facilitated aphid population growth, which increased with the duration and density of a previous infestation by aphids. Colonization success also depended on the aphids' own history. Aphids coming from high‐density populations and/or longer infestation periods produced larger populations on an (initially) uninfested plant. Pest outbreaks in a polluted environment might be expected to be modulated by the hosts' spatial‐temporal heterogeneity related to the ozone exposure and previous herbivory.     

Aphid-host plant interactions: does aphid honeydew exactly reflect the host plant amino acid composition?

Plants provide aphids with unbalanced and low concentrations of amino acids. Likely, intracellular symbionts improve the aphid nutrition by participating to the synthesis of essential amino acids. To compare the aphid amino acid uptakes from the host plant and the aphids amino acid excretion into the honeydew, host plant exudates (phloem + xylem) from infested and uninfested Vicia faba L. plants were compared to the honeydew produced by two aphid species (Acyrthosiphon pisum Harris and Megoura viciae Buckton) feeding on V. faba. Our results show that an aphid infestation modifies the amino acid composition of the infested broad bean plant since the global concentration of amino acids significantly increased in the host plant in response to aphid infestations. Specifically, the concentrations of the two amino acids glutamine and asparagine were strongly enhanced. The amino acid profiles from honeydews were similar for the two aphid species, but the concentrations found in the honeydews were generally lower than those measured in the exudates of infested plants (aphids uptakes). This work also highlights that aphids take large amounts of amino acids from the host plant, especially glutamine and asparagine, which are converted into glutamic and aspartic acids but also into other essential amino acids. The amino acid profiles differed between the host plant exudates and the aphid excretion product. Finally, this study highlights that the pea aphid, a “specialist” for the V. faba host plant, induced more important modifications into the host plant amino acid composition than the “generalist” aphid M. viciae.     

Resistance induction and herbivore virulence in the interaction between Myzus persicae (Sulzer) and a major aphid resistance gene (Rm2) from peach

In gene-for-gene host–enemy interactions, monogenic plant resistance results from pathogen recognition that initiates the induction of plant defense responses. Schematically, as the result of the on/off process of recognition, phenotypic variability in enemy virulence is expected to be qualitative, with either a failure or a success of host colonization. We focussed on a major gene from peach conferring avoidance resistance against the green peach aphid Myzus persicae. Measurements of herbivore density and time-dependent aspects of resistance induction were examined, as well as variability in the aphid’s ability to exploit the resistant host. Varying densities of infestation did not provoke differences in the aphid’s tendency to leave a plant, and a single aphid was sufficient to elicit a response. Similarly, the duration of infestation did not affect the aphid response. A brief aphid feeding time of 3 h triggered induced resistance, which became effective between 24 and 48 h after the initial attack. Induced resistance decayed over time in the absence of additional infestation. Thirty aphid genotypes collected from natural populations were tested in the laboratory. No clone could colonize the resistant host, suggesting that all of them triggered the induction of effective plant defense responses. However, we detected significant quantitative variation among clones in the tendency of aphids to leave plants. These results improve our understanding of induced resistance as a dynamic phenomenon and suggest that the potential for aphids to adapt to a major plant resistance gene may depend on factors other than the mere capacity to evade recognition.     

Induced resistance by Myzus persicae in the peach cultivar `Rubira'

The effect of a previous infestation by the green peach aphid Myzus persicae (Sulzer) on the settling behaviour and reproduction of the same aphid species was investigated in the resistant peach cultivar Rubira, and compared with that observed in the susceptible control cultivar GF305. A previous infestation of 48 h triggered induced resistance in Rubira. There were significantly fewer aphids settling on preinfested than on uninfested plants, indicating an increased rejection of Rubira as a host plant. The level of induced resistance in preinfested plants was positively related to the duration of the first infestation. In GF305, previous infestation had no detrimental effect on aphid settlement and even slightly enhanced larviposition by adult females. The aphid probing behaviour after a 48-h preinfestation was also monitored for 8 h with the electrical peneration graph (EPG) technique. On preinfested GF305, most EPG parameters indicated an enhanced host plant acceptance. On preinfested GF305, aphids produced less sieve element salivation and more continuous sap ingestion than on uninfested GF305, indicating that the previous aphids provoked changes in plant properties beneficial to the test aphids. In Rubira, a major induced factor of resistance was thought to be expressed in the sieve element as phloem sap ingestion was 4-fold shorter on preinfested than on uninfested plants. The time taken by the aphid stylets to reach a sieve element was also significantly increased on preinfested Rubira, suggesting the induction of resistance factors outside the phloem. The originality of the Rubira/M. persicae interaction is discussed in the perspective of a better understanding of plant induced responses to aphids.     

