How does the host‐specialized aphid deal with food deficiency?

Aphis gossypii Glover shows obvious host specialization, with cucurbit‐ and cotton‐specialized biotypes or host races in many regions. Because its annual natal host crops senesce earlier the cucurbit‐specialized biotype may suffer food deficiency. The method this biotype uses to overcome this challenge is still poorly understood. In order to understand the potential of the cucurbit‐specialized biotype aphids in host shift and usage, the performance of this biotype on cotton (Gossypium hirsutum), a common but poor quality host plant, was explored in this study. The cucurbit‐specialized aphids could establish populations on cotton only when these plants had at least nine leaves, and subsequent populations developed rather slowly. The presence of whitefly populations on cotton improved the success rate of cucurbit‐specialized aphids. The cucurbit‐specialized aphids were mainly distributed on the older leaves of cotton, with only a few settling on the upper leaves. The cucurbit‐specialized aphids reared on cotton for 40, 54 and 61 days still maintained strong preference for their natal host plant, cucumber (Cucumis sativus), rather than cotton, and their net reproductive rates and intrinsic rates of natural increase were dramatically lower when they were transferred onto new six‐leaf cotton plants or detached leaves. Therefore, we concluded that the cucurbit‐specialized aphids have the potential to utilize mature or whitefly‐stressed cotton plants, but that this feeding experience on cotton did not alter their specialization for cucurbits. Some cotton plants could act as a temporary host for the cucurbit‐specialized aphids to overcome food deficiency arising from senescing cucurbits.     

OLFACTORY RESPONSES OF LYSIPHLEBIA JAPONICA TO VOLATILE CHEMICALS AND FRESH LEAVES OF THE HOST PLANTS OF COTTON APHIDS IN OLFACTOMETER

Abstract The behavioral responses of the female adult parasitoids Lysiphlebia japonica to plant volatile chemicals, host plants and cotton aphids were tested in a four arm air flow olfactometer. The results indicated that the female parasitoids were strongly attracted by the odors from cotton aphids and their host plants. Damaged cotton leaves were more attractive than intact ones. Among the four olfactory stimuli from the plant volatile chemicals dissolved in liquid paraffin oil at the concentration of 0.01 (volume fraction), the female parasitoids only positively responded to benzaldehyde, but did not show any preference to β-caryophyllene and β-pinene. Although heptanol elicited the highest EAC response of the parasitoids, the behavioral test showed that it repelled the parasitoids, and β-pinene shared the same feature with heptanol at the concentration of 1 (volume fraction). Our results also showed that the female parasitoids only preferred the cotton leaf from which the mummified aphids were collected, and did not express any interest to the three plants of melon family in the olfactometer, The possible mechanisms are discussed.     

Host‐associated differentiation in a highly polyphagous,sexually reproducing insect herbivore

Insect herbivores may undergo genetic divergence on their host plants through host‐associated differentiation (HAD). Much of what we know about HAD involves insect species with narrow host ranges (i.e., specialists) that spend part or all their life cycle inside their hosts, and/or reproduce asexually (e.g., parthenogenetic insects), all of which are thought to facilitate HAD. However, sexually reproducing polyphagous insects can also exhibit HAD. Few sexually reproducing insects have been tested for HAD, and when they have insects from only a handful of potential host‐plant populations have been tested, making it difficult to predict how common HAD is when one considers the entire species' host range. This question is particularly relevant when considering insect pests, as host‐associated populations may differ in traits relevant to their control. Here, we tested for HAD in a cotton (Gossypium hirsutum) pest, the cotton fleahopper (CFH) (Pseudatomoscelis seriatus), a sexually reproducing, highly polyphagous hemipteran insect. A previous study detected one incidence of HAD among three of its host plants. We used Amplified fragment length polymorphism (AFLP) markers to assess HAD in CFH collected from an expanded array of 13 host‐plant species belonging to seven families. Overall, four genetically distinct populations were found. One genetically distinct genotype was exclusively associated with one of the host‐plant species while the other three were observed across more than one host‐plant species. The relatively low degree of HAD in CFH compared to the pea aphid, another hemipteran insect, stresses the likely importance of sexual recombination as a factor increasing the likelihood of HAD.     