Direct and Indirect Impacts of Infestation of Tomato Plant by Myzus persicae (Hemiptera: Aphididae) on Bemisia tabaci (Hemiptera: Aleyrodidae)

The impacts of infestation by the green peach aphid (Myzus persicae) on sweetpotato whitefly (Bemisia tabaci) settling on tomato were determined in seven separate experiments with whole plants and with detached leaves through manipulation of four factors: durations of aphid infestation, density of aphids, intervals between aphid removal after different durations of infestation and the time of whitefly release, and leaf positions on the plants. The results demonstrated that B. tabaci preferred to settle on the plant leaves that had not been infested by aphids when they had a choice. The plant leaves on which aphids were still present (direct effect) had fewer whiteflies than those previously infested by aphids (indirect effect). The whiteflies were able to settle on the plant which aphids had previously infested, and also could settle on leaves with aphids if no uninfested plants were available. Tests of direct factors revealed that duration of aphid infestation had a stronger effect on whitefly landing preference than aphid density; whitefly preference was the least when 20 aphids fed on the leaves for 72 h. Tests of indirect effects revealed that the major factor that affected whitefly preference for a host plant was the interval between the time of aphid removal after infestation and the time of whitefly release. The importance of the four factors that affected the induced plant defense against whiteflies can be arranged in the following order: time intervals between aphid removal and whitefly release > durations of aphid infestation > density of aphids > leaf positions on the plants. In conclusion, the density of aphid infestation and time for which they were feeding influenced the production of induced compounds by tomatoes, the whitefly responses to the plants, and reduced interspecific competition.     

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.     

Evaluation of mirid predatory bugs and release strategy for aphid control in sweet pepper

Zoophytophagous predators of the family Miridae (Heteroptera), which feed both on plant and prey, often maintain a close relationship with certain host plants. In this study, we aimed to select a suitable mirid predatory bug for aphid control in sweet pepper. Four species were compared: Macrolophus pygmaeus (Rambur), Dicyphus errans (Wolff), Dicyphus tamaninii Wagner and Deraeocoris pallens (Reuter). They were assessed on their establishment on sweet pepper plants with and without supplemental food (eggs of the flour moth Ephestia kuehniella Zeller and decapsulated cysts of the brine shrimp Artemia franciscana Kellogg) and on their effects on aphids with releases before and after aphid infestations. None of the predator species was able to control an established population of aphids on sweet pepper plants; however, the predators M. pygmaeus and D. tamaninii could successfully reduce aphid populations when released prior to an artificially introduced aphid infestation. The best results were achieved with M. pygmaeus in combination with a weekly application of supplemental food. Hence, our results demonstrate that the order and level of plant colonization by mirid predators and aphids determines how successful biological control is. Further studies are needed to evaluate the performance of mirid predatory bugs in sweet pepper crops in commercial greenhouses with multiple pests and natural enemies, in particular to understand how increased variation in food sources affects their feeding behaviour and preferences.     

Aphid populations showing differential levels of virulence on Capsicum accessions

The green peach aphid,Myzus persicae,is one of the most threatening pests in pepper cultivation and growers would benefit from resistant varietices.Previously,we identified two Capsicum acessions as susceptible and three as resistant to M.persicae using an aphid population originating from the Netherlands(NL).Later on we identified an aphid population originating from a diferent gcographical region(Switserland,SW)that was virulent on all tested Capsicum acessions.The objeetive of the current work is to describe in detail diferent aspects of the interaction between two aphid populations and two sclected Capsicum acessions(one that was susceptible[PB2013046]and one that was resistant[PB2013071]to population NL),including biochemical processes involved.Electrical penetration graph(EPG)recordings showed similar feeding activities for both aphid populations on PB2013046.On acession PB2013071 the aphid population sw was able to devote significantly more time to phloem ingestion than population NL.We also studied plant defense response and found that plants of acession PB2013046 could not induce an accumulation of reactive oxygen species and callose formation after infestation with either aphid population.However,plants of PB2013071 induced a stronger defense response after infestation by population NL than after infestation by population SW.Based on these results,population SW of M.persicae seems to have overcome the resistance of PB2013071 that prevented feeding of aphids from NL population.The potential mechanism by which SW population overcomes the resistance is discussed.     