Host‐plant use by two Orthomeria (Phasmida: Aschiphasmatini) species feeding on Macaranga myrmecophytes

Myrmecophytes depend on symbiotic ants (plant‐ants) to defend against herbivores. Although these defensive mechanisms are highly effective, some herbivorous insects can use myrmecophytes as their host‐plants. The feeding habits of these phytophages on myrmecophytes and the impacts of the plant‐ants on their feeding behavior have been poorly studied. We examined two phasmid species, Orthomeria alexis and O. cuprinus, which are known to feed on Macaranga (Euphorbiaceae) myrmecophytes in a Bornean primary forest. Our observations revealed that: (i) each phasmid species relied on two closely‐related myrmecophytic Macaranga species for its host‐plants in spite of their normal plant‐ant symbioses; and (ii) there was little overlap between their host‐plant preferences. More O. cuprinus adults and nymphs were found on new leaves, which were attended by more plant‐ants than mature leaves, while most adults and nymphs of O. alexis tended to avoid new leaves. In a feeding choice experiment under ant‐excluded conditions, O. alexis adults chose a non‐host Macaranga myrmecophyte that was more intensively defended by plant‐ants and was more palatable than their usual host‐plants almost as frequently as their usual host‐plant, suggesting that the host‐plant range of O. alexis was restricted by the presence of plant‐ants on non‐host‐plants. Phasmid behavior that appeared to minimize plant‐ant attacks is described.     

不同食料植物对棉铃虫生长发育和繁殖的影响

报道了棉花、烟草、番茄和辣椒4种植物对棉铃虫Helicoverpa armigera生长发育和繁殖的影响。棉铃虫成虫喜欢在番茄上产卵,在辣椒上的着卵量最少。初孵幼虫喜选食嫩棉叶,选食辣椒嫩叶的虫数最少。4组幼虫取食嫩叶时的平均相对生长速率都有显著差异,顺序为棉叶组>烟叶组>番茄叶组>辣椒叶组。幼虫存活率以取食棉花时最高,取食番茄时最低。棉花组成虫的产卵量最高,烟草组的产卵量最低。取食棉花的棉铃虫种群增长的速度约为取食番茄时的14倍。6龄幼虫能有效利用和转化棉铃、烟草蒴果、辣椒果实,而对番茄果实的利用和转化效率较低。棉铃虫可分别以这4种植物的不同器官为食完成世代循环。其中,棉花是最适宜的寄主,辣椒和番茄是较不适宜的寄主。     

Do defoliating insects distinguish between symmetric and asymmetric leaves within a plant?

1. Plants represent a highly heterogeneous resource for herbivores. One dimension of this heterogeneity is reflected by the within‐plant variation in the leaf fluctuating asymmetry (FA), i.e. in the magnitude of the random deviations from the symmetry in leaf shape. 2. This study is the first to test experimentally the hypothesis that variation in the quality of individual leaves for defoliating insects (11 species) within a plant (seven tree and shrub species) is associated with the FA of these leaves. 3. It was demonstrated that specialist defoliators generally distinguish between nearly symmetric (low FA) and highly asymmetric (high FA) leaves, but do not distinguish between discs cut from these leaves. Low‐FA leaves of Salix caprea, Salix myrsinifolia and Populus tremula were of better quality for insects than high‐FA leaves, as demonstrated by both preference tests and performance trials. By contrast, high‐FA leaves of Betula pubescens were of better quality for insects than low‐FA leaves, whereas insects feeding on Alnus incana showed similar responses to high‐ and low‐FA leaves. 4. It is concluded that insect herbivores can distinguish between leaves with high and low FA, and that FA may be associated with the quality of an individual leaf for insects, although the direction and strength of the effect of leaf FA on insect preference and performance vary among study systems. The ecological significance of substantial within‐plant variation in leaf FA remains to be explored.     