Occurrence of cotton aphids (Aphis gossypii) and lady beetles (Hamonia axyridis) on Hibiscus syriacus Linne: Are the aphids a pest of cucurbits?

Populations of cotton aphid on Hibiscus syriacus increased rapidly from 17 to 24 May 2007, and then decreased as its predator, the lady beetle Hamonia axyridis, increased in number. There was a 10 day time lag between peak populations of aphids and lady beetles. The infestation of aphids on H. syriacus produced some damage, but H. syriacus recovered soon after the lady beetles arrived. Cotton aphid clones from H. syriacus were transferred to other summer host plants: to five different vegetables on two dates, and to cucumber on three dates. Apart from one case where reproduction occurred on eggplant, most H. syriacus aphid clones did not survive on the vegetables. The cotton aphid on H. syriancus was prey and a food source for H. axyridis and acted to conserve natural enemies.     

Wind matters: Asymmetric distribution of aphids on host plants can be explained by stems functioning as windbreaks

Understanding how abiotic factors influence organisms at present is the necessary first step to predict how species assemblages could be affected by climate change in the future. We examined how wind, a poorly studied abiotic factor, affects the distribution and abundance of two aphid species, Uroleucon aeneum and Brachycaudus cardui (hereafter black and green aphids, respectively), that live on the thistle Carduus thoermeri (Asteraceae) in a windy region of Patagonia, Argentina. First, considering the prevailing wind direction, we described the distribution of both aphid species around plant stems. Then, we performed a bi‐factorial experiment in which we cut stems with aphids to manipulate their position respect to wind (exposed/unexposed) and to control wind incidence (protected/unprotected). Finally, using the species most affected by wind, we examined possible mechanisms through which wind could affect aphids. Both aphid species were less abundant on the side of the stem exposed to wind respect to the unexposed side; and this pattern was stronger for the black aphid. When black aphids were positioned exposed to wind and without protection, their proportion changed towards the unexposed side of the stem; while green aphids showed a weaker response to wind. Laboratory experiments demonstrated that wind triggered both the detachment of black aphids and their movement towards the unexposed side of the stem. Our results showed that wind can explain the asymmetric distribution of aphids around plants and that stems can act as windbreaks. In a less windy future scenario, aphids could expand their foraging area, reaching higher infestation rates, which could affect their role in structuring ant assemblages and their status as crop pests. This work highlights the importance of testing the effects of less studied abiotic factors to fully understand how climate change could impact on the abundance and distribution of animals in the future.     

Cucumber mosaic virus 2b proteins inhibit virus-induced aphid resistance in tobacco

Cucumber mosaic virus (CMV), which is vectored by aphids, has a tripartite RNA genome encoding five proteins. In tobacco (Nicotiana tabacum), a subgroup IA CMV strain, Fny-CMV, increases plant susceptibility to aphid infestation but a viral mutant unable to express the 2b protein (Fny-CMV∆2b) induces aphid resistance. We hypothesized that in tobacco, one or more of the four other Fny-CMV gene products (the 1a or 2a replication proteins, the movement protein, or the coat protein) are potential aphid resistance elicitors, whilst the 2b protein counteracts induction of aphid resistance. Mutation of the Fny-CMV 2b protein indicated that inhibition of virus-induced resistance to aphids (Myzus persicae) depends on amino acid sequences known to control nucleus-to-cytoplasm shuttling. LS-CMV (subgroup II) also increased susceptibility to aphid infestation but the LS-CMV∆2b mutant did not induce aphid resistance. Using reassortant viruses comprising different combinations of LS and Fny genomic RNAs, we showed that Fny-CMV RNA 1 but not LS-CMV RNA 1 conditions aphid resistance in tobacco, suggesting that the Fny-CMV 1a protein triggers resistance. However, the 2b proteins of both strains suppress aphid resistance, suggesting that the ability of 2b proteins to inhibit aphid resistance is conserved among divergent CMV strains.     