Editorial note

Generalist herbivores can face many challenges when choosing their host plant. This can be particularly difficult if their choice and performance are affected by host experience. Greenhouse whitefly, Trialeurodes vaporariorum Westwood (Hemiptera: Aleyrodidae), is an invasive generalist herbivore, which has established in year‐round greenhouses at northern latitudes where it cannot overwinter outdoors. It mainly uses crops such as cucumber (Cucumis sativus L.), tomato (Solanum lycopersicum L.), and ornamentals as host plants. However, every summer the insect escapes greenhouses and is exposed to natural vegetation. We evaluated the performance of T. vaporariorum on diverse vegetation outside greenhouses after prolonged experience of greenhouse crops. First, we surveyed the vegetation near infested greenhouses. Development success of the insect differed among wild hosts. We identified five new hosts among 12 plant species that bore pupae and were thus considered suitable as the insect's host plants. Members of the Urticaceae and Onagraceae were the most preferred and frequently inhabited by all insect life stages. The highest abundance of insects occurred in plots with low plant species richness, independent of plant family in these habitats. We then studied experimentally the impact of 1 year of preconditioning to one of three common greenhouse crops, cucumber, tomato, or poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch), on the performance of the preconditioned adults and their progeny on four wild plants. Adults from tomato and poinsettia preferred the novel host species over the species to which they were preconditioned. The whitefly population preconditioned to cucumber was the most fecund on all offered hosts. We conclude that generalist herbivores can have large variation in performance, despite polyphagy, on novel hosts as shown by the variable abundance of T. vaporariorum pupae among outdoor hosts. Furthermore, performance of whiteflies on natural vegetation was affected by experience on greenhouse crops. Based on our observations, we provide insights and recommendations for pest management.     

Aphid and host‐plant genotype × genotype interactions under elevated CO2

1. Elevated CO₂ can alter plant physiology and morphology, and these changes are expected to impact diet quality for insect herbivores. While the plastic responses of insect herbivores have been well studied, less is known about the propensity of insects to adapt to such changes. Genetic variation in insect responses to elevated CO₂ and genetic interactions between insects and their host plants may exist and provide the necessary raw material for adaptation. 2. We used clonal lines of Rhopalosiphum padi (L.) aphids to examine genotype‐specific responses to elevated CO₂. We used the host plant Schedonorus arundinaceus (tall fescue; Schreb), which is capable of asexual reproduction, to investigate host plant genotype‐specific effects and possible host plant‐by‐insect genotype interactions. The abundance and density of three R. padi genotypes on three tall fescue genotypes under three concentrations of CO₂ (ambient, 700, and 1000 ppm) in a controlled greenhouse environment were examined. 3. Aphid abundance decreased in the 700 ppm CO₂ concentration, but increased in the 1000 ppm concentration relative to ambient. The effect of CO₂ on aphid density was dependent on host plant genotype; the density of aphids in high CO₂ decreased for two plant genotypes but was unchanged in one. No interaction between aphid genotype and elevated CO₂ was found, nor did we find significant genotype‐by‐genotype interactions. 4. This study suggests that the density of R. padi aphids feeding on tall fescue may decrease under elevated CO₂ for some plant genotypes. The likely impact of genotype‐specific responses on future changes in the genetic structure of plant and insect populations is discussed.     