Aphid infestation promotes survival of a seed predator: observations and experiments on a tritrophic community module

We investigated whether aphid presence and abundance influence the survival of an endophagous pre-dispersal seed predator of the same host plant. We studied a terrestrial community module consisting of one plant (Laburnum anagyroides) and four insect species/groups (an aphid, Aphis cytisorum, a pre-dispersal seed predator bruchid, Bruchidius villosus, aphid-attending ant species, and parasitoids of the bruchid). Two complementary investigations were carried out in parallel: (a) a plant-aphid-ant complex was experimentally manipulated by excluding aphids, ants, or both for 5 years to assess their impacts on the seed predator’s survival and parasitism rate; and (b) different aphid infestation levels on randomly selected infructescences were correlated with plant traits, nutrient allocation pattern, and variables of seed predator’s survival, such as the number of eggs laid and adults emerged influenced by parasitoid activity, for 7 years. We found that ants did not affect bruchid oviposition negatively, but egg-parasitism was significantly decreased by their presence. Plant traits, such as the number of seeds and seed mass, as well as seed predator performance were negatively affected by heavy aphid infestation. Seed predator -infested seeds had no effect on the mass of remaining seeds in the pods. This study suggests that aphids were nevertheless promoting bruchid abundance and survival, depending on their infestation rate.     

Response of Solanum tuberosum to Myzus persicae infestation at different stages of foliage maturity

Young leaves of the potato Solanum tuberosum L. cultivar Kardal contain resistance factors to the green peach aphid Myzuspersicae （Sulzer） （Hemiptera： Aphididae） and normal probing behavior is impeded. However, M. persicae can survive and reproduce on mature and senescent leaves of the cv. Kardal plant without problems. We compared the settling ofM. persicae on young and old leaves and analyzed the impact of aphids settling on the plant in terms of gene expression. Settling, as measured by aphid numbers staying on young or old leaves, showed that after 21 h significantly fewer aphids were found on the young leaves. At earlier time points there were no difference between young and old leaves, suggesting that the young leaf resistance factors are not located at the surface level but deeper in the tissue. Gene expression was measured in plants at 96 h postinfestation, which is at a late stage in the interaction and in compatible interactions this is long enough for host plant acceptance to occur. In old leaves of cv. Kardal （compatible interaction）, M. persicae infestation elicited a higher number of differentially regulated genes than in young leaves. The plant response to aphid infestation included a larger number of genes induced than repressed, and the proportion of induced versus repressed genes was larger in young than in old leaves. Several genes changing expression seem to be involved in changing the metabolic state of the leaf from source to sink.     

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.     

Evaluation of induced responses, insect population growth, and host-plant fitness may change the outcome of tests of the preference-performance hypothesis: a case study

The preference‐performance hypothesis predicts that insect preference should correspond to host suitability for offspring development. We studied the pattern of within‐plant preference in the aphid Sipha flava and its consequences for offspring performance on the host‐plant Sorghum halepense, regarding the role of induced responses of plants to aphid feeding. The consequences of within‐plant preference on aphid population growth and host‐plant traits were also evaluated. Our results showed that winged and wingless aphids preferred to settle on mature rather than young leaves. In contrast, aphid individual growth rate was higher on young leaves when compared with mature leaves, suggesting that the outcome of this test rejected the preference‐performance hypothesis. However, the inclusion of the factor ‘previous aphid infestation’ changed the outcome from a maladaptive choice to a neutral one. Thus, individual growth rates of S. flava increased when aphids developed on leaves that had been previously infested. Interestingly, aphid growth rate on previously infested leaves did not differ between young and mature leaves. On the other hand, aphid population reproductive rate was higher and the percentage of winged aphids lower when infestation occurred on mature rather than young leaves. Aphid infestation reduced plant and shoot biomass, and increased leaf mortality. These negative effects on plant traits related to plant fitness were greater when aphid infestation occurred on young leaves. Likewise, whereas infestation on mature leaves did not cause a significant reduction in the number of flowering plants compared with control plants, aphid infestation on young leaves did reduce the number of plants at the flowering stage. Consequently, if both the reproductive rate of aphids in the mid‐term, and host‐plant fitness are taken into account, the results indicate that aphid preference for mature leaves may be an adaptive choice, thus supporting the preference‐performance hypothesis.     