The nocturnal larvae of a specialist folivore perform better on Chromolaena odorata leaves from a shaded environment

Increasing evidence suggests that individuals of the same plant species occurring in different microhabitats often show a degree of phenotypic and phytochemical variation. Consequently, insect herbivores associated with such plant species must deal with environment‐mediated changes or variability in the traits of their host plants. In this study, we examined the effects of habitat condition (shaded vs. full‐sun habitats) on plant traits and leaf characteristics of the invasive alien plant, Chromolaena odorata (L.) King & Robinson (Asteraceae). In addition, the performance was evaluated in two generations of a specialist folivore, Pareuchaetes insulata (Walker) (Lepidoptera: Erebidae: Arctiinae), on leaves obtained from both shaded and full‐sun habitats. The study was done in an area where the insect was introduced as a biological control agent. Leaves growing in shade were less tough, had higher water and nitrogen content, and lower total non‐structural carbohydrate, compared with leaves growing in full sun. Plants growing in shade had longer leaves and were taller, but above‐ground biomass was significantly reduced compared with plants growing in full sun. In both generations (parents and offspring), P. insulata developed faster and had larger pupal mass, increased growth rate, and higher fecundity when reared on shaded foliage compared with full‐sun foliage. Although immature survival and adult longevity did not differ between habitats, Maw's host suitability index indicated that shaded leaves were more suitable for the growth and reproduction of P. insulata. We suggest that the benefits obtained by P. insulata feeding on shaded foliage are associated with reduced toughness and enhanced nitrogen and water content of leaves. These results demonstrate that light‐mediated changes in plant traits and leaf characteristics can affect insect folivore performance.     

Effect of mechanical damage on emission of volatile organic compounds from plant leaves and implications for evaluation of host plant specificity of prospective biological control agents of weeds

Assessment of host plant specificity is a critical step in the evaluation of classical biological control agents of weeds which is necessary for avoiding possible damage to non-target plants. Volatile organic compounds (VOCs) emitted by plants likely play an important role in determining which plants attract and are accepted by a prospective arthropod agent. However, current methods to evaluate host plant specificity usually rely on empirical choice and no-choice behavioural experiments, with little knowledge about what chemical or physical attributes are stimulating the insect. We conducted experiments to measure the quantitative and qualitative effects on emission of VOCs caused by simple mechanical damage to leaves of plants known to differ in suitability and attractiveness to a prospective agent. More VOCs were detected from damaged than from undamaged leaves for all three species tested. Discriminant analysis was able to correctly distinguish the taxonomic identity of all plants based on their VOC profiles; however, the VOCs that discriminated species among undamaged leaves were completely different from those that discriminated among damaged leaves. Thus, damaged and undamaged plants present different VOC profiles to insects, which should be considered when conducting host plant specificity experiments. An unacceptable non-target plant, Centaurea cineraria, emitted all except one of the VOCs that were emitted by its preferred host plant, Centaurea solstitialis, indicating the importance of compounds that are repellant in host plant specificity. Centaurea cyanus emitted fewer VOCs than C. solstitialis, which suggests that it lacked some VOCs important for host plant recognition.     

Polyphagy and primary host plants: oviposition preference versus larval performance in the lepidopteran pest Helicoverpa armigera

Oviposition preference and several measures of offspring performance of Helicoverpa armigera (Hübner) were investigated on a subset of its host plants that were selected for their reputed importance in the field in Australia. They included cotton, pigeon pea, sweet corn, mungbean, bean and common sowthistle. Plants were at their flowering stage when presented to gravid female moths. Flowering pigeon pea evoked far more oviposition than did the other plant species and was the most preferred plant for neonate larval feeding. It also supported development of the most robust larvae and pupae, and these produced the most fecund moths. Common sowthistle and cotton were equally suitable to pigeon pea for larval development, but these two species received far fewer H. armigera eggs than did pigeon pea. Mungbean also received relatively few eggs, but it did support intermediate measures of larval growth and survival. Fewest eggs were laid on bean and it was also the least beneficial in terms of larval growth. Among the host plant species tested, only flowering pigeon pea supported a good relationship between oviposition preference of H. armigera and its subsequent offspring performance. Australian H. armigera moths are thus consistent with Indian H. armigera moths in their ovipositional behaviour and larval performance relative to pigeon pea. The results suggest that the host recognition and acceptance behaviour of this species is fixed across its geographical distribution and they support the theory that pigeon pea might be one of the primary host plants of this insect. These insights, together with published results on the sensory responses of the females to volatiles derived from the different host plant species tested here, help to explain why some plant species are primary targets for the ovipositing moths whereas others are only secondary targets of this polyphagous pest, which has a notoriously broad host range. Handling Editor: Joseph Dickens     

Plant virus infection modifies plant pigment and manipulates the host preference behavior of an insect vector

Insect-borne plant viruses may modify the phenotype of their host plants and thus influence the responses of insect vectors. When a plant virus modifies host preference behavior of a vector, it can be expected to influence the rate of virus transmission. In this study, we examined the effect of Maize Iranian mosaic virus (MIMV) infection on host preference behavior of the nymphs and adults of its vector, the small brown planthopper, Laodelphax striatellus Fallén (Hemiptera: Delphacidae), feeding on barley plants (Hordeum vulgare L., Poaceae). We found that both viruliferous nymphs and adults significantly preferred healthy plants, whereas non-viruliferous planthoppers preferred virus-infected barley. Further investigations revealed significant reductions in the chlorophyll and carotenoid contents of infected barley leaves. Based on these results, a possible association between insect host preferences and the pigment contents of the plants was observed. In summary, we suggest that host preference of L. striatellus could be affected by the propagative plant virus, possibly through association of this modification with some phenotypic traits of infected plants. These effects may have a critical impact on MIMV transmission rate, with significant implications for the development of virus epidemics.     

The use of oviposition‐induced plant cues by Trichogramma egg parasitoids

1. Female parasitoids have evolved various foraging strategies in order to find suitable hosts. Egg parasitoids have been shown to exploit plant cues induced by the deposition of host eggs. 2. The tiny wasp Trichogramma brassicae uses oviposition‐induced cues from Brussels sprouts to locate eggs of the cabbage white butterflies Pieris brassicae and Pieris rapae that differ in their egg‐laying behaviour. These plant cues are elicited by male‐derived anti‐aphrodisiac pheromones in the accessory reproductive gland (ARG) secretions of mated female butterflies. However, the closely related generalist species Trichogramma evanescens does not respond to Brussels sprout cues induced by the deposition of P. brassicae egg clutches. 3. Here we showed in two‐choice bioassays that T. evanescens wasps respond to Brussels sprout cues induced by (i) the deposition of single eggs by P. rapae, and (ii) the application of ARG secretions from either mated P. rapae females, or from virgin female butterflies in combination with P. rapae's anti‐aphrodisiac compound indole. The wasps only associatively learned to respond to Brussels sprout cues after applying indole alone by linking those cues with the presence of P. rapae eggs. 4. Our results indicate that Trichogramma wasps more commonly exploit oviposition‐induced plant cues to locate their host eggs. Generalist wasps show less specificity in their response than specialists and employ associative learning.     

Rho 1 participates in parasitoid wasp eggs maturation and host cellular immunity inhibition

Endoparasitoid wasps introduce venom into their host insects during the egg-laying stage. Venom proteins play various roles in the host physiology, development, immunity, and behavior manipulation and regulation. In this study, we identified a venom protein, MmRho1, a small guanine nucleotide-binding protein derived from ovary in the endoparasitoid wasp Microplitis mediator and found that knockdown of its expression by RNA interference caused down-regulation of vitellogenin and juvenile hormone, egg production, and cocoons formation in the female wasps. We demonstrated that MmRho1 entered the cotton bollworm's (host) hemocytes and suppressed cellular immune responses after parasitism using immunofluorescence staining. Furthermore, wasp MmRho1 interacted with the cotton bollworm's actin cytoskeleton rearrangement regulator diaphanous by yeast 2-hybrid and glutathione s-transferase pull-down. In conclusion, this study indicates that MmRho1 plays dual roles in wasp development and the suppression of the host insect cellular immune responses.     