The use of visual cues in host evaluation by aphidiid wasps

We examined host evaluation behaviour in three species of aphid parasitoids, Ephedrus californicus Baker, Monoctonus paulensis (Ashmead), and Praon pequodorum Viereck (Hymenoptera: Aphidiidae). Mated females were provided with pairwise choices among three kinds of hosts in the laboratory: (green) pea aphid, Acyrthosiphon pisum (Harris), and a green and a pink colour morph of alfalfa aphid, Macrosiphum creelii Davis. Patterns of attack and host acceptance were species-specific. Females of E. californicus did not respond to the presence of aphids prior to making antennal contact. Variations in rates of parasitization (pea aphid>green alfalfa aphid>pink alfalfa aphid) were consistent with differences in aphid defensive behaviours; no ‘preference’ for any host type was evident when aphids were anaesthetized with carbon dioxide. In M. paulensis, the order of preference (pea aphid>green alfalfa aphid>pink alfalfa aphid) did not vary when aphids were immobilized, or presented in the dark, or both. Host movement did not influence the rate of attack by M. paulensis. In contrast, the ranked order of preference in P. pequodorum varied with circumstance. In the light, females attacked pea aphid and green alfalfa aphid with equal frequency, but parasitized significantly more of the former; both kinds of aphids were attacked and parasitized at higher rates than pink alfalfa aphid. In the dark, P. pequodorum females parasitized green and pink alfalfa aphids equally and at higher rates than pea aphids. Whereas E. californicus was more successful ovipositing in immobilized hosts, P. pequodorum females attacked and laid more eggs in normal than anaesthetized aphids. Patterns of host recognition and evaluation are compared across six species representing four genera in the family Aphidiidae.     

Effects of integrating companion cropping and nitrogen application on the performance and infestation of collards by Brevicoryne brassicae

Sustainable management of cabbage aphids, Brevicoryne brassicae (L.) (Hemiptera: Aphididae), is a major goal for collard, Brassica oleracea (L.) var. acephala (Brassicaceae), growers globally. Host finding ability of insect pests is significantly affected by diversified cropping systems, and this approach is being utilized currently as a pest management tool. Soil nutrition and its interaction with the cropping systems could have a significant effect on the general performance of collards and the infestation by cabbage aphids. In a search for a sustainable cabbage aphid control, a two‐season field experiment was carried out with two intercrops, collards and chilli, Capsicum frutescens (L.) (Solanaceae), and collards and spring onions, Allium cepa (L.) (Alliaceae), and a collard monoculture. For each of the cropping systems, nitrogen (N) was applied to the soil as a top‐dress at 20, 25, 30, and 35 g per collard plant. The response factors monitored were collard yield (fresh weight) and aphid infestation on collards. Spring onion‐collard intercrop had the lowest aphid density and the highest yield. Collard monoculture had the highest aphid infestation and the lowest yield. High levels of N led to increased infestation of collards by aphids, but also led to a significant increase in the yield of collards. Significant interactions between the N rates and the cropping systems were observed on some sampling dates, with the highest yield being realized under a combination of spring onion‐collard intercrop at a N rate of 30 g per plant. High aphid density led to a decrease in the yield of collards. It was concluded that with a spring onion‐collard intercrop, the soil N level could be raised from the blanket rate of 20–30 g per plant and this would lead to an increase in yield.     

Non‐consumptive effects of a natural enemy on a non‐prey herbivore population

1. Predator–prey interactions have traditionally focused on the consumptive effects that predators have on prey. However, predators can also reduce the abundance of prey through behaviourally‐mediated non‐consumptive effects. For example, pea aphids (Acyrthosiphon pisum Harris) drop from their host plants in response to the risk of attack, reducing population sizes as a consequence of lost feeding opportunities. 2. The objective of the present study was to determine whether the non‐consumptive effects of predators could extend to non‐prey herbivore populations as a result of non‐lethal incidental interactions between herbivores and foraging natural enemies. 3. Polyculture habitats consisting of green peach aphids (Myzus persicae Sulzer) feeding on collards and pea aphids feeding on fava beans were established in greenhouse cages. Aphidius colemani Viereck, a generalist parasitoid that attacks green peach aphids but not pea aphids, was released into half of the cages and the abundance of the non‐host pea aphid was assessed. 4. Parasitoids reduced the population growth of the non‐host pea aphid by increasing the frequency of defensive drops; but this effect was dependent on the presence of green peach aphids. 5. Parasitoids probably elicited the pea aphid dropping behaviour through physical contact with pea aphids while foraging for green peach aphids. It is unlikely that pea aphids were responding to volatile alarm chemicals emitted by green peach aphids in the presence of the parasitoid. 6. In conclusion, the escape response of the pea aphid provided the opportunity for a parasitoid to have non‐target effects on an herbivore with which it did not engage in a trophic interaction. The implication is that natural enemies with narrow diet breadths have the potential to influence the abundance of a broad range of prey and non‐prey species via non‐consumptive effects.