Feeding of fall armyworm,Spodoptera frugiperda,on Bt transgenic cotton and its isoline

Studies on insect food intake and utilization are important for determining the degree of insect/plant association and host species’ resistance, and also for helping design pest management programs by providing estimates of potential economic losses, techniques for mass breeding of insects, and identifying physiological differences between species. We studied the feeding and development of fall armyworm, Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae), on transgenic (Bt) and non‐transgenic (non‐Bt) cotton. The larvae of S. frugiperda fed on Bt cotton had a longer development period (23.0 days) than those fed on non‐Bt cotton (20.2 days). Survivorship of S. frugiperda larvae fed on Bt cotton (74.1%) was lower than that of larvae fed on non‐Bt cotton (96.7%). Pupal weight of larvae fed on Bt cotton (0.042 g) was lower than that of larvae fed on non‐Bt cotton (0.061 g). The cotton cultivar significantly affected food intake, feces production, metabolization, and food assimilation by S. frugiperda larvae. However, it did not affect their weight gain. Intake of Bt‐cotton leaf (0.53 g dry weight) per S. frugiperda larva was lower than the intake of non‐Bt‐cotton leaf (0.61 g dry weight). Larvae fed on Bt‐cotton leaves produced less feces (0.25 g dry weight) than those fed on non‐Bt‐cotton leaves (0.37 g dry weight). Weight gain per S. frugiperda larva fed on Bt‐cotton leaves (0.058 g dry weight) was similar to the weight gain for larvae fed on non‐Bt‐cotton leaves (0.056 g dry weight). The cotton cultivar significantly affected the relative growth, consumption, and metabolic rates, as well as other nutritional indices: the figures were lower for larvae fed on Bt‐cotton leaves than for larvae fed on non‐transgenic cotton leaves.     

The role of herbivore‐ and plant‐related experiences in intraspecific host preference of a relatively specialized parasitoid

Parasitoids use odor cues from infested plants and herbivore hosts to locate their hosts. Specialist parasitoids of generalist herbivores are predicted to rely more on herbivorederived cues than plant-derived cues. Microplitis croceipes (Cresson)(Hymenoptera: Braconidae) is a relatively specialized larval endoparasitoid of Heliothis virescens (F.)(Lepidoptera: Noctuidae), which is a generalist herbivore on several crops including cotton and soybean. Using M. croceipes/H. virescens as a model system, we tested the following predictions about specialist parasitoids of generalist herbivores:(i) naive parasitoids will show innate responses to herbivore-emitted kairomones, regardless of host plant identity and (ii) herbivore-related experience will have a greater influence on intraspecific oviposition preference than plant-related experience. Inexperienced (naive) female M. croceipes did not discriminate between cotton-fed and soybean-fed H. virescens in oviposition choice tests, supporting our first prediction. Oviposition experience alone with either host group influenced subsequent oviposition preference while experience with infested plants alone did not elicit preference in M. croceipes, supporting our second prediction. Furthermore, associative learning of oviposition with host-damaged plants facilitated host location. I terestingly, naive parasitoids attacked more soybeathan cotton-fed host larvae in two-choice tests when a background of host-infested cotton odor was supplied, and vice versa. This suggests that plant volatiles may have created an olfactory contrast effect. We discussed ecological significance of the results and concluded that both plant- and herbivore-related experiences play important role in parasitoid host foraging.     

Phylogenetic test of speciation by host shift in leaf cone moths (Caloptilia) feeding on maples (Acer)

The traditional explanation for the exceptional diversity of herbivorous insects emphasizes host shift as the major driver of speciation. However, phylogenetic studies have often demonstrated widespread host plant conservatism by insect herbivores, calling into question the prevalence of speciation by host shift to distantly related plants. A limitation of previous phylogenetic studies is that host plants were defined at the family or genus level; thus, it was unclear whether host shifts predominate at a finer taxonomic scale. The lack of a statistical approach to test the hypothesis of host‐shift‐driven speciation also hindered studies at the species level. Here, we analyze the radiation of leaf cone moths (Caloptilia) associated with maples (Acer) using a newly developed, phylogeny‐based method that tests the role of host shift in speciation. This method has the advantage of not requiring complete taxon sampling from an entire radiation. Based on 254 host plant records for 14 Caloptilia species collected at 73 sites in Japan, we show that major dietary changes are more concentrated toward the root of the phylogeny, with host shift playing a minor role in recent speciation. We suggest that there may be other roles for host shift in promoting herbivorous insect diversification rather than facilitating speciation per se.     

Volatiles released from foliar extract of host plant enhance landing rates of gravid Polygonia c‐album females,but do not stimulate oviposition

The role of olfactory cues for host search is much less investigated in day‐active butterflies than in their relatives, the nocturnal moths. The goal of this study was to investigate whether host‐plant volatiles from foliar extracts of hop, Humulus lupulus L. (Cannabaceae), evoke electroantennographic (EAG) responses, increase landing rates, and stimulate egg‐laying behavior of gravid Polygonia c‐album L. (Lepidoptera: Nymphalidae) females. Eighty‐nine volatile compounds were detected in a non‐concentrated methanol extract of hop by gas chromatography–mass spectrometry, 11 of which elicited an EAG response. Concentration of the crude extract significantly reduced landing rates on artificial leaves treated with the sample due to loss of volatile compounds, but after landing the oviposition response of gravid females was not affected. A mixture of eight commercially available EAG‐active volatiles increased the landing rate of gravid females to their source but did not act as oviposition stimulants. Dividing the volatile compounds into two groups – consisting of (1) hexanal, (E)‐2‐hexenal, octanal, nonanal, and decanal, and (2) sulcatone, humulene, and benzyl alcohol – obliterated effectiveness, revealing synergism between compounds. Although volatiles did not stimulate oviposition, they significantly contributed to the distribution of eggs by increasing the landing rates on treated artificial leaves.     

Responses of a native beetle to novel exotic plant species with varying invasion history

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Elevated CO2 levels and herbivore damage alter host plant preferences

Interactions between the moth Spodoptera littoralis and two of its host plants, alfalfa (Medicago sativa) and cotton (Gossypium hirsutum) were examined, using plants grown under ambient (350 ppm) and elevated (700 ppm) CO₂ conditions. To determine strength and effects of herbivore‐induced responses assays were performed with both undamaged (control) and herbivore damaged plants. CO₂ and damage effects on larval host plant preferences were determined through dual‐choice bioassays. In addition, larvae were reared from hatching to pupation on experimental foliage to examine effects on larval growth and development. When undamaged plants were used S. littoralis larvae in consumed more cotton than alfalfa, and CO₂ enrichment caused a reduction in the preference for cotton. With damaged plants larvae consumed equal amounts of the two plant species (ambient CO₂ conditions), but CO₂ enrichment strongly shifted preferences towards cotton, which was then consumed three times more than alfalfa. Complementary assays showed that elevated CO₂ levels had no effect on the herbivore‐induced responses of cotton, whereas those of alfalfa were significantly increased. Larval growth was highest for larvae fed undamaged cotton irrespectively of CO₂ level, and lowest for larvae on damaged alfalfa from the high CO₂ treatment. Development time increased on damaged cotton irrespectively of CO₂ treatment, and on damaged alfalfa in the elevated CO₂ treatment. These results demonstrate that elevated CO₂ levels can cause insect herbivores to alter host plant preferences, and that effects on herbivore‐induced responses may be a key mechanism behind these processes. Furthermore, since the insects were shown to avoid foliage that reduced their physiological performance, our data suggest that behavioural host plant shifts result in partial escape from negative consequences of feeding on high CO₂ foliage. Thus, CO₂ enrichment can alter both physiology and behaviour of important insect herbivores, which in turn may to impact plant biodiversity.